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  • 1. Abramowski, A.
    et al.
    Acero, F.
    Aharonian, F.
    Akhperjanian, A. G.
    Anguener, E.
    Anton, G.
    Balenderan, S.
    Balzer, A.
    Barnacka, A.
    Becherini, Y.
    Tjus, J. Becker
    Bernloehr, K.
    Birsin, E.
    Bissaldi, E.
    Biteau, J.
    Boisson, C.
    Bolmont, J.
    Bordas, P.
    Brucker, J.
    Brun, F.
    Brun, P.
    Bulik, T.
    Carrigan, S.
    Casanova, S.
    Cerruti, M.
    Chadwick, P. M.
    Chalme-Calvet, R.
    Chaves, R. C. G.
    Cheesebrough, A.
    Chretien, M.
    Colafrancesco, S.
    Cologna, G.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Couturier, C.
    Dalton, M.
    Daniel, M. K.
    Davids, I. D.
    Degrange, B.
    Deil, C.
    deWilt, P.
    Dickinson, Hugh J.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Djannati-Atai, A.
    Domainko, W.
    Drury, L. O 'C.
    Dubus, G.
    Dutson, K.
    Dyks, J.
    Dyrda, M.
    Edwards, T.
    Egberts, K.
    Eger, P.
    Espigat, P.
    Farnier, Christian
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fegan, S.
    Feinstein, F.
    Fernandes, M. V.
    Fernandez, D.
    Fiasson, A.
    Fontaine, G.
    Foerster, A.
    Fuessling, M.
    Gajdus, M.
    Gallant, Y. A.
    Garrigoux, T.
    Gast, H.
    Giebels, B.
    Glicenstein, J. F.
    Goering, D.
    Grondin, M-H
    Grudzinska, M.
    Haeffner, S.
    Hague, J. D.
    Hahn, J.
    Harris, J.
    Heinzelmann, G.
    Henri, G.
    Hermann, G.
    Hervet, O.
    Hillert, A.
    Hinton, J. A.
    Hofmann, W.
    Hofverberg, P.
    Holler, M.
    Horns, D.
    Jacholkowska, A.
    Jahn, C.
    Jamrozy, M.
    Janiak, M.
    Jankowsky, F.
    Jung, I.
    Kastendieck, M. A.
    Katarzynski, K.
    Katz, U.
    Kaufmann, S.
    Khelifi, B.
    Kieffer, M.
    Klepser, S.
    Klochkov, D.
    Kluzniak, W.
    Kneiske, T.
    Kolitzus, D.
    Komin, Nu.
    Kosack, K.
    Krakau, S.
    Krayzel, F.
    Krueger, P. P.
    Laffon, H.
    Lamanna, G.
    Lefaucheur, J.
    Lemoine-Goumard, M.
    Lenain, J-P
    Lennarz, D.
    Lohse, T.
    Lopatin, A.
    Lu, C-C
    Marandon, V.
    Marcowith, A.
    Maurin, G.
    Maxted, N.
    Mayer, M.
    McComb, T. J. L.
    Medina, M. C.
    Mehault, J.
    Menzler, U.
    Meyer, M.
    Moderski, R.
    Mohamed, M.
    Moulin, E.
    Murach, T.
    Naumann, C. L.
    de Naurois, M.
    Nedbal, D.
    Niemiec, J.
    Nolan, S. J.
    Oakes, L.
    Ohm, S.
    Wilhelmi, E. de Ona
    Opitz, B.
    Ostrowski, M.
    Oya, I.
    Panter, M.
    Parsons, R. D.
    Arribas, M. Paz
    Pekeur, N. W.
    Pelletier, G.
    Perez, J.
    Petrucci, P-O
    Peyaud, B.
    Pita, S.
    Poon, H.
    Puehlhofer, G.
    Punch, M.
    Quirrenbach, A.
    Raab, S.
    Raue, M.
    Reimer, A.
    Reimer, O.
    Renaud, M.
    de los Reyes, R.
    Rieger, F.
    Rob, L.
    Rosier-Lees, S.
    Rowell, G.
    Rudak, B.
    Rulten, C. B.
    Sahakian, V.
    Sanchez, D. A.
    Santangelo, A.
    Schlickeiser, R.
    Schuessler, F.
    Schulz, A.
    Schwanke, U.
    Schwarzburg, S.
    Schwemmer, S.
    Sol, H.
    Spengler, G.
    Spiess, F.
    Stawarz, L.
    Steenkamp, R.
    Stegmann, C.
    Stinzing, F.
    Stycz, K.
    Sushch, I.
    Szostek, A.
    Tavernet, J-P
    Terrier, R.
    Tluczykont, M.
    Trichard, C.
    Valerius, K.
    van Eldik, C.
    Vasileiadis, G.
    Venter, C.
    Viana, A.
    Vincent, P.
    Voelk, H. J.
    Volpe, F.
    Vorster, M.
    Wagner, S. J.
    Wagner, P.
    Ward, M.
    Weidinger, M.
    White, R.
    Wierzcholska, A.
    Willmann, P.
    Woernlein, A.
    Wouters, D.
    Zacharias, M.
    Zajczyk, A.
    Zdziarski, A. A.
    Zech, A.
    Zechlin, H-S
    Perkins, J. S.
    Ojha, R.
    Stevens, J.
    Edwards, P. G.
    Kadler, M.
    HESS and Fermi-LAT discovery of gamma-rays from the blazar 1ES 1312-4232013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 434, no 3, 1889-1901 p.Article in journal (Refereed)
    Abstract [en]

    A deep observation campaign carried out by the High Energy Stereoscopic System (HESS) on Centaurus A enabled the discovery of gamma-rays from the blazar 1ES 1312-423, 2 degrees away from the radio galaxy. With a differential flux at 1 TeV of phi(1 TeV) = (1.9 +/- 0.6(stat) +/- 0.4(sys)) x 10(-13) cm(-2) s(-1) TeV-1 corresponding to 0.5 per cent of the Crab nebula differential flux and a spectral index Gamma = 2.9 +/- 0.5(stat) +/- 0.2(sys), 1ES 1312-423 is one of the faintest sources ever detected in the very high energy (E > 100 GeV) extragalactic sky. A careful analysis using three and a half years of Fermi Large Area Telescope (Fermi-LAT) data allows the discovery at high energies (E > 100 MeV) of a hard spectrum (Gamma = 1.4 +/- 0.4(stat) +/- 0.2(sys)) source coincident with 1ES 1312-423. Radio, optical, UV and X-ray observations complete the spectral energy distribution of this blazar, now covering 16 decades in energy. The emission is successfully fitted with a synchrotron self-Compton model for the non-thermal component, combined with a blackbody spectrum for the optical emission from the host galaxy.

  • 2. Abramowski, A.
    et al.
    Acero, F.
    Aharonian, F.
    Akhperjanian, A. G.
    Anton, G.
    Balzer, A.
    Barnacka, A.
    Becherini, Y.
    Becker, J.
    Bernloehr, K.
    Birsin, E.
    Biteau, J.
    Bochow, A.
    Boisson, C.
    Bolmont, J.
    Bordas, P.
    Brucker, J.
    Brun, F.
    Brun, P.
    Bulik, T.
    Buesching, I.
    Carrigan, S.
    Casanova, S.
    Cerruti, M.
    Chadwick, P. M.
    Charbonnier, A.
    Chaves, R. C. G.
    Cheesebrough, A.
    Cologna, G.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Dalton, M.
    Daniel, M. K.
    Davids, I. D.
    Degrange, B.
    Deil, C.
    Dickinson, Hugh J.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Djannati-Atai, A.
    Domainko, W.
    Drury, L. O 'C.
    Dubus, G.
    Dutson, K.
    Dyks, J.
    Dyrda, M.
    Egberts, K.
    Eger, P.
    Espigat, P.
    Fallon, L.
    Fegan, S.
    Feinstein, F.
    Fernandes, M. V.
    Fiasson, A.
    Fontaine, G.
    Foerster, A.
    Fuessling, M.
    Gallant, Y. A.
    Garrigoux, T.
    Gast, H.
    Gerard, L.
    Giebels, B.
    Glicenstein, J. F.
    Glueck, B.
    Goering, D.
    Grondin, M. -H
    Haeffner, S.
    Hague, J. D.
    Hahn, J.
    Hampf, D.
    Harris, J.
    Hauser, M.
    Heinz, S.
    Heinzelmann, G.
    Henri, G.
    Hermann, G.
    Hillert, A.
    Hinton, J. A.
    Hofmann, W.
    Hofverberg, P.
    Holler, M.
    Horns, D.
    Jacholkowska, A.
    Jahn, C.
    Jamrozy, M.
    Jung, I.
    Kastendieck, M. A.
    Katarzynski, K.
    Katz, U.
    Kaufmann, S.
    Khelifi, B.
    Klochkov, D.
    Kluzniak, W.
    Kneiske, T.
    Komin, Nu.
    Kosack, K.
    Kossakowski, R.
    Krayzel, F.
    Laffon, H.
    Lamanna, G.
    Lenain, J. -P
    Lennarz, D.
    Lohse, T.
    Lopatin, A.
    Lu, C. -C
    Marandon, V.
    Marcowith, A.
    Masbou, J.
    Maurin, G.
    Maxted, N.
    Mayer, M.
    McComb, T. J. L.
    Medina, M. C.
    Mehault, J.
    Moderski, R.
    Mohamed, M.
    Moulin, E.
    Naumann, C. L.
    Naumann-Godo, M.
    de Naurois, M.
    Nedbal, D.
    Nekrassov, D.
    Nguyen, N.
    Nicholas, B.
    Niemiec, J.
    Nolan, S. J.
    Ohm, S.
    Wilhelmi, E. de Ona
    Opitz, B.
    Ostrowski, M.
    Oya, I.
    Panter, M.
    Arribas, M. Paz
    Pekeur, N. W.
    Pelletier, G.
    Perez, J.
    Petrucci, P. -O
    Peyaud, B.
    Pita, S.
    Puehlhofer, G.
    Punch, M.
    Quirrenbach, A.
    Raue, M.
    Reimer, A.
    Reimer, O.
    Renaud, M.
    de los Reyes, R.
    Rieger, F.
    Ripken, Joachim
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Rob, L.
    Rosier-Lees, S.
    Rowell, G.
    Rudak, B.
    Rulten, C. B.
    Sahakian, V.
    Sanchez, D. A.
    Santangelo, A.
    Schlickeiser, R.
    Schulz, A.
    Schwanke, U.
    Schwarzburg, S.
    Schwemmer, S.
    Sheidaei, F.
    Skilton, J. L.
    Sol, H.
    Spengler, G.
    Stawarz, L.
    Steenkamp, R.
    Stegmann, C.
    Stinzing, F.
    Stycz, K.
    Sushch, I.
    Szostek, A.
    Tavernet, J. -P
    Terrier, R.
    Tluczykont, M.
    Valerius, K.
    van Eldik, C.
    Vasileiadis, G.
    Venter, C.
    Viana, A.
    Vincent, P.
    Voelk, H. J.
    Volpe, F.
    Vorobiov, S.
    Vorster, M.
    Wagner, S. J.
    Ward, M.
    White, R.
    Wierzcholska, A.
    Zacharias, M.
    Zajczyk, A.
    Zdziarski, A. A.
    Zech, A.
    Zechlin, H. -S
    Montmerle, T.
    HESS observations of the Carina nebula and its enigmatic colliding wind binary Eta Carinae2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 424, no 1, 128-135 p.Article in journal (Refereed)
    Abstract [en]

    The massive binary system Eta Carinae and the surrounding H ii complex, the Carina nebula, are potential particle acceleration sites from which very high energy (VHE; E= 100 GeV) ?-ray emission could be expected. This paper presents data collected during VHE ?-ray observations with the HESS telescope array from 2004 to 2010, which cover a full orbit of Eta Carinae. In the 33.1-h data set no hint of significant ?-ray emission from Eta Carinae has been found and an upper limit on the ?-ray flux of (99 per cent confidence level) is derived above the energy threshold of 470 GeV. Together with the detection of high energy (HE; 0.1 =E= 100 GeV) ?-ray emission by the Fermi Large Area Telescope up to 100 GeV, and assuming a continuation of the average HE spectral index into the VHE domain, these results imply a cut-off in the ?-ray spectrum between the HE and VHE ?-ray range. This could be caused either by a cut-off in the accelerated particle distribution or by severe ?? absorption losses in the wind collision region. Furthermore, the search for extended ?-ray emission from the Carina nebula resulted in an upper limit on the ?-ray flux of (99 per cent confidence level). The derived upper limit of 23 on the cosmic ray enhancement factor is compared with results found for the old-age mixed-morphology supernova remnant W28.

  • 3. Abramowski, A.
    et al.
    Acero, F.
    Aharonian, F.
    Benkhali, F. Ait
    Akhperjanian, A. G.
    Anguner, E.
    Anton, G.
    Balenderan, S.
    Balzer, A.
    Barnacka, A.
    Becherini, Y.
    Tjus, J. Becker
    Bernloehr, K.
    Birsin, E.
    Bissaldi, E.
    Biteau, J.
    Boettcher, M.
    Boisson, C.
    Bolmont, J.
    Bordas, P.
    Brucker, J.
    Brun, F.
    Brun, P.
    Bulik, T.
    Carrigan, S.
    Casanova, S.
    Cerruti, M.
    Chadwick, P. M.
    Chalme-Calvet, R.
    Chaves, R. C. G.
    Cheesebrough, A.
    Chretien, M.
    Clapson, A. -C
    Colafrancesco, S.
    Cologna, G.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Couturier, C.
    Cui, Y.
    Dalton, M.
    Daniel, M. K.
    Davids, I. D.
    Degrange, B.
    Deil, C.
    deWilt, P.
    Dickinson, H. J.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Djannati-Atai, A.
    Domainko, W.
    Drury, L. O 'C.
    Dubus, G.
    Dutson, K.
    Dyks, J.
    Dyrda, M.
    Edwards, T.
    Egberts, K.
    Eger, P.
    Espigat, P.
    Farnier, Christian
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fegan, S.
    Feinstein, F.
    Fernandes, M. V.
    Fernandez, D.
    Fiasson, A.
    Fontaine, G.
    Foerster, A.
    Fuessling, M.
    Gajdus, M.
    Gallant, Y. A.
    Garrigoux, T.
    Giavitto, G.
    Giebels, B.
    Glicenstein, J. F.
    Grondin, M. -H
    Grudzinska, M.
    Haeffner, S.
    Hahn, J.
    Harris, J.
    Heinzelmann, G.
    Henri, G.
    Hermann, G.
    Hervet, O.
    Hillert, A.
    Hinton, J. A.
    Hofmann, W.
    Hofverberg, P.
    Holler, M.
    Horns, D.
    Jacholkowska, A.
    Jahn, C.
    Jamrozy, M.
    Janiak, M.
    Jankowsky, F.
    Jung, I.
    Kastendieck, M. A.
    Katarzynski, K.
    Katz, U.
    Kaufmann, S.
    Khelifi, B.
    Kieffer, M.
    Klepser, S.
    Klochkov, D.
    Kluzniak, W.
    Kneiske, T.
    Kolitzus, D.
    Komin, Nu.
    Kosack, K.
    Krakau, S.
    Krayzel, F.
    Krueger, P. P.
    Laffon, H.
    Lamanna, G.
    Lefaucheur, J.
    Lemiere, A.
    Lemoine-Goumard, M.
    Lenain, J. -P
    Lennarz, D.
    Lohse, T.
    Lopatin, A.
    Lu, C. -C
    Marandon, V.
    Marcowith, A.
    Marx, R.
    Maurin, G.
    Maxted, N.
    Mayer, M.
    McComb, T. J. L.
    Mehault, J.
    Meintjes, P. J.
    Menzler, U.
    Meyer, Manuel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Moderski, R.
    Mohamed, M.
    Moulin, E.
    Murach, T.
    Naumann, C. L.
    de Naurois, M.
    Niemiec, J.
    Nolan, S. J.
    Oakes, L.
    Ohm, S.
    Wilhelmi, E. de Ona
    Opitz, B.
    Ostrowski, M.
    Oya, I.
    Panter, M.
    Parsons, R. D.
    Arribas, M. Paz
    Pekeur, N. W.
    Pelletier, G.
    Perez, J.
    Petrucci, P. -O
    Peyaud, B.
    Pita, S.
    Poon, H.
    Puehlhofer, G.
    Punch, M.
    Quirrenbach, A.
    Raab, S.
    Raue, M.
    Reimer, A.
    Reimer, O.
    Renaud, M.
    de los Reyes, R.
    Rieger, F.
    Rob, L.
    Romoli, C.
    Rosier-Lees, S.
    Rowell, G.
    Rudak, B.
    Rulten, C. B.
    Sahakian, V.
    Sanchez, D. A.
    Santangelo, A.
    Schlickeiser, R.
    Schuessler, F.
    Schulz, A.
    Schwanke, U.
    Schwarzburg, S.
    Schwemmer, S.
    Sol, H.
    Spengler, G.
    Spies, F.
    Stawarz, L.
    Steenkamp, R.
    Stegmann, C.
    Stinzing, F.
    Stycz, K.
    Sushch, I.
    Szostek, A.
    Tavernet, J. -P
    Tavernier, T.
    Taylor, A. M.
    Terrier, R.
    Tluczykont, M.
    Trichard, C.
    Valerius, K.
    van Eldik, C.
    van Soelen, B.
    Vasileiadis, G.
    Venter, C.
    Viana, A.
    Vincent, P.
    Voelk, H. J.
    Volpe, F.
    Vorster, M.
    Vuillaume, T.
    Wagner, S. J.
    Wagner, P.
    Ward, M.
    Weidinger, M.
    Weitzel, Q.
    White, R.
    Wierzcholska, A.
    Willmann, P.
    Woernlein, A.
    Wouters, D.
    Zabalza, V.
    Zacharias, M.
    Zajczyk, A.
    Zdziarski, A. A.
    Zech, A.
    Zechlin, H. -S
    Discovery of the VHE gamma-ray source HESS J1832-093 in the vicinity of SNR G22.7-0.22015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 446, no 2, 1163-1169 p.Article in journal (Refereed)
    Abstract [en]

    The region around the supernova remnant (SNR) W41 contains several TeV sources and has prompted the HESS Collaboration to perform deep observations of this field of view. This resulted in the discovery of the new very high energy (VHE) source HESS J1832-093, at the position RA = 18(h)32(m)50(s) +/- 3(stat)(s) +/- 2(syst)(s), Dec = -9 degrees 22'36 '' +/- 32(stat)'' +/- 20(syst)'' (J2000), spatially coincident with a part of the radio shell of the neighbouring remnant G22.7-0.2. The photon spectrum is well described by a power law of index Gamma = 2.6 +/- 0.3(stat) +/- 0.1(syst) and a normalization at 1 TeV of Phi(0) = (4.8 +/- 0.8(stat) +/- 1.0(syst)) x 10(-13) cm(-2) s(-1) TeV-1. The location of the gamma-ray emission on the edge of the SNR rim first suggested a signature of escaping cosmic rays illuminating a nearby molecular cloud. Then a dedicated XMM-Newton observation led to the discovery of a new X-ray point source spatially coincident with the TeV excess. Two other scenarios were hence proposed to identify the nature of HESS J1832-093. Gamma-rays from inverse Compton radiation in the framework of a pulsar wind nebula scenario or the possibility of gamma-ray production within a binary system are therefore also considered. Deeper multiwavelength observations will help to shed new light on this intriguing VHE source.

  • 4. Abramowski, A.
    et al.
    Aharonian, F.
    Benkhali, F. Ait
    Akhperjanian, A. G.
    Anguener, E.
    Anton, G.
    Balenderan, S.
    Balzer, A.
    Barnacka, A.
    Becherini, Y.
    Tjus, J. Becker
    Bernloehr, K.
    Birsin, E.
    Bissaldi, E.
    Biteau, J.
    Boettcher, M.
    Boisson, C.
    Bolmont, J.
    Bordas, P.
    Brucker, J.
    Brun, F.
    Brun, P.
    Bulik, T.
    Carrigan, S.
    Casanova, S.
    Cerruti, M.
    Chadwick, P. M.
    Chalme-Calvet, R.
    Chaves, R. C. G.
    Cheesebrough, A.
    Chretien, M.
    Colafrancesco, S.
    Cologna, G.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Couturier, C.
    Cui, Y.
    Dalton, M.
    Daniel, M. K.
    Davids, I. D.
    Degrange, B.
    Deil, C.
    deWilt, P.
    Dickinson, Hugh J.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Djannati-Atai, A.
    Domainko, W.
    Drury, L. O ' C
    Dubus, G.
    Dutson, K.
    Dyks, J.
    Dyrda, M.
    Edwards, T.
    Egberts, K.
    Eger, P.
    Espigat, P.
    Farnier, Christian
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fegan, S.
    Feinstein, F.
    Fernandes, M. V.
    Fernandez, D.
    Fiasson, A.
    Fontaine, G.
    Foerster, A.
    Fuessling, M.
    Gajdus, M.
    Gallant, Y. A.
    Garrigoux, T.
    Giavitto, G.
    Giebels, B.
    Glicenstein, J. F.
    Grondin, M. -H
    Grudzinska, M.
    Haeffner, S.
    Hahn, J.
    Harris, J.
    Heinzelmann, G.
    Henri, G.
    Hermann, G.
    Hervet, O.
    Hillert, A.
    Hinton, J. A.
    Hofmann, W.
    Hofverberg, P.
    Holler, M.
    Horns, D.
    Jacholkowska, A.
    Jahn, C.
    Jamrozy, M.
    Janiak, M.
    Jankowsky, F.
    Jung, I.
    Kastendieck, M. A.
    Katarzynski, K.
    Katz, U.
    Kaufmann, S.
    Khelifi, B.
    Kieffer, M.
    Klepser, S.
    Klochkov, D.
    Kluzniak, W.
    Kneiske, T.
    Kolitzus, D.
    Komin, Nu
    Kosack, K.
    Krakau, S.
    Krayzel, F.
    Krueger, P. P.
    Laffon, H.
    Lamanna, G.
    Lefaucheur, J.
    Lemiere, A.
    Lemoine-Goumard, M.
    Lenain, J. -P
    Lennarz, D.
    Lohse, T.
    Lopatin, A.
    Lu, C. -C
    Marandon, V.
    Marcowith, A.
    Marx, R.
    Maurin, G.
    Maxted, N.
    Mayer, M.
    McComb, T. J. L.
    Mehault, J.
    Meintjes, P. J.
    Menzler, U.
    Meyer, M.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Moderski, R.
    Mohamed, M.
    Moulin, E.
    Murach, T.
    Naumann, C. L.
    de Naurois, M.
    Niemiec, J.
    Nolan, S. J.
    Oakes, L.
    Ohm, S.
    Wilhelmi, E. de Ona
    Opitz, B.
    Ostrowski, M.
    Oya, I.
    Panter, M.
    Parsons, R. D.
    Arribas, M. Paz
    Pekeur, N. W.
    Pelletier, G.
    Perez, J.
    Petrucci, P. -O
    Peyaud, B.
    Pita, S.
    Poon, H.
    Puehlhofer, G.
    Punch, M.
    Quirrenbach, A.
    Raab, S.
    Raue, M.
    Reimer, A.
    Reimer, O.
    Renaud, M.
    de los Reyes, R.
    Rieger, F.
    Rob, L.
    Romoli, C.
    Rosier-Lees, S.
    Rowell, G.
    Rudak, B.
    Rulten, C. B.
    Sahakian, V.
    Sanchez, D. A.
    Santangelo, A.
    Schlickeiser, R.
    Schuessler, F.
    Schulz, A.
    Schwanke, U.
    Schwarzburg, S.
    Schwemmer, S.
    Sol, H.
    Spengler, G.
    Spies, F.
    Stawarz, L.
    Steenkamp, R.
    Stegmann, C.
    Stinzing, F.
    Stycz, K.
    Sushch, I.
    Szostek, A.
    Tavernet, J. -P
    Tavernier, T.
    Taylor, A. M.
    Terrier, R.
    Tluczykont, M.
    Trichard, C.
    Valerius, K.
    van Eldik, C.
    van Soelen, B.
    Vasileiadis, G.
    Venter, C.
    Viana, A.
    Vincent, P.
    Voelk, H. J.
    Volpe, F.
    Vorster, M.
    Vuillaume, T.
    Wagner, S. J.
    Wagner, P.
    Ward, M.
    Weidinger, M.
    Weitzel, Q.
    White, R.
    Wierzcholska, A.
    Willmann, P.
    Woernlein, A.
    Wouters, D.
    Zabalza, V.
    Zacharias, M.
    Zajczyk, A.
    Zdziarski, A. A.
    Zech, A.
    Zechlin, H. -S
    TeV gamma-ray observations of the young synchrotron-dominated SNRs G1.9+0.3 and G330.2+1.0 with HESS2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 441, no 1, 790-799 p.Article in journal (Refereed)
    Abstract [en]

    The non-thermal nature of the X-ray emission from the shell-type supernova remnants (SNRs) G1.9+0.3 and G330.2+1.0 is an indication of intense particle acceleration in the shock fronts of both objects. This suggests that the SNRs are prime candidates for very-high-energy (VHE; E > 0.1 TeV) gamma-ray observations. G1.9+0.3, recently established as the youngest known SNR in the Galaxy, also offers a unique opportunity to study the earliest stages of SNR evolution in the VHE domain. The purpose of this work is to probe the level of VHE gamma-ray emission from both SNRs and use this to constrain their physical properties. Observations were conducted with the H. E. S. S. (High Energy Stereoscopic System) Cherenkov Telescope Array over a more than six-year period spanning 2004-2010. The obtained data have effective livetimes of 67 h for G1.9+0.3 and 16 h for G330.2+1.0. The data are analysed in the context of the multiwavelength observations currently available and in the framework of both leptonic and hadronic particle acceleration scenarios. No significant gamma-ray signal from G1.9+0.3 or G330.2+1.0 was detected. Upper limits (99 per cent confidence level) to the TeV flux from G1.9+0.3 and G330.2+1.0 for the assumed spectral index Gamma = 2.5 were set at 5.6 x 10(-1)3 cm(-2) s(-1) above 0.26 TeV and 3.2 x 10(-12) cm(-2) s(-1) above 0.38 TeV, respectively. In a one-zone leptonic scenario, these upper limits imply lower limits on the interior magnetic field to B-G1.9 greater than or similar to 12 mu G for G1.9+0.3 and to B-G330 greater than or similar to 8 mu G for G330.2+1.0. In a hadronic scenario, the low ambient densities and the large distances to the SNRs result in very low predicted fluxes, for which the H.E.S.S. upper limits are not constraining.

  • 5. Abramowski, A.
    et al.
    Aharonian, F.
    Benkhali, F. Ait
    Akhperjanian, A. G.
    Anguener, E.
    Anton, G.
    Balenderan, S.
    Balzer, A.
    Barnacka, A.
    Becherini, Y.
    Tjus, J. Becker
    Bernloehr, K.
    Birsin, E.
    Bissaldi, E.
    Biteau, J.
    Boettcher, M.
    Boisson, C.
    Bolmont, J.
    Bordas, P.
    Brucker, J.
    Brun, F.
    Brun, P.
    Bulik, T.
    Carrigan, S.
    Casanova, S.
    Cerruti, M.
    Chadwick, P. M.
    Chalme-Calvet, R.
    Chaves, R. C. G.
    Cheesebrough, A.
    Chretien, M.
    Colafrancesco, S.
    Cologna, G.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Couturier, C.
    Cui, Y.
    Dalton, M.
    Daniel, M. K.
    Davids, I. D.
    Degrange, B.
    Deil, C.
    deWilt, P.
    Dickinson, Hugh J.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Djannati-Ataie, A.
    Domainko, W.
    Drury, L. O 'C.
    Dubus, G.
    Dutson, K.
    Dyks, J.
    Dyrda, M.
    Edwards, T.
    Egberts, K.
    Eger, P.
    Espigat, P.
    Farnier, Christian
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fegan, S.
    Feinstein, F.
    Fernandes, M. V.
    Fernandez, D.
    Fiasson, A.
    Fontaine, G.
    Foerster, A.
    Fuessling, M.
    Gajdus, M.
    Gallant, Y. A.
    Garrigoux, T.
    Giavitto, G.
    Giebels, B.
    Glicenstein, J. F.
    Grondin, M. -H
    Grudzinska, M.
    Haeffner, S.
    Hahn, J.
    Harris, J.
    Heinzelmann, G.
    Henri, G.
    Hermann, G.
    Hervet, O.
    Hillert, A.
    Hinton, J. A.
    Hofmann, W.
    Hofverberg, P.
    Holler, M.
    Horns, D.
    Jacholkowska, A.
    Jahn, C.
    Jamrozy, M.
    Janiak, M.
    Jankowsky, F.
    Jung, I.
    Kastendieck, M. A.
    Katarzynski, K.
    Katz, U.
    Kaufmann, S.
    Khelifi, B.
    Kieffer, M.
    Klepser, S.
    Klochkov, D.
    Kluzniak, W.
    Kneiske, T.
    Kolitzus, D.
    Komin, Nu.
    Kosack, K.
    Krakau, S.
    Krayzel, F.
    Krueger, P. P.
    Laffon, H.
    Lamanna, G.
    Lefaucheur, J.
    Lemiere, A.
    Lemoine-Goumard, M.
    Lenain, J. -P
    Lennarz, D.
    Lohse, T.
    Lopatin, A.
    Lu, C. -C
    Marandon, V.
    Marcowith, A.
    Marx, R.
    Maurin, G.
    Maxted, N.
    Mayer, M.
    McComb, T. J. L.
    Mehault, J.
    Meintjes, P. J.
    Menzler, U.
    Meyer, Manuel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Moderski, R.
    Mohamed, M.
    Moulin, E.
    Murach, T.
    Naumann, C. L.
    de Naurois, M.
    Niemiec, J.
    Nolan, S. J.
    Oakes, L.
    Ohm, S.
    Wilhelmi, E. de Ona
    Opitz, B.
    Ostrowski, M.
    Oya, I.
    Panter, M.
    Parsons, R. D.
    Arribas, M. Paz
    Pekeur, N. W.
    Pelletier, G.
    Perez, J.
    Petrucci, P. -O
    Peyaud, B.
    Pita, S.
    Poon, H.
    Puehlhofer, G.
    Punch, M.
    Quirrenbach, A.
    Raab, S.
    Raue, M.
    Reimer, A.
    Reimer, O.
    Renaud, M.
    de los Reyes, R.
    Rieger, F.
    Rob, L.
    Romoli, C.
    Rosier-Lees, S.
    Rowell, G.
    Rudak, B.
    Rulten, C. B.
    Sahakian, V.
    Sanchez, D. A.
    Santangelo, A.
    Schlickeiser, R.
    Schuessler, F.
    Schulz, A.
    Schwanke, U.
    Schwarzburg, S.
    Schwemmer, S.
    Sol, H.
    Spengler, G.
    Spies, F.
    Stawarz, L.
    Steenkamp, R.
    Stegmann, C.
    Stinzing, F.
    Stycz, K.
    Sushch, I.
    Szostek, A.
    Tavernet, J. -P
    Tavernier, T.
    Taylor, A. M.
    Terrier, R.
    Tluczykont, M.
    Trichard, C.
    Valerius, K.
    van Eldik, C.
    van Soelen, B.
    Vasileiadis, G.
    Venter, C.
    Viana, A.
    Vincent, P.
    Vink, J.
    Voelk, H. J.
    Volpe, F.
    Vorster, M.
    Vuillaume, T.
    Wagner, S. J.
    Wagner, P.
    Ward, M.
    Weidinger, M.
    Weitzel, Q.
    White, R.
    Wierzcholska, A.
    Willmann, P.
    Woernlein, A.
    Wouters, D.
    Zabalza, V.
    Zacharias, M.
    Zajczyk, A.
    Zdziarski, A. A.
    Zech, A.
    Zechlin, H. -S
    HESS J1640-465-an exceptionally luminous TeV gamma-ray supernova remnant2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 439, no 3, 2828-2836 p.Article in journal (Refereed)
    Abstract [en]

    The results of follow-up observations of the TeV gamma-ray source HESS J1640-465 from 2004 to 2011 with the High Energy Stereoscopic System (HESS) are reported in this work. The spectrum is well described by an exponential cut-off power law with photon index Gamma = 2.11 +/- 0.09(stat) +/- 0.10(sys), and a cut-off energy of E-2 = 6.0(-1.2)(+2.0) TeV. The TeV emission is significantly extended and overlaps with the northwestern part of the shell of the SNR G338.3-0.0. The new HESS results, a re-analysis of archival XMM-Newton data and multiwavelength observations suggest that a significant part of the gamma-ray emission from HESS J1640-465 originates in the supernova remnant shell. In a hadronic scenario, as suggested by the smooth connection of the GeV and TeV spectra, the product of total proton energy and mean target density could be as high as W(p)n(H) similar to 4 x 10(52)(d/10kpc)(2) erg cm(-3).

  • 6. Acero, F.
    et al.
    Aharonian, F.
    Akhperjanian, A. G.
    Anton, G.
    de Almeida, U. Barres
    Bazer-Bachi, A. R.
    Becherini, Y.
    Behera, B.
    Bernloehr, K.
    Bochow, A.
    Boisson, C.
    Bolmont, J.
    Borrel, V.
    Braun, I.
    Brucker, J.
    Brun, F.
    Brun, P.
    Buehler, R.
    Bulik, T.
    Buesching, I.
    Boutelier, T.
    Chadwick, P. M.
    Charbonnier, A.
    Chaves, R. C. G.
    Cheesebrough, A.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Chounet, L. -M
    Clapson, A. C.
    Coignet, G.
    Dalton, M.
    Daniel, M. K.
    Davids, I. D.
    Degrange, B.
    Deil, C.
    Dickinson, H. J.
    Djannati-Atai, A.
    Domainko, W.
    Drury, L. O 'C.
    Dubois, F.
    Dubus, G.
    Dyks, J.
    Dyrda, M.
    Egberts, K.
    Eger, P.
    Espigat, P.
    Fallon, L.
    Farnier, C.
    Fegan, S.
    Feinstein, F.
    Fiasson, A.
    Foerster, A.
    Fontaine, G.
    Fuessling, M.
    Gabici, S.
    Gallant, Y. A.
    Gerard, L.
    Gerbig, D.
    Giebels, B.
    Glicenstein, J. F.
    Glueck, B.
    Goret, P.
    Goering, D.
    Hauser, M.
    Heinz, S.
    Heinzelmann, G.
    Henri, G.
    Hermann, G.
    Hinton, J. A.
    Hoffmann, A.
    Hofmann, W.
    Hofverberg, P.
    Holleran, M.
    Hoppe, S.
    Horns, D.
    Jacholkowska, A.
    de Jager, O. C.
    Jahn, C.
    Jung, I.
    Katarzynski, K.
    Katz, U.
    Kaufmann, S.
    Kerschhaggl, M.
    Khangulyan, D.
    Khelifi, B.
    Keogh, D.
    Klochkov, D.
    Kluzniak, W.
    Kneiske, T.
    Komin, Nu.
    Kosack, K.
    Kossakowski, R.
    Lamanna, G.
    Lenain, J. -P
    Lohse, T.
    Marandon, V.
    Martineau-Huynh, O.
    Marcowith, A.
    Masbou, J.
    Maurin, D.
    McComb, T. J. L.
    Medina, M. C.
    Mehault, J.
    Moderski, R.
    Moulin, E.
    Naumann-Godo, M.
    de Naurois, M.
    Nedbal, D.
    Nekrassov, D.
    Nicholas, B.
    Niemiec, J.
    Nolan, S. J.
    Ohm, S.
    Olive, J-F
    Wilhelmi, E. de Ona
    Orford, K. J.
    Ostrowski, M.
    Panter, M.
    Arribas, M. Paz
    Pedaletti, G.
    Pelletier, G.
    Petrucci, P. -O
    Pita, S.
    Puehlhofer, G.
    Punch, M.
    Quirrenbach, A.
    Raubenheimer, B. C.
    Raue, M.
    Rayner, S. M.
    Reimer, O.
    Renaud, M.
    Rieger, F.
    Ripken, Joachim
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Rob, L.
    Rosier-Lees, S.
    Rowell, G.
    Rudak, B.
    Rulten, C. B.
    Ruppel, J.
    Ryde, F.
    Sahakian, V.
    Santangelo, A.
    Schlickeiser, R.
    Schoeck, F. M.
    Schoenwald, A.
    Schwanke, U.
    Schwarzburg, S.
    Schwemmer, S.
    Shalchi, A.
    Sikora, M.
    Skilton, J. L.
    Sol, H.
    Stawarz, L.
    Steenkamp, R.
    Stegmann, C.
    Stinzing, F.
    Superina, G.
    Sushch, I.
    Szostek, A.
    Tam, P. H.
    Tavernet, J. -P
    Terrier, R.
    Tibolla, O.
    Tluczykont, M.
    van Eldik, C.
    Vasileiadis, G.
    Venter, C.
    Venter, L.
    Vialle, J. P.
    Vincent, P.
    Vivier, M.
    Voelk, H. J.
    Volpe, F.
    Wagner, S. J.
    Ward, M.
    Zdziarski, A. A.
    Zech, A.
    Localizing the VHE gamma-ray source at the Galactic Centre2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 402, no 3, 1877-1882 p.Article in journal (Refereed)
    Abstract [en]

    The inner 10 pc of our Galaxy contains many counterpart candidates of the very high energy (VHE; > 100 GeV) gamma-ray point source HESS J1745-290. Within the point spread function of the H.E.S.S. measurement, at least three objects are capable of accelerating particles to VHE and beyond and of providing the observed gamma-ray flux. Previous attempts to address this source confusion were hampered by the fact that the projected distances between these objects were of the order of the error circle radius of the emission centroid (34 arcsec, dominated by the pointing uncertainty of the H.E.S.S. instrument). Here we present H.E.S.S. data of the Galactic Centre region, recorded with an improved control of the instrument pointing compared to H.E.S.S. standard pointing procedures. Stars observed during gamma-ray observations by optical guiding cameras mounted on each H.E.S.S. telescope are used for off-line pointing calibration, thereby decreasing the systematic pointing uncertainties from 20 to 6 arcsec per axis. The position of HESS J1745-290 is obtained by fitting a multi-Gaussian profile to the background-subtracted gamma-ray count map. A spatial comparison of the best-fitting position of HESS J1745-290 with the position and morphology of candidate counterparts is performed. The position is, within a total error circle radius of 13 arcsec, coincident with the position of the supermassive black hole Sgr A* and the recently discovered pulsar wind nebula candidate G359.95-0.04. It is significantly displaced from the centroid of the supernova remnant Sgr A East, excluding this object with high probability as the dominant source of the VHE gamma-ray emission.

  • 7. Adamo, A.
    et al.
    Smith, L. J.
    Gallagher, J. S.
    Bastian, N.
    Ryon, J.
    Westmoquette, M. S.
    Konstantopoulos, I. S.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Larsen, S. S.
    Silva-Villa, E.
    Charlton, J. C.
    Weisz, D. R.
    Revealing a ring-like cluster complex in a tidal tail of the starburst galaxy NGC 21462012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 426, no 2, 1185-1194 p.Article in journal (Refereed)
    Abstract [en]

    We report the discovery of a ring-like cluster complex in the starburst galaxy NGC?2146. The Ruby Ring, so named due to its appearance, shows a clear ring-like distribution of star clusters around a central object. It is located in one of the tidal streams that surround the galaxy. NGC?2146 is part of the Snapshot Hubble U-band Cluster Survey (SHUCS). The WFC3/F336W data have added critical information to the available archival Hubble Space Telescope imaging set of NGC?2146, allowing us to determine ages, masses and extinctions of the clusters in the Ruby Ring. These properties have then been used to investigate the formation of this extraordinary system. We find evidence of a spatial and temporal correlation between the central cluster and the clusters in the ring. The latter are about 4?Myr younger than the central cluster, which has an age of 7?Myr. This result is supported by the Ha emission which is strongly coincident with the ring, and weaker at the position of the central cluster. From the derived total Ha luminosity of the system, we constrain the star formation rate density to be quite high (SSFR = 0.47?M??yr-1?kpc-2). The Ruby Ring is the product of an intense and localized burst of star formation, similar to the extended cluster complexes observed in M?51 and the Antennae, but more impressive because it is quite isolated. The central cluster contains only 5 per cent of the total stellar mass in the clusters that are determined within the complex. The ring-like morphology, the age spread and the mass ratio support a triggering formation scenario for this complex. We discuss the formation of the Ruby Ring in a collect and collapse framework. The predictions made by this model agree quite well with the estimated bubble radius and expansion velocity produced by the feedback from the central cluster, making the Ruby Ring an interesting case of triggered star formation.

  • 8.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kruijssen, J. M. D.
    Bastian, N.
    Silva-Villa, E.
    Ryon, J.
    Probing the role of the galactic environment in the formation of stellar clusters, using M83 as a test bench2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 452, no 1, 246-260 p.Article in journal (Refereed)
    Abstract [en]

    We present a study of the M83 cluster population, covering the disc of the galaxy between radii of 0.45 and 4.5 kpc. We aim to probe the properties of the cluster population as a function of distance from the galactic centre. We observe a net decline in cluster formation efficiency (Gamma, i.e. amount of star formation happening in bound clusters) from about 26 per cent in the inner region to 8 per cent in the outer part of the galaxy. The recovered Gamma values within different regions of M83 follow the same Gamma versus star formation rate density relation observed for entire galaxies. We also probe the initial cluster mass function (ICMF) as a function of galactocentric distance. We observe a significant steepening of the ICMF in the outer regions (from -1.90 +/- 0.11 to -2.70 +/- 0.14) and for the whole galactic cluster population (slope of -2.18 +/- 0.07) of M83. We show that this change of slope reflects a more fundamental change of the 'truncation mass' at the high-mass end of the distribution. This can be modelled as a Schechter function of slope -2 with an exponential cutoff mass (M-c) that decreases significantly from the inner to the outer regions (from 4.00 to 0.25 x 10(5) M-circle dot) while the galactic M-c is approximate to 1.60 x 10(5) M-circle dot. The trends in Gamma and ICMF are consistent with the observed radial decrease of the Sigma (H-2), hence in gas pressure. As gas pressure declines, cluster formation becomes less efficient. We conclude that the host galaxy environment appears to regulate (1) the fraction of stars locked in clusters and (2) the upper mass limit of the ICMF, consistently described by a near-universal slope -2 truncated at the high-mass end.

  • 9.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Probing cluster formation under extreme conditions: massive star clusters in blue compact galaxies2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 417, no 3, 1904-1912 p.Article in journal (Refereed)
    Abstract [en]

    The numerous and massive young star clusters in blue compact galaxies (BCGs) are used to investigate the properties of their hosts. We test whether BCGs follow claimed relations between cluster populations and their hosts, such as the fraction of the total luminosity contributed by the clusters as function of the mean star formation rate (SFR) density, the V-band luminosity of the brightest youngest cluster as related to the mean host SFR and the cluster formation efficiency (i.e. the fraction of star formation happening in star clusters) versus the density of the SFR. We find that BCGs follow the trends, supporting a scenario where cluster formation and environmental properties of the host are correlated. They occupy, in all the diagrams, the regions of higher SFRs, as expected by the extreme nature of the starbursts operating in these systems. We find that the star clusters contribute almost to the 20 per cent of the UV luminosity of the hosts. We suggest that the BCG starburst environment has most likely favoured the compression and collapse of the giant molecular clouds, enhancing the local star formation efficiency, so that massive clusters have been formed. The estimated cluster formation efficiency supports this scenario. BCGs have a cluster formation efficiency comparable to luminous IR galaxies and spiral starburst nuclei (the averaged value is similar to 35 per cent) which is much higher than the 8-10 per cent reported for quiescent spirals and dwarf star-forming galaxies.

  • 10.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hayes, Matthew
    The Massive Star Clusters in the Dwarf Merger ESO 185-IG13: is the Red Excess Ubiquitous in Starbursts?2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, no 3, 1793-1812 p.Article in journal (Other academic)
    Abstract [en]

    We have investigated the starburst properties of the luminous blue compact galaxy ESO 185-IG13. The galaxy has been imaged with the high resolution cameras onboard to the Hubble Space Telescope. From the UV to the IR, the data reveal a system shaped by hundreds of young star clusters, and fine structures, like a tidal stream and a shell. The presence of numerous clusters and the perturbed morphology indicate that the galaxy has been involved in a recent merger event. Using previous simulations of shell formation in galaxy mergers we constrain potential progenitors of ESO 185-IG13. The analysis of the star cluster population is used to investigate the properties of the present starburst and to date the final merger event, which has produced hundreds of clusters younger than 100 Myr. We have found a peak of cluster formation only 3.5 Myr old. A large fraction of these clusters will not survive after 10-20 Myr, due to the "infant mortality" caused by gas expulsion. However, this sample of clusters represents an unique chance to investigate the youngest phases of cluster evolution. As already observed in the analog blue compact galaxy Haro 11, a fraction of young clusters are affected by a flux excess at wavelengths longer than 8000 \AA. Ages, masses, and extinctions of clusters with this NIR excess are estimated from UV and optical data. We discuss similarities and differences of the observed NIR excess in ESO 185-IG13 clusters with other cases in the literature. The cluster ages and masses are used to distinguish among the potential causes of the excess. We observe, as in Haro 11, that the use of the IR and the (commonly used) I band data results in overestimates of age and mass in clusters affected by the NIR excess. This has important implications for a number of related studies of star clusters.

  • 11.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hayes, Matthew
    Observatoire Astronomique de l'Université de Genève.
    Cumming, Robert
    Stockholm University, Faculty of Science, Department of Astronomy.
    Micheva, Genoveva
    Stockholm University, Faculty of Science, Department of Astronomy.
    Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, MNRAS, Vol. 407, no 2, 870-890 p.Article in journal (Refereed)
    Abstract [en]

    We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO350-IG038) by means of its stellar cluster population. The central starburst region has been observed in eight different high-resolution Hubble Space Telescope (HST) wavebands, sampling the stellar and gas components from UV to near-infrared. Photometric imaging of the galaxy was also carried out at 2.16μm by NaCo AO instrument at the ESO Very Large Telescope. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters located in the active star-forming regions and compared them with single stellar population models (suitable for physical properties of very young cluster population) in order to derive ages, masses and extinctions of the star clusters. The cluster age distribution we recover confirms that the present starburst has lasted for 40Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. We looked for possible relations between cluster ages, extinctions and masses. Extinction tends to diminish as a function of the cluster age, but the spread is large and reaches the highest dispersion for clusters in partial embedded phases (<5Myr). A fraction of low-mass (below 104 Msolar), very young (1-3Myr) clusters is missing, either because they are embedded in the parental molecular cloud and heavily extinguished, or because of blending with neighbouring clusters. The range of the cluster masses is wide; we observe that more than 30 per cent of the clusters have masses above 105 Msolar, qualifying them as super star clusters. Almost half of the cluster sample is affected by flux excesses at wavelengths >8000Å which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40Myr we discuss possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 has produced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. The present cluster formation efficiency is ~38 per cent of the galactic star formation rate.

  • 12.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Papaderos, P.
    Bergvall, N.
    Rich, R. M.
    Micheva, Genoveva
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Star cluster formation and evolution in Mrk 930: properties of a metal-poor starburst2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 415, no 3, 2388-2406 p.Article in journal (Refereed)
    Abstract [en]

    We present the analysis of the large population of star clusters in the blue compact galaxy (BCG) Mrk 930. The study has been conducted by means of a photometric analysis of multiband data obtained with the Hubble Space Telescope (HST). We have reconstructed the spectral energy distributions of the star clusters and estimated the age, mass and extinction for a representative sample. Similar to previous studies of star clusters in BCGs, we observe a very young cluster population with 70 per cent of the systems formed less than 10 Myr ago. In Mrk 930, the peak in the star cluster age distribution at 4 Myr is corroborated by the presence of Wolf-Rayet spectral features, and by the observed optical and infrared (IR) line ratios [OIII]/H beta and [Ne III]/[Ne II]. The recovered extinction in these very young clusters shows large variations, with a decrease at older ages. It is likely that our analysis is limited to the optically brightest objects (i.e. systems only partially embedded in their natal cocoons; the deeply embedded clusters being undetected). We map the extinction across the galaxy using low-resolution spectra and the H alpha-to-H beta ratio, as obtained from ground-based narrow band imaging. These results are compared with the extinction distribution recovered from the clusters. We find that the mean optical extinction derived in the starburst regions is close to the averaged value observed in the clusters [more than 80 per cent of the systems have E(B - V) <= 0.2mag], but locally, do not trace the more extinguished clusters. Previous HST studies of BCGs have revealed a population of young and extremely red super star clusters. We detect a considerable fraction of clusters affected by a red excess also in Mrk 930. The nature of the red excess, which turns up at near-IR wavelengths (I band and longwards), remains unknown. We compare the cluster formation history and the star formation history, the latter derived from the fit of spectral population synthesis models to the spectra. We find a general agreement between the two independently estimated quantities. Using the cluster properties, we perform a study of the host environmental properties. We find that the cluster formation efficiency (the fraction of star formation happening in clusters) is significantly higher, suggesting a key role of the environment for the formation of these massive objects.

  • 13.
    Amanullah, Rahman
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Johansson, Joel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ferretti, Raphael
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Papadogiannakis, Seméli
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Petrushevska, Tanja
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Brown, P. J.
    Cao, Y.
    Contreras, C.
    Dahle, H.
    Elias-Rosa, N.
    Fynbo, J. P. U.
    Gorosabel, J.
    Guaita, L.
    Hangard, Laura
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Howell, D. A.
    Hsiao, E. Y.
    Kankare, E.
    Kasliwal, M.
    Leloudas, G.
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mattila, S.
    Nugent, P.
    Phillips, M. M.
    Sandberg, Andreas
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Stanishev, V.
    Sullivan, M.
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Asadi, Saghar
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Herrero-Illana, R.
    Jensen, J. J.
    Karhunen, K.
    Lazarevic, S.
    Varenius, E.
    Santos, P.
    Sridhar, S. Seethapuram
    Wallström, S. H. J.
    Wiegert, J.
    Diversity in extinction laws of Type Ia supernovae measured between 0.2 and 2 mu m2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 453, no 3, 3300-3328 p.Article in journal (Refereed)
    Abstract [en]

    We present ultraviolet (UV) observations of six nearby Type Ia supernovae (SNe Ia) obtained with the Hubble Space Telescope, three of which were also observed in the near-IR (NIR) with Wide-Field Camera 3. UV observations with the Swift satellite, as well as ground-based optical and NIR data provide complementary information. The combined data set covers the wavelength range 0.2-2 mu m. By also including archival data of SN 2014J, we analyse a sample spanning observed colour excesses up to E(B - V) = 1.4 mag. We study the wavelength-dependent extinction of each individual SN and find a diversity of reddening laws when characterized by the total-to-selective extinction R-V. In particular, we note that for the two SNe with E(B - V) greater than or similar to 1 mag, for which the colour excess is dominated by dust extinction, we find R-V = 1.4 +/- 0.1 and R-V = 2.8 +/- 0.1. Adding UV photometry reduces the uncertainty of fitted R-V by similar to 50 per cent allowing us to also measure R-V of individual low-extinction objects which point to a similar diversity, currently not accounted for in the analyses when SNe Ia are used for studying the expansion history of the Universe.

  • 14. Amarsi, A. M.
    et al.
    Asplund, M.
    Collet, R.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Non-LTE oxygen line formation in 3D hydrodynamic model stellar atmospheres2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 455, no 4, 3735-3751 p.Article in journal (Refereed)
    Abstract [en]

    The O (Iota) 777 nm lines are among the most commonly used diagnostics for the oxygen abundances in the atmospheres of FGK-type stars. However, they form in conditions that are far from local thermodynamic equilibrium (LTE). We explore the departures from LTE of atomic oxygen, and their impact on O I lines, across the STAGGER-grid of three-dimensional hydrodynamic model atmospheres. For the O (Iota) 777 nm triplet, we find significant departures from LTE. These departures are larger in stars with larger effective temperatures, smaller surface gravities, and larger oxygen abundances. We present grids of predicted 3D non-LTE based equivalent widths for the O (Iota) 616 nm, [O (Iota)] 630 nm, [O (I)] 636 nm, and O (Iota) 777 nm lines, as well as abundance corrections to 1D LTE based results.

  • 15. Amarsi, A. M.
    et al.
    Asplund, M.
    Collet, R.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    The Galactic chemical evolution of oxygen inferred from 3D non-LTE spectral-line-formation calculations2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 454, no 1, L11-L15 p.Article in journal (Refereed)
    Abstract [en]

    We revisit the Galactic chemical evolution of oxygen, addressing the systematic errors inherent in classical determinations of the oxygen abundance that arise from the use of one-dimensional (1D) hydrostatic model atmospheres and from the assumption of local thermodynamic equilibrium (LTE). We perform detailed 3D non-LTE radiative-transfer calculations for atomic oxygen lines across a grid of 3D hydrodynamic STAGGER model atmospheres for dwarfs and subgiants. We apply our grid of predicted line strengths of the [O I] 630 nm and O I 777 nm lines using accurate stellar parameters from the literature. We infer a steep decay in [O/Fe] for [Fe/H] greater than or similar to -1.0, a plateau [O/Fe] approximate to 0.5 down to [Fe/H] approximate to -2.5, and an increasing trend for [Fe/H] less than or similar to -2.5. Our 3D non-LTE calculations yield overall concordant results from the two oxygen abundance diagnostics.

  • 16. Arthur, S. J.
    et al.
    Henney, W. J.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    De Colle, F.
    Vazquez-Semadeni, E.
    Radiation-magnetohydrodynamic simulations of H II regions and their associated PDRs in turbulent molecular clouds2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, no 2, 1747-1768 p.Article in journal (Refereed)
    Abstract [en]

    We present the results of radiation-magnetohydrodynamic simulations of the formation and expansion of H II regions and their surrounding photodissociation regions (PDRs) in turbulent, magnetized, molecular clouds on scales of up to 4 pc. We include the effects of ionizing and non-ionizing ultraviolet radiation and X-rays from population synthesis models of young star clusters. For all our simulations we find that the H II region expansion reduces the disordered component of the magnetic field, imposing a large-scale order on the field around its border, with the field in the neutral gas tending to lie along the ionization front, while the field in the ionized gas tends to be perpendicular to the front. The highest pressure-compressed neutral and molecular gas is driven towards approximate equipartition between thermal, magnetic and turbulent energy densities, whereas lower pressure neutral/molecular gas bifurcates into, on the one hand, quiescent, magnetically dominated regions and, on the other hand, turbulent, demagnetized regions. The ionized gas shows approximate equipartition between thermal and turbulent energy densities, but with magnetic energy densities that are 1-3 orders of magnitude lower. A high velocity dispersion (similar to 8 km s(-1)) is maintained in the ionized gas throughout our simulations, despite the mean expansion velocity being significantly lower. The magnetic field does not significantly brake the large-scale H II region expansion on the length and time-scales accessible to our simulations, but it does tend to suppress the smallest scale fragmentation and radiation-driven implosion of neutral/molecular gas that forms globules and pillars at the edge of the H II region. However, the relative luminosity of ionizing and non-ionizing radiation has a much larger influence than the presence or absence of the magnetic field. When the star cluster radiation field is relatively soft (as in the case of a lower mass cluster, containing an earliest spectral type of B0.5), then fragmentation is less vigorous and a thick, relatively smooth PDR forms.

  • 17. Asad, K. M. B.
    et al.
    Koopmans, L. V. E.
    Jelic, V.
    Pandey, V. N.
    Ghosh, A.
    Abdalla, F. B.
    Bernardi, G.
    Brentjens, M. A.
    de Bruyn, A. G.
    Bus, S.
    Ciardi, B.
    Chapman, E.
    Daiboo, S.
    Fernandez, E. R.
    Harker, G.
    Iliev, I. T.
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Martinez-Rubi, O.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mevius, M.
    Offringa, A. R.
    Patil, A. H.
    Schaye, J.
    Thomas, R. M.
    van der Tol, S.
    Vedantham, H. K.
    Yatawatta, S.
    Zaroubi, S.
    Polarization leakage in epoch of reionization windows - I. Low Frequency Array observations of the 3C196 field2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 451, no 4, 3709-3727 p.Article in journal (Refereed)
    Abstract [en]

    Detection of the 21-cm signal coming from the epoch of reionization (EoR) is challenging especially because, even after removing the foregrounds, the residual Stokes <italic toggle=yes>I maps contain leakage from polarized emission that can mimic the signal. Here, we discuss the instrumental polarization of Low Frequency Array (LOFAR) and present realistic simulations of the leakages between Stokes parameters. From the LOFAR observations of polarized emission in the 3C196 field, we have quantified the level of polarization leakage caused by the nominal model beam of LOFAR, and compared it with the EoR signal using power spectrum analysis. We found that at 134-166 MHz, within the central 4A degrees of the field the (<italic toggle=yes>Q, <italic toggle=yes>U) -> <italic toggle=yes>I leakage power is lower than the EoR signal at <italic toggle=yes>k < 0.3 Mpc(-1). The leakage was found to be localized around a Faraday depth of 0, and the rms of the leakage as a fraction of the rms of the polarized emission was shown to vary between 0.2 and 0.3 per cent, both of which could be utilized in the removal of leakage. Moreover, we could define an 'EoR window' in terms of the polarization leakage in the cylindrical power spectrum above the point spread function (PSF)-induced wedge and below <italic toggle=yes>k(ayen) similar to 0.5 Mpc(-1), and the window extended up to <italic toggle=yes>k(ayen) similar to 1 Mpc(-1) at all <italic toggle=yes>k(aSyen) when 70 per cent of the leakage had been removed. These LOFAR results show that even a modest polarimetric calibration over a field of view of a parts per thousand(2) 4A degrees in the future arrays like Square Kilometre Array will ensure that the polarization leakage remains well below the expected EoR signal at the scales of 0.02-1 Mpc(-1).

  • 18.
    Asadi, Saghar
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Freeland, Emily
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Probing cold dark matter subhaloes with simulated ALMA observations of macrolensed sub-mm galaxies2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 472, no 1, 129-140 p.Article in journal (Refereed)
    Abstract [en]

    If the dark matter haloes of galaxies contain large numbers of subhaloes as predicted by the Lambda cold dark matter model, these subhaloes are expected to appear in strong galaxy-galaxy lens systems as small-scale perturbations in individual images. We simulate observations of multiply lensed sub-mm galaxies at z similar to 2 as a probe of the dark matter halo of a lens galaxy at z similar to 0.5. We present detection limits for dark substructures based on a visibility plane analysis of simulated Atacama Large Millimeter/submillimeter Array (ALMA) data in bands 7, 8 and 9. We explore two effects: local surface brightness anomalies on angular scales similar to the Einstein radius and the astrometric shift of macroimages. This improves the sensitivity of our lens modelling to the mass of the lens perturber. We investigate the sensitivity of the detection of low-mass subhaloes to the projected position of the subhalo on the image plane as well as the source structure and inner density profile of the lens. We demonstrate that, using the most extended ALMA configuration, pseudo-Jaffe subhaloes can be detected with 99 per cent confidence down to M = 10(7)M(circle dot) . We show how the detection threshold for the three ALMA bands depends on the projected position of the subhalo with respect to the lensed images and conclude that, despite the highest nominal angular resolution, band 9 provides the poorest sensitivity due to observational noise. All simulations use the ALMA Full ops most extended ALMA configuration setup in CASA.

  • 19. Ashworth, G.
    et al.
    Fumagalli, M.
    Krumholz, M. R.
    Adamo, Angela
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Calzetti, D.
    Chandar, R.
    Cignoni, M.
    Dale, D.
    Elmegreen, B. G.
    Gallagher, J. S.
    Gouliermis, D. A.
    Grasha, K.
    Grebel, E. K.
    Johnson, K. E.
    Lee, J.
    Tosi, M.
    Wofford, A.
    Exploring the IMF of star clusters: a joint SLUG and LEGUS effort2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, no 2, 2464-2480 p.Article in journal (Refereed)
    Abstract [en]

    We present the implementation of a Bayesian formalism within the Stochastically Lighting Up Galaxies (SLUG) stellar population synthesis code, which is designed to investigate variations in the initial mass function (IMF) of star clusters. By comparing observed cluster photometry to large libraries of clusters simulated with a continuously varying IMF, our formalism yields the posterior probability distribution function (PDF) of the cluster mass, age and extinction, jointly with the parameters describing the IMF. We apply this formalism to a sample of star clusters from the nearby galaxy NGC 628, for which broad-band photometry in five filters is available as part of the Legacy ExtraGalactic UV Survey (LEGUS). After allowing the upper-end slope of the IMF (a3) to vary, we recover PDFs for the mass, age and extinction that are broadly consistent with what is found when assuming an invariant Kroupa IMF. However, the posterior PDF for a3 is very broad due to a strong degeneracy with the cluster mass, and it is found to be sensitive to the choice of priors, particularly on the cluster mass. We find only a modest improvement in the constraining power of a3 when adding Ha photometry from the companion Ha-LEGUS survey. Conversely, Ha photometry significantly improves the age determination, reducing the frequency of multi-modal PDFs. With the aid of mock clusters, we quantify the degeneracy between physical parameters, showing how constraints on the cluster mass that are independent of photometry can be used to pin down the IMF properties of star clusters.

  • 20.
    Axelsson, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Borgonovo, Luis
    Stockholm University, Faculty of Science, Department of Astronomy.
    The width of gamma-ray burst spectra2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 447, no 4, 3150-3154 p.Article in journal (Refereed)
    Abstract [en]

    The emission processes active in the highly relativistic jets of gamma-ray bursts (GRBs) remain unknown. In this paper, we propose a new measure to describe spectra: the width of the EFE spectrum, a quantity dependent only on finding a good fit to the data. We apply this to the full sample of GRBs observed by Fermi/Gamma-ray Burst Monitor (GBM) and Compton Gamma-ray Observatory/Burst and Transient Source Experiment (BATSE). The results from the two instruments are fully consistent. We find that the median widths of spectra from long and short GRBs are significantly different (chance probability < 10(-6)). The width does not correlate with either duration or hardness, and this is thus a new, independent distinction between the two classes. Comparing the measured spectra with widths of spectra from fundamental emission processes - synchrotron and blackbody radiation - the results indicate that a large fraction of GRB spectra are too narrow to be explained by synchrotron radiation from a distribution of electron energies: for example, 78 per cent of long GRBs and 85 per cent of short GRBs are incompatible with the minimum width of standard slow cooling synchrotron emission from a Maxwellian distribution of electrons, with fast cooling spectra predicting even wider spectra. Photospheric emission can explain the spectra if mechanisms are invoked to give a spectrum much broader than a blackbody.

  • 21.
    Axelsson, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Done, Chris
    Hjalmarsdotter, Linnea
    An imperfect double: probing the physical origin of the low-frequency quasi-periodic oscillation and its harmonic in black hole binaries2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 438, no 1, 657-662 p.Article in journal (Refereed)
    Abstract [en]

    We extract the spectra of the strong low-frequency quasi-periodic oscillation (QPO) and its harmonic during the rising phase of an outburst in the black hole binary XTE J1550-564. We compare these frequency-resolved spectra to the time-averaged spectrum and the spectrum of the rapid (<0.1 s) variability. The spectrum of the time-averaged emission can be described by a disc, a Compton upscattered tail and its reflection. The QPO spectrum is very similar to the spectrum of the most rapid variability, implying it arises in the innermost regions of the flow. It contains little detectable disc, and its Compton spectrum is generally harder and shows less reflection than in the time-averaged emission. The harmonic likewise contains little detectable disc component, but has a Compton spectrum which is systematically softer than the QPO, softer even than the Compton tail in the time-averaged emission. We interpret these results in the context of the truncated disc model, where the inner disc is replaced by a hot flow. The QPO can arise in this picture from vertical (Lense-Thirring) precession of the entire hot inner flow, and its harmonic can be produced by the angular dependence of Compton scattering within the hot flow. We extend these models to include stratification of the hot flow, so that it is softer (lower optical depth) at larger radii closer to the truncated disc, and harder (higher optical depth) in the innermost parts of the flow where the rapid variability is produced. The different optical depth with radius gives rise to different angular dependence of the Comptonized emission, weighting the fundamental to the inner parts of the hot flow, and the harmonic to the outer. This is the first model which can explain both the spectrum of the QPO, and its harmonic, in a self consistent geometry.

  • 22.
    Axelsson, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hjalmarsdotter, Linnea
    Done, Chris
    Fast variability as a probe of the smallest regions around accreting black holes2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 431, no 2, 1987-1994 p.Article in journal (Refereed)
    Abstract [en]

    We extract the spectra of the fastest variability (above 10 Hz) from the black hole XTE J1550-564 during a transition from hard to soft state on the rise to outburst. We confirm previous results that the rapid variability contains no significant disc component despite this being strongly present in the total spectrum of the softer observations. We model ionized reflection significantly better than previous work, and show that this is also suppressed in the rapid variability spectrum compared to the total emission. This is consistent with the fast variability having its origin in a hot inner flow close to the black hole rather than in the accretion disc or in a corona above it. However, the rapid variability spectrum is not simply the same as the total Comptonized emission. It is always significantly harder, by an amount which increases as the spectrum softens during the outburst. This adds to evidence from time lags that the Comptonization region is inhomogeneous, with harder spectra produced closest to the black hole, the same region which produces the fastest variability.

  • 23.
    Axelsson, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Larsson, Stefan
    Hjalmarsdotter, Linnea
    The aperiodic broad-band X-ray variability of Cygnus X-32009In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 394, no 3, 1544-1550 p.Article in journal (Refereed)
    Abstract [en]

    We study the soft X-ray variability of Cygnus X-3. By combining data from the All-Sky Monitor and Proportional Counter Array instruments on the RXTE satellite with EXOSAT/Medium Energy (ME) detector observations, we are able to analyse the power density spectrum (PDS) of the source from 10−9 to 0.1 Hz, thus covering time-scales from seconds to years. As the data on the longer time-scales are unevenly sampled, we combine traditional power spectral techniques with simulations to analyse the variability in this range. The PDS at higher frequencies (≳10−3 Hz) are for the first time compared for all states of this source. We find that it is for all states well described by a power law, with index ∼−2 in the soft states and a tendency for a less steep power law in the hard state. At longer time-scales, we study the effect of the state transitions on the PDS, and find that the variability below ∼10−7 Hz is dominated by the transitions. Furthermore, we find no correlation between the length of a high/soft-state episode and the time since the previous high/soft state. On intermediate time-scales, we find evidence for a break in the PDS at time-scales of the order of the orbital period. This may be interpreted as evidence for the existence of a tidal resonance in the accretion disc around the compact object, and constraining the mass ratio to M2/M1≲ 0.3.

  • 24. Bacchus, E.
    et al.
    Parry, I. R.
    Oppenheimer, R.
    Aguilar, J.
    Beichman, C.
    Brenner, D.
    Burruss, R.
    Cady, E.
    Luszcz-Cook, S.
    Crepp, J.
    Dekany, R.
    Gianninas, A.
    Hillenbrand, L.
    Kilic, M.
    King, D.
    Lockhart, T. G.
    Matthews, C. T.
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy. American Museum of Natural History, USA; California Institute of Technology, USA.
    Pueyo, L.
    Rice, E. L.
    Roberts, L. C.
    Sivaramakrishnan, A.
    Soummer, R.
    Vasisht, G.
    Veicht, A.
    Zhai, C.
    Zimmerman, N. T.
    Project 1640 observations of the white dwarf HD 114174 B2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, no 4, 4796-4805 p.Article in journal (Refereed)
    Abstract [en]

    We present the first near infrared spectrum of the faint white dwarf companion HD 114174 B, obtained with Project 1640. Our spectrum, covering the Y, J and H bands, combined with previous TaRgetting bENchmark-objects with Doppler Spectroscopy (TRENDS) photometry measurements, allows us to place further constraints on this companion. We suggest two possible scenarios; either this object is an old, low-mass, cool H atmosphere white dwarf with T-eff similar to 3800 K or a high-mass white dwarf with T-eff > 6000 K, potentially with an associated cool (T-eff similar to 700 K) brown dwarf or debris disc resulting in an infrared excess in the L' band. We also provide an additional astrometry point for 2014 June 12 and use the modelled companion mass combined with the radial velocity and direct imaging data to place constraints on the orbital parameters for this companion.

  • 25. Bagla, J. S.
    et al.
    Khandai, Nishikanta
    Datta, Kanan K.
    Stockholm University, Faculty of Science, Department of Astronomy.
    H i as a probe of the large-scale structure in the post-reionization universe2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 407, no 1, 567-580 p.Article in journal (Refereed)
    Abstract [en]

    We model the distribution of neutral hydrogen (H i) in the post-reionization universe. This model uses gravity-only N-body simulations and an ansatz to assign H i to dark matter haloes that is consistent with observational constraints and theoretical models. We resolve the smallest haloes that are likely to host H i in the simulations; care is also taken to ensure that any errors due to the finite size of the simulation box are small. We then compute the smoothed one-point probability distribution function and the power spectrum of fluctuations in H i. This is compared with other predictions that have been made using different techniques. We highlight the significantly high bias for the H i distribution at small scales. This aspect has not been discussed before. We then discuss the prospects of the detection with the Murchison Widefield Array (MWA), Giant Meterwave Radio Telescope (GMRT) and the hypothetical MWA5000. The MWA5000 can detect visibility correlations at large angular scales at all redshifts in the post-reionization era. The GMRT can detect visibility correlations at lower redshifts; specifically there is a strong case for a survey at z similar or equal to 1.3. We also discuss prospects for direct detection of rare peaks in the H i distribution using the GMRT. We show that direct detection should be possible with an integration time that is comparable to, or even less than, the time required for a statistical detection. Specifically, it is possible to make a statistical detection of the H i distribution by measuring the visibility correlation and direct detection of rare peaks in the H i distribution at z similar or equal to 1.3 with the GMRT in less than 1000 h of observations.

  • 26. Barbarino, C.
    et al.
    Dall'Ora, M.
    Botticella, M. T.
    Della Valle, M.
    Zampieri, L.
    Maund, J. R.
    Pumo, M. L.
    Jerkstrand, A.
    Benetti, S.
    Elias-Rosa, N.
    Fraser, M.
    Gal-Yam, A.
    Hamuy, M.
    Inserra, C.
    Knapic, C.
    LaCluyze, A. P.
    Molinaro, M.
    Ochner, P.
    Pastorello, A.
    Pignata, G.
    Reichart, D. E.
    Ries, C.
    Riffeser, A.
    Schmidt, B.
    Schmidt, M.
    Smareglia, R.
    Smartt, S. J.
    Smith, K.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sullivan, M.
    Tomasella, L.
    Turatto, M.
    Valenti, S.
    Yaron, O.
    Young, D.
    SN 2012ec: mass of the progenitor from PESSTO follow-up of the photospheric phase2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 448, no 3, 2312-2331 p.Article in journal (Refereed)
    Abstract [en]

    We present the results of a photometric and spectroscopic monitoring campaign of SN 2012ec, which exploded in the spiral galaxy NGC 1084, during the photospheric phase. The photometric light curve exhibits a plateau with luminosity L = 0.9 x 10(42) erg s(-1) and duration similar to 90 d, which is somewhat shorter than standard Type II-P supernovae (SNe). We estimate the nickel mass M(Ni-56) = 0.040 +/- 0.015 M-circle dot from the luminosity at the beginning of the radioactive tail of the light curve. The explosion parameters of SN 2012ec were estimated from the comparison of the bolometric light curve and the observed temperature and velocity evolution of the ejecta with predictions from hydrodynamical models. We derived an envelope mass of 12.6 M-circle dot, an initial progenitor radius of 1.6 x 10(13) cm and an explosion energy of 1.2 foe. These estimates agree with an independent study of the progenitor star identified in pre-explosion images, for which an initial mass of M = 14-22 M-circle dot was determined. We have applied the same analysis to two other Type II-P SNe (SNe 2012aw and 2012A), and carried out a comparison with the properties of SN 2012ec derived in this paper. We find a reasonable agreement between the masses of the progenitors obtained from pre-explosion images and masses derived from hydrodynamical models. We estimate the distance to SN 2012ec with the standardized candle method (SCM) and compare it with other estimates based on other primary and secondary indicators. SNe 2012A, 2012aw and 2012ec all follow the standard relations for the SCM for the use of Type II-P SNe as distance indicators.

  • 27. Bastian, N.
    et al.
    Adamo, Angela
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gieles, M.
    Lamers, H. J. G. L. M.
    Larsen, S. S.
    Silva-Villa, E.
    Smith, L. J.
    Kotulla, R.
    Konstantopoulos, I. S.
    Trancho, G.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Evidence for environmentally dependent cluster disruption in M832011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 417, no 1, l6-L10 p.Article in journal (Refereed)
    Abstract [en]

    Using multiwavelength imaging from the Wide Field Camera 3 on the Hubble Space Telescope we study the stellar cluster populations of two adjacent fields in the nearby face-on spiral galaxy, M83. The observations cover the galactic centre and reach out to similar to 6 kpc, thereby spanning a large range of environmental conditions, ideal for testing empirical laws of cluster disruption. The clusters are selected by visual inspection to be centrally concentrated, symmetric and resolved on the images. We find that a large fraction of objects detected by automated algorithms (e. g. SEXTRACTOR or DAOFIND) are not clusters, but rather are associations. These are likely to disperse into the field on time-scales of tens of Myr due to their lower stellar densities and not due to gas expulsion (i.e. they were never gravitationally bound). We split the sample into two discrete fields (inner and outer regions of the galaxy) and search for evidence of environmentally dependent cluster disruption. Colour-colour diagrams of the clusters, when compared to simple stellar population models, already indicate that a much larger fraction of the clusters in the outer field are older by tens of Myr than in the inner field. This impression is quantified by estimating each cluster's properties (age, mass and extinction) and comparing the age/mass distributions between the two fields. Our results are inconsistent with 'universal' age and mass distributions of clusters, and instead show that the ambient environment strongly affects the observed populations.

  • 28. Bastian, N.
    et al.
    Adamo, Angela
    Stockholm University, Faculty of Science, Department of Astronomy. Max Planck Institut für Astronomie, Germany.
    Schirmer, M.
    Hollyhead, K.
    Beletsky, Y.
    Carraro, G.
    Davies, B.
    Gieles, M.
    Silva-Villa, E.
    The effect of spatial resolution on optical and near-IR studies of stellar clusters: implications for the origin of the red excess2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 444, no 4, 3829-3836 p.Article in journal (Refereed)
    Abstract [en]

    Recent ground-based near-IR (NIR) studies of stellar clusters in nearby galaxies have suggested that young clusters remain embedded for 7-10 Myr in their progenitor molecular cloud, in conflict with optical-based studies which find that clusters are exposed after 1-3 Myr. Here, we investigate the role that spatial resolution plays in this apparent conflict. We use a recent catalogue of young (< 10 Myr) massive (> 5000M(circle dot)) clusters in the nearby spiral galaxy, M83, along with Hubble Space Telescope (HST) imaging in the optical and NIR, and ground-based NIR imaging, to see how the colours (and hence estimated properties such as age and extinction) are affected by the aperture size employed, in order to simulate studies of differing resolution. We find that the NIR is heavily affected by the resolution, and when aperture sizes > 40 pc are used, all young/blue clusters move redwards in colour space, which results in their appearance as heavily extincted clusters. However, this is due to contamination from nearby sources and nebular emission, and is not an extinction effect. Optical colours are much less affected by resolution. Due to the larger effect of contamination in the NIR, we find that, in some cases, clusters will appear to show NIR excess when large (> 20 pc) apertures are used. Our results explain why few young (< 6 Myr), low-extinction (AV < 1 mag) clusters have been found in recent ground-based NIR studies of cluster populations, while many such clusters have been found in higher resolution HST-based studies. Additionally, resolution effects appear to (at least partially) explain the origin of the NIR excess that has been found in a number of extragalactic young massive clusters.

  • 29. Benetti, S.
    et al.
    Nicholl, M.
    Cappellaro, E.
    Pastorello, A.
    Smartt, S. J.
    Elias-Rosa, N.
    Drake, A. J.
    Tomasella, L.
    Turatto, M.
    Harutyunyan, A.
    Taubenberger, S.
    Hachinger, S.
    Morales-Garoffolo, A.
    Chen, T. -W
    Djorgovski, S. G.
    Fraser, M.
    Gal-Yam, A.
    Inserra, C.
    Mazzali, P.
    Pumo, M. L.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Valenti, S.
    Young, D. R.
    Dennefeld, M.
    Le Guillou, L.
    Fleury, M.
    Leget, P. -F
    The supernova CSS121015:004244+132827: a clue for understanding superluminous supernovae2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 441, no 1, 289-303 p.Article in journal (Refereed)
    Abstract [en]

    We present optical photometry and spectra of the superluminous Type II/IIn supernova (SN) CSS121015: 004244+132827 (z = 0.2868) spanning epochs from -30 d (rest frame) to more than 200 d after maximum. CSS121015 is one of the more luminous SNe ever found and one of the best observed. The photometric evolution is characterized by a relatively fast rise to maximum (similar to 40 d in the SN rest frame), and by a linear post-maximum decline. The light curve shows no sign of a break to an exponential tail. A broad Ha is first detected at similar to+40 d (rest frame). Narrow, barely resolved Balmer and [O III] 5007 angstrom lines, with decreasing strength, are visible along the entire spectral evolution. The spectra are very similar to other superluminous supernovae (SLSNe) with hydrogen in their spectrum, and also to SN 2005gj, sometimes considered Type Ia interacting with H-rich circumstellar medium. The spectra are also similar to a subsample of H-deficient SLSNe. We propose that the properties of CSS121015 are consistent with the interaction of the ejecta with a massive, extended, opaque shell, lost by the progenitor decades before the final explosion, although a magnetar-powered model cannot be excluded. Based on the similarity of CSS121015 with other SLSNe (with and without H), we suggest that the shocked-shell scenario should be seriously considered as a plausible model for both types of SLSN.

  • 30. Bento, J.
    et al.
    Schmidt, B.
    Hartman, J. D.
    Bakos, G. A.
    Ciceri, Simona
    Stockholm University, Faculty of Science, Department of Astronomy. Max Plank Institute for Astronomy, Heidelberg, Germany.
    Brahm, R.
    Bayliss, D.
    Espinoza, N.
    Zhou, G.
    Rabus, M.
    Bhatti, W.
    Penev, K.
    Csubry, Z.
    Jordan, A.
    Mancini, L.
    Henning, T.
    de Val-Borro, M.
    Tinney, C. G.
    Wright, D. J.
    Durkan, S.
    Suc, V.
    Noyes, R.
    Lazar, J.
    Papp, I.
    Sari, P.
    HATS-22b, HATS-23b and HATS-24b: three new transiting super-Jupiters from the HATSouth project2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, no 1, 835-848 p.Article in journal (Refereed)
    Abstract [en]

    We report the discovery of three moderately high-mass transiting hot Jupiters from the HATSouth survey: HATS-22b, HATS-23b and HATS-24b. These planets add to the number of known planets in the similar to 2MJ regime. HATS-22b is a 2.74 +/- 0.11MJ mass and 0.953(-0.029)(+0.048) R-J radius planet orbiting a V = 13.455 +/- 0.040 sub-solar mass (M-* = 0.759 +/- 0.019M(circle dot); R-* = 0.759 +/- 0.019 R-circle dot) K-dwarf host star on an eccentric (e = 0.079 +/- 0.026) orbit. This planet's high planet-to-stellar mass ratio is further evidence that migration mechanisms for hot Jupiters may rely on exciting orbital eccentricities that bring the planets closer to their parent stars followed by tidal circularization. HATS-23b is a 1.478 +/- 0.080M(J) mass and 1.69 +/- 0.24 R-J radius planet on a grazing orbit around a V = 13.901 +/- 0.010 G-dwarf with properties very similar to those of the Sun (M* = 1.115 +/- 0.054; R-* = 1.145 +/- 0.070). HATS24b orbits a moderately bright V = 12.830 +/- 0.010 F-dwarf star (M-* = 1.218 +/- 0.036M circle dot; R-* = 1.194(-0.041)(+0.066) R circle dot). This planet has a mass of 2.39+0.21 -0.12MJ and an inflated radius of 1.516(-0.065)(+0.085) R-J.

  • 31. Bergfors, C.
    et al.
    Brandner, W.
    Bonnefoy, M.
    Schlieder, J.
    Janson, Markus
    Stockholm University, Faculty of Science, Department of Astronomy.
    Henning, Th.
    Chauvin, G.
    Characterization of close visual binaries from the AstraLux Large M Dwarf Survey2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, no 3, 2576-2585 p.Article in journal (Refereed)
    Abstract [en]

    We present Very Large Telescope/Spectrograph for INtegral Field Observations in the Near Infrared (VLT/SINFONI) J, H + K spectra of seven close visual pairs in M dwarf binary/triple systems, discovered or observed by the AstraLux M dwarf survey. We determine the spectral types to within +/- 1.0 subclasses from comparison to template spectra and the strength of K-band water absorption, and derive effective temperatures. The results are compared to optical spectral types of the unresolved binary/multiple systems, and we confirm that our photometric method to derive spectral types in the AstraLux M dwarf survey is accurate. We look for signs of youth such as chromospheric activity and low surface gravity, and find an age in the range 0.25-1 Gyr for the GJ 852 system. Strong Li absorption is detected in optical spectra of the triple system J024902 obtained with the Fiberfed Extended Range Optical Spectrograph (FEROS) at the European Southern Observatory (ESO)-Max-Planck-Gesellschaft (MPG) 2.2 m telescope. The equivalent width of the absorption suggests an age consistent with the beta Pic moving group. However, further observations are needed to establish group membership. Ongoing orbital monitoring will provide dynamical masses and thus calibration of evolutionary models for low mass stars.

  • 32. Bergvall, Nils
    et al.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Caldwell, Brady
    The red haloes of SDSS low surface brightness disc galaxies2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 405, no 4, 2697-2716 p.Article in journal (Refereed)
    Abstract [en]

    The faint stellar haloes of galaxies contain key information about the oldest stars and the process of galaxy formation. A previous study of stacked SDSS images of disc galaxies has revealed a halo with an abnormally red r - i colour, seemingly inconsistent with our current understanding of the stellar populations inhabiting stellar haloes. Measurements of this type are, however, plagued by large uncertainties which calls for follow-up studies. Here, we investigate the statistical properties of the faint envelopes of low surface brightness disc galaxies to look for further support for a red excess. A total of 1510 nearly edge-on, bulgeless low surface brightness galaxies were selected from the SDSS Data Release 5, rescaled to the same apparent size, aligned and stacked. This procedure allows us to reach a surface brightness of mu(r) similar to 31 mag arcsec-2. After a careful assessment of instrumental light scattering effects in the stacked images, we derive median and average radial surface brightness and colour profiles in g, r and i. The sample is then divided into three subsamples according to g - r colour. All three samples exhibit a red colour excess in r - i in the thick disc/halo region. The halo colours of the full sample, g - r = 0.60 +/- 0.15 and r - i = 0.80 +/- 0.15, are found to be incompatible with the colours of any normal type of stellar population. The fact that no similar colour anomaly is seen at comparable surface brightness levels along the disc rules out a sky subtraction residual as the source of the extreme colours. A number of possible explanations for these abnormally red haloes are discussed. We find that two different scenarios - dust extinction of extragalactic background light and a stellar population with a very bottom-heavy initial mass function - appear to be broadly consistent with our observations and with similar red excesses reported in the haloes of other types of galaxies.

  • 33. Bhat, Pallavi
    et al.
    Subramanian, Kandaswamy
    Brandenburg, Axel
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Colorado, USA.
    A unified large/small-scale dynamo in helical turbulence2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 461, no 1, 240-247 p.Article in journal (Refereed)
    Abstract [en]

    We use high resolution direct numerical simulations (DNS) to show that helical turbulence can generate significant large-scale fields even in the presence of strong small-scale dynamo action. During the kinematic stage, the unified large/ small-scale dynamo grows fields with a shapeinvariant eigenfunction, with most power peaked at small scales or large k, as in Subramanian & Brandenburg. Nevertheless, the large-scale field can be clearly detected as an excess power at small k in the negatively polarized component of the energy spectrum for a forcing with positively polarized waves. Its strength (B) over bar, relative to the total rms field Brms, decreases with increasing magnetic Reynolds number, Re-M. However, as the Lorentz force becomes important, the field generated by the unified dynamo orders itself by saturating on successively larger scales. The magnetic integral scale for the positively polarized waves, characterizing the smallscale field, increases significantly from the kinematic stage to saturation. This implies that the small-scale field becomes as coherent as possible for a given forcing scale, which averts the Re-M-dependent quenching of (B) over bar /B-rms. These results are obtained for 1024(3) DNS with magnetic Prandtl numbers of PrM = 0.1 and 10. For PrM = 0.1, B/ Brms grows from about 0.04 to about 0.4 at saturation, aided in the final stages by helicity dissipation. For Pr-M = 10, (B) over bar /B-rms grows from much less than 0.01 to values of the order the 0.2. Our results confirm that there is a unified large/ small-scale dynamo in helical turbulence.

  • 34.
    Bisbas, Thomas G.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University College London, UK; Max-Planck-Institut für Extraterrestrische Physik, Germany; University of Florida, USA.
    Haworth, Thomas J.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Cambridge, UK.
    Barlow, M. J.
    Viti, S.
    Harries, T. J.
    Bell, T.
    Yates, J. A.
    TORUS-3DPDR: a self-consistent code treating three-dimensional photoionization and photodissociation regions2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 454, no 3, 2828-2843 p.Article in journal (Refereed)
    Abstract [en]

    The interaction of ionizing and far-ultraviolet radiation with the interstellar medium is of great importance. It results in the formation of regions in which the gas is ionized, beyond which are photodissociation regions (PDRs) in which the gas transitions to its atomic and molecular form. Several numerical codes have been implemented to study these two main phases of the interstellar medium either dynamically or chemically. In this paper we present TORUS-3DPDR, a new self-consistent code for treating the chemistry of three-dimensional photoionization and photodissociation regions. It is an integrated code coupling the two codes TORUS, a hydrodynamics and Monte Carlo radiation transport code, and 3D-PDR, a PDRs code. The new code uses a Monte Carlo radiative transfer scheme to account for the propagation of the ionizing radiation including the diffusive component as well as a ray-tracing scheme based on the HEALPIX package in order to account for the escape probability and column density calculations. Here, we present the numerical techniques we followed and we show the capabilities of the new code in modelling three-dimensional objects including single or multiple sources. We discuss the effects introduced by the diffusive component of the ultraviolet field in determining the thermal balance of PDRs as well as the effects introduced by a multiple sources treatment of the radiation field. With this new code, three-dimensional synthetic observations for the major cooling lines are possible, for making feasible a detailed comparison between hydrodynamical simulations and observations.

  • 35.
    Blasco-Herrera, J.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fathi, Kambiz
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Font, J.
    Beckman, J. E.
    H alpha kinematics of 11 starburst galaxies selected from the Sloan Digital Sky Survey2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 435, no 3, 1958-1983 p.Article in journal (Refereed)
    Abstract [en]

    We present analysis of Fabry-Perot interferometric observations for a sample of 11 starburst galaxies with redshift z less than or similar to 0.03 selected from the Sloan Digital Sky Survey. We have targeted the H alpha lambda 6562.78 emission line at spatial sampling of 0.2arcsec-0.4 arcsec and at spectral resolution of R similar to 16 000. Studying two-dimensional maps for the velocity, velocity dispersion, line emission and continuum deliver new insights into the nature of starburst and postburst galaxies. The velocity fields are fitted to an exponential disc model, providing scalelengths and masses, to be compared with the photometric scalelength and the mass estimated from the velocity dispersions. From the line emission, the star formation rates are also calculated and compared with values in the literature. We have found that between 55 and 64 per cent of our sample show large-scale dominating rotation, while 27 per cent show local rotation typically restricted to the central part of each system. The morphology and line profiles in the centre of eight objects are consistent with a recent merger, while another two seem to be involved in a accretion process which, for the moment, has not disrupted their kinematics.

  • 36.
    Blasco-Herrera, Javier
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Fathi, Kambiz
    Stockholm University, Faculty of Science, Department of Physics, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Beckman, J.
    Gutierrez, L.
    Lundgren, A.
    Epinat, B.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Physics, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Font, J.
    Hernandez, O.
    de Denus-Baillargeon, M. -M
    Carignan, C.
    An improved method for statistical studies of the internal kinematics of H ii regions: the case of M832010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 407, no 4, 2519-2529 p.Article in journal (Refereed)
    Abstract [en]

    We present the integrated H alpha emission line profile for 157 H ii regions in the central 3.4 x 3.4 arcmin2 of the galaxy M83 (NGC 5236). Using the Fabry-Perot interferometer GH alpha FaS, on the 4.2-m William Herschel Telescope on La Palma, we show the importance of a good characterization of the instrumental response function for the study of line profile shapes. The luminosity-velocity dispersion relation is also studied, and in the log(L)-log(Sigma) plane we do not find a linear relation, but an upper envelope with equation log(L-H alpha) = 0.9 log(Sigma) + 38.1. For the adopted distance of 4.5 Mpc, the upper envelope appears at the luminosity L = 1038.5 erg s-1, in full agreement with previous studies of other galaxies, reinforcing the idea of using H ii regions as standard candles.

  • 37. Bosman, Sarah E. I.
    et al.
    Becker, George D.
    Haehnelt, Martin G.
    Hewett, Paul C.
    McMahon, Richard G.
    Mortlock, Daniel J.
    Stockholm University, Faculty of Science, Department of Astronomy. Imperial College London, UK.
    Simpson, Chris
    Venemans, Bram P.
    A deep search for metals near redshift 7: the line of sight towards ULAS J1120+06412017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 470, no 2, 1919-1934 p.Article in journal (Refereed)
    Abstract [en]

    We present a search for metal absorption line systems at the highest redshifts to date using a deep (30 h) Very Large Telescope/X=Shooter spectrum of the z = 7.084 quasi-stellar object (QSO) ULAS J1120+0641. We detect seven intervening systems at z > 5.5, with the highest redshift system being a C IV absorber at z = 6.51. We find tentative evidence that the mass density of C IV remains flat or declines with redshift at z < 6, while the number density of C II systems remains relatively flat over 5 < z < 7. These trends are broadly consistent with models of chemical enrichment by star formation-driven winds that include a softening of the ultraviolet background towards higher redshifts. We find a larger number of weak (W-rest < 0.3 angstrom) Mg II systems over 5.9 < z < 7.0 than predicted by a power-law fit to the number density of stronger systems. This is consistent with trends in the number density of weak Mg II systems at z less than or similar to 2.5, and suggests that the mechanisms that create these absorbers are already in place at z similar to 7. Finally, we investigate the associated narrow Si IV, C IV and N V absorbers located near the QSO redshift, and find that at least one component shows evidence of partial covering of the continuum source.

  • 38.
    Brandenburg, Axel
    Stockholm University, Faculty of Science, Department of Astronomy.
    Magnetic field evolution in simulations with Euler potentials2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 401, no 1, 347-354 p.Article in journal (Refereed)
    Abstract [en]

    Using two- and three-dimensional hydromagnetic simulations for a range of different flows, including laminar and turbulent ones, it is shown that solutions expressing the field in terms of Euler potentials (EP) are in general incorrect if the EP are evolved with an artificial diffusion term. In three dimensions, standard methods using the magnetic vector potential are found to permit dynamo action when the EP give decaying solutions. With an imposed field, the EP method yields excessive power at small scales. This effect is more exaggerated in the dynamic case, suggesting an unrealistically reduced feedback from the Lorentz force. The EP approach agrees with standard methods only at early times when magnetic diffusivity did not have time to act. It is demonstrated that the usage of EP with even a small artificial magnetic diffusivity does not converge to a proper solution of hydromagnetic turbulence. The source of this disagreement is not connected with magnetic helicity or the three-dimensionality of the magnetic field, but is simply due to the fact that the non-linear representation of the magnetic field in terms of EP that depend on the same coordinates is incompatible with the linear diffusion operator in the induction equation.

  • 39.
    Brandenburg, Axel
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy.
    Candelaresi, Simon
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy.
    Chatterjee, Piyali
    Small-scale magnetic helicity losses from a mean-field dynamo2009In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 398, no 3, 1414-1422 p.Article in journal (Refereed)
    Abstract [en]

    Using mean-field models with a dynamical quenching formalism, we show that in finite domains magnetic helicity fluxes associated with small-scale magnetic fields are able to alleviate catastrophic quenching. We consider fluxes that result from advection by a mean flow, the turbulent mixing down the gradient of mean small-scale magnetic helicity density or the explicit removal which may be associated with the effects of coronal mass ejections in the Sun. In the absence of shear, all the small-scale magnetic helicity fluxes are found to be equally strong for both large- and small-scale fields. In the presence of shear, there is also an additional magnetic helicity flux associated with the mean field, but this flux does not alleviate catastrophic quenching. Outside the dynamo-active region, there are neither sources nor sinks of magnetic helicity, so in a steady state this flux must be constant. It is shown that unphysical behaviour emerges if the small-scale magnetic helicity flux is forced to vanish within the computational domain.

  • 40. Bulla, M.
    et al.
    Sim, S. A.
    Kromer, Markus
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Polarization spectral synthesis for Type Ia supernova explosion models2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 450, no 1, 967-981 p.Article in journal (Refereed)
    Abstract [en]

    We present a Monte Carlo radiative transfer technique for calculating synthetic spectropolarimetry for multidimensional supernova explosion models. The approach utilizes 'virtual-packets' that are generated during the propagation of the Monte Carlo quanta and used to compute synthetic observables for specific observer orientations. Compared to extracting synthetic observables by direct binning of emergent Monte Carlo quanta, this virtual-packet approach leads to a substantial reduction in the Monte Carlo noise. This is not only vital for calculating synthetic spectropolarimetry (since the degree of polarization is typically very small) but also useful for calculations of light curves and spectra. We first validate our approach via application of an idealized test code to simple geometries. We then describe its implementation in the Monte Carlo radiative transfer code ARTIS and present test calculations for simple models for Type Ia supernovae. Specifically, we use the well-known one-dimensional W7 model to verify that our scheme can accurately recover zero polarization from a spherical model, and to demonstrate the reduction in Monte Carlo noise compared to a simple packet-binning approach. To investigate the impact of aspherical ejecta on the polarization spectra, we then use ARTIS to calculate synthetic observables for prolate and oblate ellipsoidal models with Type Ia supernova compositions.

  • 41. Bulla, M.
    et al.
    Sim, S. A.
    Kromer, Markus
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Seitenzahl, I. R.
    Fink, M.
    Ciaraldi-Schoolmann, F.
    Röpke, K.
    Hillebrandt, W.
    Pakmor, R.
    Ruiter, A. J.
    Taubenberger, S.
    Predicting polarization signatures for double-detonation and delayed-detonation models of Type Ia supernovae2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 462, no 1, 1039-1056 p.Article in journal (Refereed)
    Abstract [en]

    Calculations of synthetic spectropolarimetry are one means to test multidimensional explosion models for Type Ia supernovae. In a recent paper, we demonstrated that the violent merger of a 1.1 and 0.9 M-circle dot white dwarf binary system is too asymmetric to explain the low polarization levels commonly observed in normal Type Ia supernovae. Here, we present polarization simulations for two alternative scenarios: the sub-Chandrasekhar mass double-detonation and the Chandrasekhar mass delayed-detonation model. Specifically, we study a 2D double-detonation model and a 3D delayed-detonation model, and calculate polarization spectra for multiple observer orientations in both cases. We find modest polarization levels (<1 per cent) for both explosion models. Polarization in the continuum peaks at similar to 0.1-0.3 per cent and decreases aftermaximum light, in excellent agreement with spectropolarimetric data of normal Type Ia supernovae. Higher degrees of polarization are found across individual spectral lines. In particular, the synthetic Si II lambda 6355 profiles are polarized at levels that match remarkably well the values observed in normal Type Ia supernovae, while the low degrees of polarization predicted across the O I lambda 7774 region are consistent with the non-detection of this feature in current data. We conclude that our models can reproduce many of the characteristics of both flux and polarization spectra for well-studied Type Ia supernovae, such as SN 2001el and SN 2012fr. However, the two models considered here cannot account for the unusually high level of polarization observed in extreme cases such as SN 2004dt.

  • 42. Bulla, M.
    et al.
    Sim, S. A.
    Pakmor, R.
    Kromer, Markus
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Taubenberger, S.
    Röpke, F. K.
    Hillebrandt, W.
    Seitenzahl, I. R.
    Type Ia supernovae from violent mergers of carbon-oxygen white dwarfs: polarization signatures2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 455, no 1, 1060-1070 p.Article in journal (Refereed)
    Abstract [en]

    The violent merger of two carbon-oxygen white dwarfs has been proposed as a viable progenitor for some Type Ia supernovae. However, it has been argued that the strong ejecta asymmetries produced by this model might be inconsistent with the low degree of polarization typically observed in Type Ia supernova explosions. Here, we test this claim by carrying out a spectropolarimetric analysis for the model proposed by Pakmor et al. for an explosion triggered during the merger of a 1.1 and 0.9 M-circle dot carbon-oxygen white dwarf binary system. Owing to the asymmetries of the ejecta, the polarization signal varies significantly with viewing angle. We find that polarization levels for observers in the equatorial plane are modest (less than or similar to 1 per cent) and show clear evidence for a dominant axis, as a consequence of the ejecta symmetry about the orbital plane. In contrast, orientations out of the plane are associated with higher degrees of polarization and departures from a dominant axis. While the particular model studied here gives a good match to highly polarized events such as SN 2004dt, it has difficulties in reproducing the low polarization levels commonly observed in normal Type Ia supernovae. Specifically, we find that significant asymmetries in the element distribution result in a wealth of strong polarization features that are not observed in the majority of currently available spectropolarimetric data of Type Ia supernovae. Future studies will map out the parameter space of the merger scenario to investigate if alternative models can provide better agreement with observations.

  • 43.
    Burgess, J. Michael
    et al.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). KTH Royal Institute of Technology, Sweden.
    Ryde, Felix
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). KTH Royal Institute of Technology, Sweden.
    Yu, Hoi-Fung
    Taking the band function too far: a tale of two alpha's2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 451, no 2, 1511-1521 p.Article in journal (Refereed)
    Abstract [en]

    The long standing problem of identifying the emission mechanism operating in gamma-ray bursts (GRBs) has produced a myriad of possible models that have the potential of explaining the observations. Generally, the empirical Band function is fit to the observed gamma-ray data and the fit parameters that are used to infer which radiative mechanisms are at work in GRB outflows. In particular, the distribution of the Band function's low-energy power-law index, alpha, has led to the so-called synchrotron 'line-of-death' (LOD) which is a statement that the distribution cannot be explained by the simplest of synchrotron models alone. As an alternatively fitting model, a combination of a blackbody in addition to the Band function is used, which in many cases provide a better or equally good fit. It has been suggested that such fits would be able to alleviate the LOD problem for synchrotron emission in GRBs. However, these conclusions rely on the Band function's ability to fit a synchrotron spectrum within the observed energy band. In order to investigate if this is the case, we simulate synchrotron and synchrotron+blackbody spectra and fold them through the instrumental response of the Fermi Gamma-ray Burst Monitor (GBM). We then perform a standard data analysis by fitting the simulated data with both Band and Band+blackbody models. We find two important results: the synchrotron LOD is actually more severe than the original predictions: alpha(LOD) similar to -0.8. Moreover, we find that intrinsic synchrotron+blackbody emission is insufficient to account for the entire observed alpha distribution. This implies that some other emission mechanism(s) are required to explain a large fraction of observed GRBs.

  • 44.
    Bäckström, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Gurell, Jonas
    Stockholm University, Faculty of Science, Department of Physics.
    Royen, Peder
    Stockholm University, Faculty of Science, Department of Physics.
    Mannervik, Sven
    Stockholm University, Faculty of Science, Department of Physics.
    Norlin, L.
    Blackwell-Whitehead, R.
    Hartman, H.
    Nilsson, H.
    The FERRUM project: metastable lifetimes in Cr II2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 420, no 2, 1636-1639 p.Article in journal (Refereed)
    Abstract [en]

    Parity forbidden radiative transitions from metastable levels are observed in spectra of low-density astrophysical plasmas. These lines are used as probes of the physical conditions, made possible due to the long lifetime of their upper level. In a joint effort, the FERRUM project aims to obtain new and accurate atomic data for the iron-group elements, and part of this project concerns forbidden lines. The radiative lifetimes of the metastable energy levels 3 d4(a 3 D)4 s c4 D 5/2 and 3 d4(a 3 D)4 s c4 D 7/2 of singly ionized chromium have been measured. The experiment has been performed at the ion storage ring CRYRING. We employed a laser-probing technique developed for measuring long lifetimes. In this article, we present the lifetimes of these levels to be t5/2= 1.28(16) s and t7/2= 1.37(7) s, respectively. A comparison with previous theoretical work shows good agreement and the result is discussed in a theoretical context.

  • 45. Chamandy, Luke
    et al.
    Singh, Nishant K.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Max Planck Institute for Solar system Research, Germany.
    A new constraint on mean-field galactic dynamo theory2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, no 3, 3657-3662 p.Article in journal (Refereed)
    Abstract [en]

    Appealing to an analytical result from mean-field theory, we show, using a generic galaxy model, that galactic dynamo action can be suppressed by small-scale magnetic fluctuations. This is caused by the magnetic analogue of the Radler or Omega x J effect, where rotation-induced corrections to the mean-field turbulent transport result in what we interpret to be an effective reduction of the standard a effect in the presence of small-scale magnetic fields.

  • 46. Chauvin, M.
    et al.
    Florén, Hans-Gustav
    Stockholm University, Faculty of Science, Department of Astronomy.
    Jackson, M.
    Kamae, T.
    Kawano, T.
    Kiss, M.
    Kole, M.
    Mikhalev, V.
    Moretti, E.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Rydström, S.
    Takahashi, H.
    Iyudin, A.
    Arimoto, M.
    Fukazawa, Y.
    Kataoka, J.
    Kawai, N.
    Mizuno, T.
    Ryde, F.
    Tajima, H.
    Takahashi, T.
    Pearce, M.
    Observation of polarized hard X-ray emission from the Crab by the PoGOLite Pathfinder2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, no 1, l84-L88 p.Article in journal (Refereed)
    Abstract [en]

    We have measured the linear polarization of hard X-ray emission from the Crab in a previously unexplored energy interval, 20-120 keV. The introduction of two new observational parameters, the polarization fraction and angle stands to disentangle geometrical and physical effects, thereby providing information on the pulsar wind geometry and magnetic field environment. Measurements are conducted using the PoGOLite Pathfinder - a balloon-borne polarimeter. Polarization is determined by measuring the azimuthal Compton scattering angle of incident X-rays in an array of plastic scintillators housed in an anticoincidence well. The polarimetric response has been characterized prior to flight using both polarized and unpolarized calibration sources. We address possible systematic effects through observations of a background field. The measured polarization fraction for the integrated Crab light curve is 18.4(-10.6)(+9.8) per cent, corresponding to an upper limit (99 per cent credibility) of 42.4 per cent, for a polarization angle of (149.2 +/- 16.0)degrees.

  • 47.
    Chiappo, Andrea
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cohen-Tanugi, J.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Strigari, L. E.
    Anderson, Brandon
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sánchez-Conde, Miguel A.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Dwarf spheroidal J-factors without priors: A likelihood-based analysis for indirect dark matter searches2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 466, no 1, 669-676 p.Article in journal (Refereed)
    Abstract [en]

    Line-of-sight integrals of the squared density, commonly called the J-factor, are essential for inferring dark matter (DM) annihilation signals. The J-factors of DM-dominated dwarf spheroidal satellite galaxies (dSphs) have typically been derived using Bayesian techniques, which for small data samples implies that a choice of priors constitutes a non-negligible systematic uncertainty. Here we report the development of a new fully frequentist approach to construct the profile likelihood of the J-factor. Using stellar kinematic data from several classical and ultra-faint dSphs, we derive the maximum likelihood value for the J-factor and its confidence intervals. We validate this method, in particular its bias and coverage, using simulated data from the Gaia Challenge. We find that the method possesses good statistical properties. The J-factors and their uncertainties are generally in good agreement with the Bayesian-derived values, with the largest deviations restricted to the systems with the smallest kinematic data sets. We discuss improvements, extensions, and future applications of this technique.

  • 48. Childress, M. J.
    et al.
    Hillier, D. J.
    Seitenzahl, I.
    Sullivan, M.
    Maguire, K.
    Taubenberger, S.
    Scalzo, R.
    Ruiter, A.
    Blagorodnova, N.
    Camacho, Y.
    Castillo, J.
    Elias-Rosa, N.
    Fraser, M.
    Gal-Yam, A.
    Graham, M.
    Howell, D. A.
    Inserra, C.
    Jha, S. W.
    Kumar, S.
    Mazzali, P. A.
    McCully, C.
    Morales-Garoffolo, A.
    Pandya, V.
    Polshaw, J.
    Schmidt, B.
    Smartt, S.
    Smith, K. W.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Spyromilio, J.
    Tucker, B.
    Valenti, S.
    Walton, N.
    Wolf, C.
    Yaron, O.
    Young, D. R.
    Yuan, F.
    Zhang, B.
    Measuring nickel masses in Type Ia supernovae using cobalt emission in nebular phase spectra2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 454, no 4, 3816-3842 p.Article in journal (Refereed)
    Abstract [en]

    The light curves of Type Ia supernovae (SNe Ia) are powered by the radioactive decay of Ni-56 to Co-56 at early times, and the decay of Co-56 to Fe-56 from similar to 60 d after explosion. We examine the evolution of the [Co III] lambda 5893 emission complex during the nebular phase for SNe Ia with multiple nebular spectra and show that the line flux follows the square of the mass of Co-56 as a function of time. This result indicates both efficient local energy deposition from positrons produced in Co-56 decay and long-term stability of the ionization state of the nebula. We compile SN Ia nebular spectra from the literature and present 21 new late-phase spectra of 7 SNe Ia, including SN 2014J. From these we measure the flux in the [Co III] lambda 5893 line and remove its well-behaved time dependence to infer the initial mass of Ni-56 (M-Ni, produced in the explosion. We then examine Ni-56 yields for different SN Ia ejected masses (M-ej-calculated using the relation between light-curve width and ejected mass) and find that the Ni-56 masses of SNe Ia fall into two regimes: for narrow light curves (low stretch s similar to 0.7-0.9), M-Ni is clustered near MN, 0.4 Me and shows a shallow increase as Mei increases from similar to 1 to 1.4 M-circle dot; at high stretch, M-ej clusters at the Chandrasekhar mass (1.4 M-circle dot) while M-Ni, spans a broad range from 0.6 to 1.2 M-circle dot. This could constitute evidence for two distinct SN Ia explosion mechanisms.

  • 49. Christensen, Lise
    et al.
    Laursen, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Richard, Johan
    Hjorth, Jens
    Milvang-Jensen, Bo
    Dessauges-Zavadsky, Miroslava
    Limousin, Marceau
    Grillo, Claudio
    Ebeling, Harald
    Gravitationally lensed galaxies at 2 < z < 3.5: direct abundance measurements of Ly alpha emitters2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 427, no 3, 1973-1982 p.Article in journal (Refereed)
    Abstract [en]

    Strong gravitational lensing magnifies the flux from distant galaxies, allowing us to detect emission lines that would otherwise fall below the detection threshold for medium-resolution spectroscopy. Here we present the detection of temperature-sensitive oxygen emission lines from three galaxies at 2 ? z ? 3.5, which enables us to directly determine the oxygen abundances and thereby double the number of galaxies at z > 2 for which this has been possible. The three galaxies have similar to 10?per?cent solar oxygen abundances in agreement with strong emission-line diagnostics. Carbon and nitrogen ratios relative to oxygen are subsolar as expected for young metal-poor galaxies. Two of the galaxies are Lyman a (Lya) emitters with rest-frame equivalent widths of 20 and 40 angstrom, respectively, and their high magnification factors allow us for the first time to gain insight into the physical characteristics of high-redshift Lya emitters. Using constraints from the physical properties of the galaxies, we accurately reproduce their line profiles with radiative transfer models. The models show a relatively small outflow in agreement with the observed small velocity offsets between nebular emission and interstellar absorption lines.

  • 50. Christensen, Lise
    et al.
    Richard, Johan
    Hjorth, Jens
    Milvang-Jensen, Bo
    Laursen, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Limousin, Marceau
    Dessauges-Zavadsky, Miroslava
    Grillo, Claudio
    Ebeling, Harald
    The low-mass end of the fundamental relation for gravitationally lensed star-forming galaxies at 1 < z < 62012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 427, no 3, 1953-1972 p.Article, review/survey (Refereed)
    Abstract [en]

    We present Very Large Telescope/X-shooter spectra of 13 galaxies in the redshift range 1 ? z ? 6, which are strongly lensed by massive galaxy clusters. Spectroscopic redshifts are measured for nine galaxies, while three sources have redshifts determined from continuum breaks in their spectra. The stellar masses of the galaxies span four orders of magnitude between 107 and 1011?M? and have luminosities at 1500 angstrom rest frame between 0.004 and 9L* after correcting for the magnification. This allows us to probe a variety of galaxy types from young, low-mass starburst galaxies to massive evolved galaxies. The lensed galaxies with stellar masses less than 1010?M? have a large scatter compared to the fundamental relation between stellar mass, star formation rates and oxygen abundances. We provide a modified fit to the fundamental relation for low-mass, low-metallicity galaxies with a weaker dependence of the metallicity on either the star formation rate or stellar mass compared to low-redshift, high-mass and high-metallicity Sloan Digital Sky Survey galaxies.

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