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  • 51. Barbiellini, G.
    et al.
    Bastieri, D.
    Bechtol, K.
    Bellazzini, R.
    Blandford, R. D.
    Borgland, A. W.
    Bregeon, J.
    Bruel, P.
    Buehler, R.
    Buson, S.
    Caliandro, G. A.
    Cameron, R. A.
    Caraveo, P. A.
    Cavazzuti, E.
    Cecchi, C.
    Chaves, R. C. G.
    Chekhtman, A.
    Cheung, C. C.
    Chiang, J.
    Ciprini, S.
    Claus, R.
    Cohen-Tanugi, J.
    D'Ammando, F.
    de Angelis, A.
    Dermer, C. D.
    Digel, S. W.
    do Couto e Silva, E.
    Drell, P. S.
    Drlica-Wagner, A.
    Favuzzi, C.
    Focke, W. B.
    Franckowiak, A.
    Fukazawa, Y.
    Fusco, P.
    Gargano, F.
    Gasparrini, D.
    Germani, S.
    Giglietto, N.
    Giommi, P.
    Giordano, F.
    Giroletti, M.
    Glanzman, T.
    Godfrey, G.
    Grenier, I. A.
    Grove, J. E.
    Guiriec, S.
    Hadasch, D.
    Hayashida, M.
    Hays, E.
    Hughes, R. E.
    Jackson, M. S.
    Jogler, T.
    Knoedlseder, J.
    Kuss, M.
    Lande, J.
    Larsson, Stefan
    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).
    Longo, F.
    Loparco, F.
    Lovellette, M. N.
    Lubrano, P.
    Mazziotta, M. N.
    Mehault, J.
    Michelson, P. F.
    Mizuno, T.
    Moiseev, A. A.
    Monte, C.
    Monzani, M. E.
    Morselli, A.
    Moskalenko, I. V.
    Murgia, S.
    Nemmen, R.
    Nuss, E.
    Ohsugi, T.
    Omodei, N.
    Orienti, M.
    Orlando, E.
    Paneque, D.
    Perkins, J. S.
    Piron, F.
    Pivato, G.
    Prokhorov, D.
    Raino, S.
    Razzano, M.
    Razzaque, S.
    Reimer, A.
    Reimer, O.
    Ritz, S.
    Romoli, C.
    Sanchez-Conde, M.
    Sanchez, D. A.
    Sgro, C.
    Siskind, E. J.
    Spandre, G.
    Spinelli, P.
    Takahashi, H.
    Tanaka, T.
    Tibaldo, L.
    Tinivella, M.
    Tosti, G.
    Troja, E.
    Usher, T. L.
    Vandenbroucke, J.
    Vasileiou, V.
    Vianello, G.
    Vitale, V.
    Waite, A. P.
    Winer, B. L.
    Wood, K. S.
    Yang, Z.
    Fermi LARGE AREA TELESCOPE OBSERVATIONS OF BLAZAR 3C 279 OCCULTATIONS BY THE SUN2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 784, no 2, p. 118-Article in journal (Refereed)
    Abstract [en]

    Observations of occultations of bright. gamma-ray sources by the Sun may reveal predicted pair halos around blazars and/or new physics, such as, e.g., hypothetical light dark matter particles-axions. We use Fermi Gamma-Ray Space Telescope (Fermi) data to analyze four occultations of blazar 3C 279 by the Sun on October 8 each year from 2008 to 2011. A combined analysis of the observations of these occultations allows a point-like source at the position of 3C 279 to be detected with significance of approximate to 3 sigma, but does not reveal any significant excess over the flux expected from the quiescent Sun. The likelihood ratio test rules out complete transparency of the Sun to the blazar. gamma-ray emission at a 3s confidence level.

  • 52. Buson, S.
    et al.
    Longo, F.
    Larsson, Stefan
    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).
    Cutini, S.
    Finke, J.
    Ciprini, S.
    Ojha, R.
    D'Ammando, F.
    Donato, D.
    Thompson, D. J.
    Desiante, R.
    Bastieri, D.
    Wagner, S.
    Hauser, M.
    Fuhrmann, L.
    Dutka, M.
    Mueller, C.
    Kadler, M.
    Angelakis, E.
    Zensus, J. A.
    Stevens, J.
    Blanchard, J. M.
    Edwards, P. G.
    Lovell, J. E. J.
    Gurwell, M. A.
    Wehrle, A. E.
    Zook, A.
    Unusual flaring activity in the blazar PKS 1424-418 during 2008-20112014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 569, p. A40-Article in journal (Refereed)
    Abstract [en]

    Context. Blazars are a subset of active galactic nuclei (AGN) with jets that are oriented along our line of sight. Variability and spectral energy distribution (SED) studies are crucial tools for understanding the physical processes responsible for observed AGN emission. Aims. We report peculiar behavior in the bright gamma-ray blazar PKS 1424 418 and use its strong variability to reveal information about the particle acceleration and interactions in the jet. Methods. Correlation analysis of the extensive optical coverage by the ATOM telescope and nearly continuous gamma-ray coverage by the Fermi Large Area Telescope is combined with broadband, time-dependent modeling of the SED incorporating supplemental information from radio and X-ray observations of this blazar. Results. We analyse in detail four bright phases at optical-GeV energies. These flares of PKS 1424-418 show high correlation between these energy ranges, with the exception of one large optical flare that coincides with relatively low gamma-ray activity. Although the optical/gamma-ray behavior of PKS 1424-418 shows variety, the multiwavelength modeling indicates that these differences can largely be explained by changes in the flux and energy spectrum of the electrons in the jet that are radiating. We find that for all flares the SED is adequately represented by a leptonic model that includes inverse Compton emission from external radiation fields with similar parameters. Conclusions. Detailed studies of individual blazars like PKS 1424 418 during periods of enhanced activity in different wavebands are helping us identify underlying patterns in the physical parameters in this class of AGN.

  • 53. Cheung, C. C.
    et al.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Scargle, J. D.
    Amin, M. A.
    Blandford, R. D.
    Bulmash, D.
    Chiang, J.
    Ciprini, S.
    Corbet, R. H. D.
    Falco, E. E.
    Marshall, P. J.
    Wood, D. L.
    Ajello, M.
    Bastieri, D.
    Chekhtman, A.
    D'Ammando, F.
    Giroletti, M.
    Grove, J. E.
    Lott, B.
    Ojha, R.
    Orienti, M.
    Perkins, J. S.
    Razzano, M.
    Smith, A. W.
    Thompson, D. J.
    Wood, K. S.
    FERMI LARGE AREA TELESCOPE DETECTION OF GRAVITATIONAL LENS DELAYED gamma-RAY FLARES FROM BLAZAR B0218+3572014In: ASTROPHYSICAL JOURNAL LETTERS, ISSN 2041-8205, Vol. 782, no 2, p. L14-Article in journal (Refereed)
    Abstract [en]

    Using data from the Fermi Large Area Telescope (LAT), we report the first clear gamma-ray measurement of a delay between flares from the gravitationally lensed images of a blazar. The delay was detected in B0218+357, a known double-image lensed system, during a period of enhanced gamma-ray activity with peak fluxes consistently observed to reach >20-50x its previous average flux. An auto-correlation function analysis identified a delay in the gamma-ray data of 11.46 +/- 0.16 days (1 sigma) that is similar to 1 day greater than previous radio measurements. Considering that it is beyond the capabilities of the LAT to spatially resolve the two images, we nevertheless decomposed individual sequences of superposing gamma-ray flares/delayed emissions. In three such similar to 8-10 day-long sequences within a similar to 4 month span, considering confusion due to overlapping flaring emission and flux measurement uncertainties, we found flux ratios consistent with similar to 1, thus systematically smaller than those from radio observations. During the first, best-defined flare, the delayed emission was detailed with a Fermi pointing, and we observed flux doubling timescales of similar to 3-6 hr implying as well extremely compact gamma-ray emitting regions.

  • 54. Cutini, S.
    et al.
    Ciprini, S.
    Orienti, M.
    Tramacere, A.
    D'Ammando, F.
    Verrecchia, F.
    Polenta, G.
    Carrasco, L.
    D'Elia, V.
    Giommi, P.
    Gonzalez-Nuevo, J.
    Grandi, P.
    Harrison, D.
    Hays, E.
    Larsson, Stefan
    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).
    Lahteenmaki, A.
    Leon-Tavares, J.
    Lopez-Caniego, M.
    Natoli, P.
    Ojha, R.
    Partridge, B.
    Porras, A.
    Reyes, L.
    Recias, E.
    Torresill, E.
    Radio-gamma-ray connection and spectral evolution in 4C+49.22 (S4 1150+49): the Fermi, Swift and Planck view2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 445, no 4, p. 4316-4334Article in journal (Refereed)
    Abstract [en]

    The Large Area Telescope on board the Fermi Gamma-ray Space Telescope detected a strong gamma-ray flare on 2011 May 15 from a source identified as 4C +49.22, a flat spectrum radio quasar (FSRQ) also known as S4 1150+49. This blazar, characterized by a prominent radio-optical-X-ray jet, was in a low gamma-ray activity state during the first years of Fermi observations. Simultaneous observations during the quiescent, outburst and post-flare gamma-ray states were obtained by Swift, Planck and optical-IR-radio telescopes (Instituto Nacional de Astrofisica, Optica y Electronica, Catalina Sky Survey, Very Long Baseline Array [VLBA], Metsahovi). The flare is observed from microwave to X-ray bands with correlated variability and the Fermi, Swift and Planck data for this FSRQ show some features more typical of BL Lac objects, like the synchrotron peak in the optical band that outshines the thermal blue-bump emission, and the X-ray spectral softening. Multi-epoch VLBA observations show the ejection of a new component close in time with the GeV gamma-ray flare. The radio-to-gamma-ray spectral energy distribution is modelled and fitted successfully for the outburst and the post-flare epochs using either a single flaring blob with two emission processes (synchrotron self-Compton (SSC), and external-radiation Compton), and a two-zone model with SSC-only mechanism.

  • 55. D'Ammando, F.
    et al.
    Antolini, E.
    Tosti, G.
    Finke, J.
    Ciprini, S.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Ajello, M.
    Covino, S.
    Gasparrini, D.
    Gurwell, M.
    Hauser, M.
    Romano, P.
    Schinzel, F.
    Wagner, S. J.
    Impiombato, D.
    Perri, M.
    Persic, M.
    Pian, E.
    Polenta, G.
    Sbarufatti, B.
    Treves, A.
    Vercellone, S.
    Wehrle, A.
    Zook, A.
    Long-term monitoring of PKS 0537-441 with Fermi-LAT and multiwavelength observations2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 431, no 3, p. 2481-2492Article in journal (Refereed)
    Abstract [en]

    We report on multiwavelength observations of the blazar PKS 0537-441 (z = 0.896) obtained from microwaves through gamma-rays by Submillimeter Array, Rapid Eye Mounting, Automatic Telescope for Optical Monitoring (ATOM), Swift and Fermi mostly during 2008 August-2010 April. Strong variability has been observed in gamma-rays, with two major flaring episodes (2009 July and 2010 March) and a harder-when-brighter behaviour, quite common for flat spectrum radio quasars and low-synchrotron-peaked BL Lacertae objects (BL Lacs), in 2010 March. In the same way, the spectral energy distribution (SED) of the source cannot be modelled by a simple synchrotron self-Compton model, as opposed to many BL Lacs, but the addition of an external Compton component of seed photons from a dust torus is needed. The 230 GHz light curve showed an increase simultaneous with the gamma-ray one, indicating co-spatiality of the mm and gamma-ray emission region likely at large distance from the central engine. The low, average, and high activity SED of the source could be fit changing only the electron distribution parameters, but two breaks in the electron distribution are necessary. The ensuing extra spectral break, located at near-infrared (NIR)-optical frequencies, together with that in gamma-rays seem to indicate a common origin, most likely due to an intrinsic feature in the underlying electron distribution. An overall correlation between the gamma-ray band with the R band and K band has been observed with no significant time lag. On the other hand, when inspecting the light curves on short time-scales some differences are evident. In particular, flaring activity has been detected in NIR and optical bands with no evident gamma-ray counterparts in 2009 September and November. Moderate variability has been observed in X-rays with no correlation between flux and photon index. An increase of the detected X-ray flux with no counterpart at the other wavelengths has been observed in 2008 October, suggesting once more a complex correlation between the emission at different energy bands.

  • 56. Fuhrmann, L.
    et al.
    Angelakis, E.
    Zensus, J. A.
    Nestoras, I.
    Marchili, N.
    Pavlidou, V.
    Karamanavis, V.
    Ungerechts, H.
    Krichbaum, T. P.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Lee, S. S.
    Max-Moerbeck, W.
    Myserlis, I.
    Pearson, T. J.
    Readhead, A. C. S.
    Richards, J. L.
    Sievers, A.
    Sohn, B. W.
    The F-GAMMA programme: multi-frequency study of active galactic nuclei in the Fermi era Programme description and the first 2.5 years of monitoring2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 596, article id A45Article in journal (Refereed)
    Abstract [en]

    Context. To fully exploit the scientific potential of the Fermi mission for the physics of active galactic nuclei (AGN), we initiated the F-GAMMA programme. Between 2007 and 2015 the F-GAMMA was the prime provider of complementary multi-frequency monitoring in the radio regime. Aims. We quantify the radio variability of gamma-ray blazars. We investigate its dependence on source class and examine whether the radio variability is related to the gamma-ray loudness. Finally, we assess the validity of a putative correlation between the two bands. Methods. The F-GAMMA performed monthly monitoring of a sample of about 60 sources at up to twelve radio frequencies between 2.64 and 228.39 GHz. We perform a time series analysis on the first 2.5-yr data set to obtain variability parameters. A maximum likelihood analysis is used to assess the significance of a correlation between radio and gamma-ray fluxes. Results. We present light curves and spectra (coherent within ten days) obtained with the Effelsberg 100 m and IRAM 30 m telescopes. All sources are variable across all frequency bands with amplitudes increasing with frequency up to rest frame frequencies of around 60-80 GHz as expected by shock-in-jet models. Compared to flat-spectrum radio quasars (FSRQs), BL Lacertae objects (BL Lacs) show systematically lower variability amplitudes, brightness temperatures, and Doppler factors at lower frequencies, while the difference vanishes towards higher ones. The time scales appear similar for the two classes. The distribution of spectral indices appears flatter or more inverted at higher frequencies for BL Lacs. Evolving synchrotron self-absorbed components can naturally account for the observed spectral variability. We find that the Fermi-detected sources show larger variability amplitudes, brightness temperatures, and Doppler factors than non-detected ones. Flux densities at 86.2 and 142.3 GHz correlate with 1 GeV fluxes at a significance level better than 3 sigma, implying that gamma rays are produced very close to the mm-band emission region.

  • 57. Fuhrmann, L.
    et al.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Chiang, J.
    Angelakis, E.
    Zensus, J. A.
    Nestoras, I.
    Krichbaum, T. P.
    Ungerechts, H.
    Sievers, A.
    Pavlidou, V.
    Readhead, A. C. S.
    Max-Moerbeck, W.
    Pearson, T. J.
    Detection of significant cm to sub-mm band radio and gamma-ray correlated variability in Fermi bright blazars2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 441, no 3, p. 1899-1909Article in journal (Refereed)
    Abstract [en]

    The exact location of the gamma-ray emitting region in blazars is still controversial. In order to attack this problem we present first results of a cross-correlation analysis between radio (11 cm to 0.8 mm wavelength, F-GAMMA programme) and gamma-ray (0.1-300 GeV) similar to 3.5 yr light curves of 54 Fermi-bright blazars. We perform a source stacking analysis and estimate significances and chance correlations using mixed source correlations. Our results reveal: (i) the first highly significant multiband radio and. gamma-ray correlations (radio lagging gamma rays) when averaging over the whole sample, (ii) average time delays (source frame: 76 +/- 23 to 7 +/- 9 d), systematically decreasing from cm to mm/sub-mm bands with a frequency dependence tau(r,gamma)(v) proportional to v(-1), in good agreement with jet opacity dominated by synchrotron self-absorption, (iii) a bulk gamma-ray production region typically located within/upstream of the 3 mm core region (tau(3mm),(gamma) = 12 +/- 8 d), (iv) mean distances between the region of. gamma-ray peak emission and the radio 'tau = 1 photosphere' decreasing from 9.8 +/- 3.0 pc (11 cm) to 0.9 +/- 1.1 pc (2 mm) and 1.4 +/- 0.8 pc (0.8 mm), (v) 3 mm/gamma-ray correlations in nine individual sources at a significance level where one is expected by chance (probability: 4 x 10(-6)), (vi) opacity and 'time lag core shift' estimates for quasar 3C 454.3 providing a lower limit for the distance of the bulk gamma-ray production region from the supermassive black hole (SMBH) of similar to 0.8-1.6 pc, i.e. at the outer edge of the broad-line region (BLR) or beyond. A 3 mm tau = 1 surface at similar to 2-3 pc from the jet base (i.e. well outside the 'canonical BLR') finally suggests that BLR material extends to several parsec distances from the SMBH.

  • 58. González-Galán, A.
    et al.
    Kuulkers, E.
    Kretschmar, P.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Postnov, K.
    Kochetkova, A.
    Finger, M. H.
    Spin period evolution of GX 1+42012In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 537, article id A66Article in journal (Refereed)
    Abstract [en]

    Aims. We aim both to complement the existing data on the spin history of the peculiar accreting X-ray pulsar GX 1+4 with more past and current data from BeppoSAX, INTEGRAL, and Fermi and to interpret the evolution in the framework of accretion theory.

    Methods. We used source light curves obtained from BeppoSAX/WFC and INTEGRAL/ISGRI to derive pulse periods using an epoch-folding analysis. Fermi/GBM data were analysed by fitting a constant plus a Fourier expansion to background-subtracted rates, and maximizing the Y-2 statistic. We completed the sample with hard X-ray light curves from Swift/BAT. The data were checked for correlations between flux and changes of the pulsar spin on different timescales.

    Results. The spin-down of the pulsar continues with a constant change in frequency, i.e., an apparently accelerating change in the period. Over the past three decades, the pulse period has increased by about similar to 50%. Short-term fluctuations on top of this long-term trend do show anti-correlation with the source flux. Possible explanations of the observed long-term frequency and its dependence on flux are discussed.

  • 59. Hayashida, M.
    et al.
    Madejski, G. M.
    Nalewajko, K.
    Sikora, M.
    Wehrle, A. E.
    Ogle, P.
    Collmar, W.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, Department of Astronomy.
    Fukazawa, Y.
    Itoh, R.
    Chiang, J.
    Stawarz, L.
    Blandford, R. D.
    Richards, J. L.
    Max-Moerbeck, W.
    Readhead, A.
    Buehler, R.
    Cavazzuti, E.
    Ciprini, S.
    Gehrels, N.
    Reimer, A.
    Szostek, A.
    Tanaka, T.
    Tosti, G.
    Uchiyama, Y.
    Kawabata, K. S.
    Kino, M.
    Sakimoto, K.
    Sasada, M.
    Sato, S.
    Uemura, M.
    Yamanaka, M.
    Greiner, J.
    Kruehler, T.
    Rossi, A.
    Macquart, J. P.
    Bock, D. C. -J
    Villata, M.
    Raiteri, C. M.
    Agudo, I.
    Aller, H. D.
    Aller, M. F.
    Arkharov, A. A.
    Bach, U.
    Benitez, E.
    Berdyugin, A.
    Blinov, D. A.
    Blumenthal, K.
    Boettcher, M.
    Buemi, C. S.
    Carosati, D.
    Chen, W. P.
    Di Paola, A.
    Dolci, M.
    Efimova, N. V.
    Forne, E.
    Gomez, J. L.
    Gurwell, M. A.
    Heidt, J.
    Hiriart, D.
    Jordan, B.
    Jorstad, S. G.
    Joshi, M.
    Kimeridze, G.
    Konstantinova, T. S.
    Kopatskaya, E. N.
    Koptelova, E.
    Kurtanidze, O. M.
    Lahteenmaki, A.
    Lamerato, A.
    Larionov, V. M.
    Larionova, E. G.
    Larionova, L. V.
    Leto, P.
    Lindfors, E.
    Marscher, A. P.
    McHardy, I. M.
    Molina, S. N.
    Morozova, D. A.
    Nikolashvili, M. G.
    Nilsson, K.
    Reinthal, R.
    Roustazadeh, P.
    Sakamoto, T.
    Sigua, L. A.
    Sillanpaa, A.
    Takalo, L.
    Tammi, J.
    Taylor, B.
    Tornikoski, M.
    Trigilio, C.
    Troitsky, I. S.
    Umana, G.
    THE STRUCTURE AND EMISSION MODEL OF THE RELATIVISTIC JET IN THE QUASAR 3C 279 INFERRED FROM RADIO TO HIGH-ENERGY gamma-RAY OBSERVATIONS IN 2008-20102012In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 754, no 2, p. 114-Article in journal (Refereed)
    Abstract [en]

    We present time-resolved broadband observations of the quasar 3C 279 obtained from multi-wavelength campaigns conducted during the first two years of the Fermi Gamma-ray Space Telescope mission. While investigating the previously reported gamma-ray/optical flare accompanied by a change in optical polarization, we found that the optical emission appears to be delayed with respect to the gamma-ray emission by about 10 days. X-ray observations reveal a pair of isolated flares separated by similar to 90 days, with only weak gamma-ray/optical counterparts. The spectral structure measured by Spitzer reveals a synchrotron component peaking in the mid-infrared band with a sharp break at the far-infrared band during the gamma-ray flare, while the peak appears in the millimeter (mm)/submillimeter (sub-mm) band in the low state. Selected spectral energy distributions are fitted with leptonic models including Comptonization of external radiation produced in a dusty torus or the broad-line region. Adopting the interpretation of the polarization swing involving propagation of the emitting region along a curved trajectory, we can explain the evolution of the broadband spectra during the gamma-ray flaring event by a shift of its location from similar to 1 pc to similar to 4 pc from the central black hole. On the other hand, if the gamma-ray flare is generated instead at sub-pc distance from the central black hole, the far-infrared break can be explained by synchrotron self-absorption. We also model the low spectral state, dominated by the mm/sub-mm peaking synchrotron component, and suggest that the corresponding inverse-Compton component explains the steady X-ray emission.

  • 60.
    Hjalmarsdotter, Linnea
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zdziarski, Andrzej
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Beckmann, Volker
    McCollough, Michael
    Hanikainen, Diana
    Vilhu, Osmi
    The nature of the hard state of Cygnus X-32008In: Monthly Notices of the Royal Astronomical Society, Vol. 384, no 1, p. 278-290Article in journal (Refereed)
    Abstract [en]

    The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ~20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf-Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE/ASM and CGRO/BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher L/LE than in the usual advection models, or the mass of the compact object is >~20Msolar, possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation.

  • 61. Iyudin, A
    et al.
    Andra, 2
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Gamma-Ray Probe of the QSO's Obscured Evolution2008In: Relativistic Astrophysics Legacy and Cosmology - Einstein's, 2008, p. 230-Conference paper (Other academic)
    Abstract [en]

    The obscured phase of QSOs, as well as their accretion history, can be best followed by observing QSOs bright in >10 MeV gamma-rays. By analysing the resonant absorption troughs in spectral energy distribution of flaring QSOs one can measure the (baryonic) absorbing column and baryonic content of the QSO host galaxy,while the flare strength will give information on the accretion rate of the QSO powering supermassive black hole. By measuring the baryonic absorbing column for QSOs at different redshifts one can follow the early obscured evolution of AGN at redshifts up to z~6

  • 62. Iyudin, A. F.
    et al.
    Pakhomov, Yu. V.
    Chugai, N. N.
    Greiner, J.
    Axelsson, Magnus
    Stockholm University, Faculty of Science, Department of Astronomy.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ryabchikova, T. A.
    Search for broad absorption lines in spectra of stars in the field of supernova remnant RX J0852.0-4622 (Vela Jr.)2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 519, p. A86-Article in journal (Refereed)
    Abstract [en]

    Aims. Supernova remnant (SNR) RX J0852.0-4622 is one of the youngest and is most likely the closest among known Galactic SNRs. It was detected in X-rays, the Ti-44 gamma-line, and radio. We obtain and analyze medium-resolution spectra of 14 stars in the direction towards the SNR RX J0852.0-4622 in an attempt to detect broad absorption lines of unshocked ejecta against background stars. Methods. Spectral synthesis is performed for all the stars in the wavelength range of 3740-4020 angstrom to extract the broad absorption lines of Ca II related to the SNR RX J0852.0-4622. Results. We do not detect any broad absorption line and place a 3 sigma upper limit on the relative depths of <0.04 for the broad Ca II absorption produced by the SNR. We detect narrow low and high velocity absorption components of Ca II. High velocity vertical bar V-LSR vertical bar similar to 100-140 km s(-1) components are attributed to radiative shocks in clouds engulfed by the old Vela SNR. The upper limit to the absorption line strength combined with the width and flux of the Ti-44 gamma-ray line 1.16 MeV lead us to conclude that SNR RX J0852.0-4622 was probably produced by an energetic SN Ic explosion.

  • 63. Kamae, Tuneyoshi
    et al.
    Andersson, Viktor
    Arimoto, Makoto
    Axelsson, Magnus
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bettolo, Cecilia Marini
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bogaert, Gilles
    Carlson, Per
    Craig, William
    Ekeberg, Tomas
    Engdegdrd, Olle
    Fukazawa, Yasushi
    Gunji, Shuichi
    Hjalmarsdotter, Linnea
    Stockholm University, Faculty of Science, Department of Astronomy.
    Iwan, Bianca
    Kanai, Yoshikazu
    Kataoka, Jun
    Kawai, Nobuyuki
    Kazejev, Jaroslav
    Kiss, Mozsi
    Klamra, Wlodzimierz
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Madejski, Grzegorz
    Mizuno, Tsunefumi
    Ng, Johnny
    Pearce, Mark
    Ryde, Felix
    Suhonen, Markus
    TaJima, Hiroyasu
    Takahashi, Hiromitsu
    Takahashi, Tadayuki
    Tanaka, Takuya
    Thurston, Timothy
    Ueno, Masaru
    Varneri, Gary
    Yamamoto, Kazuhide
    Yamashita, Yuichiro
    Ylinen, Tomi
    Yoshida, Hiroaki
    PoGOLite - A high sensitivity balloon-borne soft gamma-ray polarimeter2008In: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 30, no 2, p. 72-84Article in journal (Refereed)
    Abstract [en]

    We describe a new balloon-borne instrument (PoGOLite) capable of detecting 10% polarisation from 200 mCrab point-like sources between 25 and 80 keV in one 6-h flight. Polarisation measurements in the soft gamma-ray band are expected to provide a powerful probe into high energy emission mechanisms as well as the distribution of magnetic fields, radiation fields and interstellar matter. Synchrotron radiation, inverse Compton scattering and propagation through high magnetic fields are likely to produce high degrees of polarisation in the energy band of the instrument. We demonstrate, through tests at accelerators, with radioactive sources and through computer simulations, that PoGOLite will be able to detect degrees of polarisation as predicted by models for several classes of high energy sources. At present, only exploratory polarisation measurements have been carried out in the soft gamma-ray band. Reduction of the large background produced by cosmic-ray particles while securing a large effective area has been the greatest challenge. PoGOLite uses Compton scattering and photo-absorption in an array of 217 well-type phoswich detector cells made of plastic and BGO scintillators surrounded by a BGO anticoincidence shield and a thick polyethylene neutron shield. The narrow Held of view (FWHM = 1.25 msr, 2.0 deg x 2.0 deg) obtained with detector cells and the use of thick background shields warrant a large effective area for polarisation measurements (similar to 228 cm(2) at E = 40 keV) without sacrificing the signal-to-noise ratio. Simulation studies for an atmospheric overburden of 3-4 g/cm(2) indicate that neutrons and gamma-rays entering the PDC assembly through the shields are dominant backgrounds. Off-line event selection based on recorded phototube waveforms and Compton kinematics reduce the background to that expected for a similar to 100 mCrab source between 25 and 50 keV. A 6-h observation of the Crab pulsar will differentiate between the Polar Cap/Slot Gap, Outer Gap, and Caustic models with greater than 5 sigma significance; and also cleanly identify the Compton reflection component in the Cygnus X-1 hard state. Long-duration flights will measure the dependence of the polarisation across the cyclotron absorption line in Hercules X-1. A scaled-down instrument will be flown as a pathfinder mission from the north of Sweden in 2010. The first science flight is planned to take place shortly thereafter. 

  • 64. Kanai, Y.
    et al.
    Ueno, M.
    Kataoka, J.
    Arimoto, M.
    Kawai, N.
    Yamamoto, K.
    Mizuno, T.
    Fukazawa, Y.
    Kiss, M.
    Ylinen, T.
    Bettolo, C. Marini
    Carlson, P.
    Klamra, W.
    Pearce, M.
    Chen, P.
    Craig, B.
    Kamae, T.
    Madejski, G.
    Ng, J. S. T.
    Rogers, R.
    Tajima, H.
    Thurston, T. S.
    Saito, Y.
    Takahashi, T.
    Gunji, S.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ryde, F.
    Stockholm University.
    Bogaert, G.
    Kishimoto, S.
    Beam test of a prototype phoswich detector assembly for the PoGOLite astronomical soft gamma-ray polarimeter2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 570, no 1, p. 61-71Article in journal (Refereed)
    Abstract [en]

    We report about the beam test on a prototype of the balloon-based astronomical soft gamma-ray polarimeter, PoGOLite (Polarized Gamma-ray Observer-Light Version) conducted at KEK Photon Factory, a synchrotron radiation facility in Japan. The synchrotron beam was set at 30, 50, and 70 keV and its polarization was monitored by a calibrated polarimeter. The goal of the experiment was to validate the flight design of the polarimeter. PoGOLite is designed to measure polarization by detecting a Compton scattering and the subsequent photo-absorption in an array of 217 well-type phoswich detector cells (PDCs). The test setup included a first flight model PDC and a front-end electronics to select and reconstruct valid Compton scattering events. The experiment has verified that the flight PDC can detect recoil electrons and select valid Compton scattering events down to 30 keV from background. The measure azimuthal modulations (34.4%, 35.8% and 37.2% at 30, 50, and 70 keV, respectively) agreed within 10% (relative) with the predictions by Geant4 implemented with dependence on the initial and final photon polarizations.

  • 65. Kiss, M.
    et al.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    Bettolo, C.
    Bogaert, G.
    Florén, Hans-Gustav
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    Fukazawa, Y.
    Gunji, S.
    Hjalmarsdotter, Linnea
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    Kamae, T.
    Kanai, Y.
    Kataoka, J.
    Kawai, N.
    Klamra, W.
    Kurita, K.
    Madejski, G.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    Pearce, Mark
    Ryde, Felix
    Rydström, S.
    Tajima, H.
    Takahashi, H.
    Takahashi, T.
    Tanaka, T.
    Ueno, M.
    Umeki, Y.
    Varner, G.
    Yoshida, H.
    The PoGOLite balloon-borne soft gamma-ray polarimeter2008In: COOL DISCS, HOT FLOWS: The Varying Faces of Accreting Compact Objects, 2008, p. 225-232Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Linearly polarized radiation in the hard X-ray/soft gamma-ray band is expected from a large variety of astronomical sources. We discuss the importance of polarimetric studies for several classes of sources-pulsars, accreting black holes, magnetic neutron stars and jets from active galaxies-and then describe PoGOLite, a balloon-borne instrument which is currently under construction and will be able to measure the polarization of electromagnetic radiation from such extra-solar objects in the energy range 25-80 keV.

  • 66.
    Larsson, Josefin
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ryde, F.
    Lundman, C.
    McGlynn, S.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ohno, M.
    Yamaoka, K.
    Spectral components in the bright, long GRB 061007: properties of the photosphere and the nature of the outflow2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, no 3, p. 2642-2649Article in journal (Refereed)
    Abstract [en]

    We present a time-resolved spectral analysis of the bright, long GRB 061007 (z = 1.261) using Swift and Suzaku data. We find that the prompt emission of the burst can be equally well explained by a photospheric component together with a power law as by a Band function, and we explore the implications of the former model. The photospheric component, which we model with a multicolour blackbody, dominates the spectra and has a very stable shape throughout the burst. This component provides a natural explanation for the hardness-intensity correlation seen within the burst and also allows us to estimate the bulk Lorentz factor and the radius of the photosphere. The power-law component dominates the fit at high energies and has a nearly constant slope of -1.5. We discuss the possibility that this component is of the same origin as the high-energy power laws recently observed in some Fermi bursts.

  • 67. Lott, B.
    et al.
    Escande, L.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Ballet, J.
    An adaptive-binning method for generating constant-uncertainty/constant-significance light curves with Fermi-LAT data2012In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 544, p. A6-Article in journal (Refereed)
    Abstract [en]

    Aims. We present a method enabling the creation of constant-uncertainty/constant-significance light curves with the data of the Fermi-Large Area Telescope (LAT). The adaptive-binning method enables more information to be encapsulated within the light curve than with the fixed-binning method. Although primarily developed for blazar studies, it can be applied to any sources. Methods. This method allows the starting and ending times of each interval to be calculated in a simple and quick way during a first step. The reported mean flux and spectral index (assuming the spectrum is a power-law distribution) in the interval are calculated via the standard LAT analysis during a second step. Results. The absence of major caveats associated with this method has been established by means of Monte-Carlo simulations. We present the performance of this method in determining duty cycles as well as power-density spectra relative to the traditional fixed-binning method.

  • 68. Mizuno, T.
    et al.
    Kanai, Y.
    Kataoka, J.
    Kiss, M.
    Kurita, K.
    Pearce, M.
    Tajima, H.
    Takahashi, H.
    Tanaka, T.
    Ueno, M.
    Umeki, Y.
    Yoshida, H.
    Arimoto, M.
    Axelsson, Magnus
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bettolo, C. Marini
    Bogaert, G.
    Chen, P.
    Craig, W.
    Fukazawa, Y.
    Gunji, S.
    Kamae, T.
    Katsuta, J.
    Kawai, N.
    Kishimoto, S.
    Klamra, W.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Madejski, G.
    Ng, J. S. T.
    Ryde, F.
    Rydström, S.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Takahashi, T.
    Thurston, T. S.
    Varner, G.
    A Monte Carlo method for calculating the energy response of plastic scintillators to polarized photons below 100 keV2009In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 600, no 3, p. 609-617Article in journal (Refereed)
    Abstract [en]

    The energy response of plastic scintillators (Eljen Technology EJ-204) to polarized soft gamma-ray photons below 100 keV has been studied, primarily for the balloon-borne polarimeter, PoGOLite. The response calculation includes quenching effects due to low-energy recoil electrons and the position dependence of the light collection efficiency in a 20 cm long scintillator rod. The broadening of the pulse-height spectrum, presumably caused by light transportation processes inside the scintillator, as well as the generation and multiplication of photoelectrons in the photomultiplier tube, were studied experimentally and have also been taken into account. A Monte Carlo simulation based on the Geant4 toolkit was used to model photon interactions in the scintillators. When using the polarized Compton/Rayleigh scattering processes previously corrected by the authors, scintillator spectra and angular distributions of scattered polarized photons could clearly be reproduced, in agreement with the results obtained at a synchrotron beam test conducted at the KEK Photon Factory. Our simulation successfully reproduces the modulation factor, defined as the ratio of the amplitude to the mean of the distribution of the azimuthal scattering angles, within similar to 5% (relative). Although primarily developed for the PoGOLite mission, the method presented here is also relevant for other missions aiming to measure polarization from astronomical objects using plastic scintillator scatterers. 

  • 69. Mészáros, Attila
    et al.
    Bagoly, Zsolt
    Klose, S
    Ryde, Felix
    Stockholm University, Faculty of Science, Department of Astronomy.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Balázs, L. G.
    Horváth, István
    Borgonovo, Luis
    Stockholm University, Faculty of Science, Department of Astronomy.
    On the origin of the dark gamma-ray bursts2005In: Nuovo Cimento C, Vol. 28, no 3, p. 311-314Article in journal (Refereed)
  • 70. Pletsch, H. J.
    et al.
    Guillemot, L.
    Fehrmann, H.
    Allen, B.
    Kramer, M.
    Aulbert, C.
    Ackermann, M.
    Ajello, M.
    de Angelis, A.
    Atwood, W. B.
    Baldini, L.
    Ballet, J.
    Barbiellini, G.
    Bastieri, D.
    Bechtol, K.
    Bellazzini, R.
    Borgland, A. W.
    Bottacini, E.
    Brandt, T. J.
    Bregeon, J.
    Brigida, M.
    Bruel, P.
    Buehler, R.
    Buson, S.
    Caliandro, G. A.
    Cameron, R. A.
    Caraveo, P. A.
    Casandjian, J. M.
    Cecchi, C.
    Celik, Oe
    Charles, E.
    Chaves, R. C. G.
    Cheung, C. C.
    Chiang, J.
    Ciprini, S.
    Claus, R.
    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).
    Cutini, S.
    D'Ammando, F.
    Dermer, C. D.
    Digel, S. W.
    Drell, P. S.
    Drlica-Wagner, A.
    Dubois, R.
    Dumora, D.
    Favuzzi, C.
    Ferrara, E. C.
    Franckowiak, A.
    Fukazawa, Y.
    Fusco, P.
    Gargano, F.
    Gehrels, N.
    Germani, S.
    Giglietto, N.
    Giordano, F.
    Giroletti, M.
    Godfrey, G.
    Grenier, I. A.
    Grondin, M. -H
    Grove, J. E.
    Guiriec, S.
    Hadasch, D.
    Hanabata, Y.
    Harding, A. K.
    den Hartog, P. R.
    Hayashida, M.
    Hays, E.
    Hill, A. B.
    Hou, X.
    Hughes, R. E.
    Johannesson, G.
    Jackson, M. S.
    Jogler, T.
    Johnson, A. S.
    Johnson, W. N.
    Kataoka, J.
    Kerr, M.
    Knoedlseder, J.
    Kuss, M.
    Lande, J.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Latronico, L.
    Lemoine-Goumard, M.
    Longo, F.
    Loparco, F.
    Lovellette, M. N.
    Lubrano, P.
    Massaro, F.
    Mayer, M.
    Mazziotta, M. N.
    McEnery, J. E.
    Mehault, J.
    Michelson, P. F.
    Mitthumsiri, W.
    Mizuno, T.
    Monzani, M. E.
    Morselli, A.
    Moskalenko, I. V.
    Murgia, S.
    Nakamori, T.
    Nemmen, R.
    Nuss, E.
    Ohno, M.
    Ohsugi, T.
    Omodei, N.
    Orienti, M.
    Orlando, E.
    de Palma, F.
    Paneque, D.
    Perkins, J. S.
    Piron, F.
    Pivato, G.
    Porter, T. A.
    Raino, S.
    Rando, R.
    Ray, P. S.
    Razzano, M.
    Reimer, A.
    Reimer, O.
    Reposeur, T.
    Ritz, S.
    Romani, R. W.
    Romoli, C.
    Sanchez, D. A.
    Parkinson, P. M. Saz
    Schulz, A.
    Sgro, C.
    do Couto e Silva, E.
    Siskind, E. J.
    Smith, D. A.
    Spandre, G.
    Spinelli, P.
    Suson, D. J.
    Takahashi, H.
    Tanaka, T.
    Thayer, J. B.
    Thayer, J. G.
    Thompson, D. J.
    Tibaldo, L.
    Tinivella, M.
    Troja, E.
    Usher, T. L.
    Vandenbroucke, J.
    Vasileiou, V.
    Vianello, G.
    Vitale, V.
    Waite, A. P.
    Winer, B. L.
    Wood, K. S.
    Wood, M.
    Yang, Zhaoyu
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zimmer, Stephan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Binary Millisecond Pulsar Discovery via Gamma-Ray Pulsations2012In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 338, no 6112, p. 1314-1317Article in journal (Refereed)
    Abstract [en]

    Millisecond pulsars, old neutron stars spun up by accreting matter from a companion star, can reach high rotation rates of hundreds of revolutions per second. Until now, all such recycled rotation-powered pulsars have been detected by their spin-modulated radio emission. In a computing-intensive blind search of gamma-ray data from the Fermi Large Area Telescope (with partial constraints from optical data), we detected a 2.5-millisecond pulsar, PSR J1311-3430. This unambiguously explains a formerly unidentified gamma-ray source that had been a decade-long enigma, confirming previous conjectures. The pulsar is in a circular orbit with an orbital period of only 93 minutes, the shortest of any spin-powered pulsar binary ever found.

  • 71. Preece, R.
    et al.
    Burgess, J. Michael
    von Kienlin, A.
    Bhat, P. N.
    Briggs, M. S.
    Byrne, D.
    Chaplin, V.
    Cleveland, W.
    Collazzi, A. C.
    Connaughton, V.
    Diekmann, A.
    Fitzpatrick, G.
    Foley, S.
    Gibby, M.
    Giles, M.
    Goldstein, A.
    Greiner, J.
    Gruber, D.
    Jenke, P.
    Kippen, R. M.
    Kouveliotou, C.
    McBreen, S.
    Meegan, C.
    Paciesas, W. S.
    Pelassa, V.
    Tierney, D.
    van der Horst, A. J.
    Wilson-Hodge, C.
    Xiong, S.
    Younes, G.
    Yu, H. -F
    Ackermann, M.
    Ajello, M.
    Axelsson, Magnus
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Baldini, L.
    Barbiellini, G.
    Baring, M. G.
    Bastieri, D.
    Bellazzini, R.
    Bissaldi, E.
    Bonamente, E.
    Bregeon, J.
    Brigida, M.
    Bruel, P.
    Buehler, R.
    Buson, S.
    Caliandro, G. A.
    Cameron, R. A.
    Caraveo, P. A.
    Cecchi, C.
    Charles, E.
    Chekhtman, A.
    Chiang, J.
    Chiaro, G.
    Ciprini, S.
    Claus, R.
    Cohen-Tanugi, J.
    Cominsky, L. R.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    D'Ammando, F.
    de Angelis, A.
    de Palma, F.
    Dermer, C. D.
    Desiante, R.
    Digel, S. W.
    Di Venere, L.
    Drell, P. S.
    Drlica-Wagner, A.
    Favuzzi, C.
    Franckowiak, A.
    Fukazawa, Y.
    Fusco, P.
    Gargano, F.
    Gehrels, N.
    Germani, S.
    Giglietto, N.
    Giordano, F.
    Giroletti, M.
    Godfrey, G.
    Granot, J.
    Grenier, I. A.
    Guiriec, S.
    Hadasch, D.
    Hanabata, Y.
    Harding, A. K.
    Hayashida, M.
    Iyyani, Shabnam
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Jogler, T.
    Joannesson, G.
    Kawano, T.
    Knoedlseder, J.
    Kocevski, D.
    Kuss, M.
    Lande, J.
    Larsson, J.
    Larsson, Stefan
    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).
    Latronico, L.
    Longo, F.
    Loparco, F.
    Lovellette, M. N.
    Lubrano, P.
    Mayer, M.
    Mazziotta, M. N.
    Michelson, P. F.
    Mizuno, T.
    Monzani, M. E.
    Moretti, E.
    Morselli, A.
    Murgia, S.
    Nemmen, R.
    Nuss, E.
    Nymark, T.
    Ohno, M.
    Ohsugi, T.
    Okumura, A.
    Omodei, N.
    Orienti, M.
    Paneque, D.
    Perkins, J. S.
    Pesce-Rollins, M.
    Piron, F.
    Pivato, G.
    Porter, T. A.
    Racusin, J. L.
    Raino, S.
    Rando, R.
    Razzano, M.
    Razzaque, S.
    Reimer, A.
    Reimer, O.
    Ritz, S.
    Roth, M.
    Ryde, F.
    Sartori, A.
    Scargle, J. D.
    Schulz, A.
    Sgro, C.
    Siskind, E. J.
    Spandre, G.
    Spinelli, P.
    Suson, D. J.
    Tajima, H.
    Takahashi, H.
    Thayer, J. G.
    Thayer, J. B.
    Tibaldo, L.
    Tinivella, M.
    Torres, D. F.
    Tosti, G.
    Troja, E.
    Usher, T. L.
    Vandenbroucke, J.
    Vasileiou, V.
    Vianello, G.
    Vitale, V.
    Werner, M.
    Winer, B. L.
    Wood, K. S.
    Zhu, S.
    The First Pulse of the Extremely Bright GRB 130427A: A Test Lab for Synchrotron Shocks2014In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 343, no 6166, p. 51-54Article in journal (Refereed)
    Abstract [en]

    Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever observed. The initial pulse up to 2.5 seconds is possibly the brightest well-isolated pulse observed to date. A fine time resolution spectral analysis shows power-law decays of the peak energy from the onset of the pulse, consistent with models of internal synchrotron shock pulses. However, a strongly correlated power-law behavior is observed between the luminosity and the spectral peak energy that is inconsistent with curvature effects arising in the relativistic outflow. It is difficult for any of the existing models to account for all of the observed spectral and temporal behaviors simultaneously.

  • 72. Ryde, F.
    et al.
    Axelsson, Magnus
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zhang, B. B.
    McGlynn, S.
    Pe'er, A.
    Lundman, C.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Physics.
    Battelino, M.
    Zhang, B.
    Bissaldi, E.
    Bregeon, J.
    Briggs, M. S.
    Chiang, J.
    de Palma, F.
    Guiriec, S.
    Larsson, Josefin
    Stockholm University, Faculty of Science, Department of Astronomy.
    Longo, F.
    McBreen, S.
    Omodei, N.
    Petrosian, V.
    Preece, R.
    van der Horst, A. J.
    IDENTIFICATION AND PROPERTIES OF THE PHOTOSPHERIC EMISSION IN GRB090902B2010In: The Astrophysical Journal Letters, ISSN 2041-8205, Vol. 709, no 2, p. l172-L177Article in journal (Refereed)
    Abstract [en]

    The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover the spectral range from 8 keV to >300 GeV. Here we show that the prompt burst spectrum is consistent with emission from the jet photosphere combined with nonthermal emission described by a single power law with photon index -1.9. The photosphere gives rise to a strong quasi-blackbody spectrum which is somewhat broader than a single Planck function and has a characteristic temperature of similar to 290 keV. We model the photospheric emission with a multicolor blackbody, and its shape indicates that the photospheric radius increases at higher latitudes. We derive the averaged photospheric radius R-ph = (1.1 +/- 0.3) x 10(12) Y-1/4 cm and the bulk Lorentz factor of the flow, which is found to vary by a factor of 2 and has a maximal value of Gamma = 750 Y-1/4. Here, Y is the ratio between the total fireball energy and the energy emitted in the gamma rays. We find that during the first quarter of the prompt phase the photospheric emission dominates, which explains the delayed onset of the observed flux in the LAT compared to the GBM. We interpret the broadband emission as synchrotron emission at R similar to 4 x 10(15) cm. Our analysis emphasizes the importance of having high temporal resolution when performing spectral analysis on gamma-ray bursts, since there is strong spectral evolution.

  • 73. Ryde, Felix
    et al.
    Pe'er, Asaf
    Nymark, Tanja
    Axelsson, Magnus
    Moretti, Elena
    Lundman, Christoffer
    Battelino, Milan
    Bissaldi, Elisabetta
    Chiang, James
    Jackson, Miranda S.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Longo, Francesco
    McGlynn, Sinead
    Omodei, Nicola
    Observational evidence of dissipative photospheres in gamma-ray bursts2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 415, no 4, p. 3693-3705Article in journal (Refereed)
    Abstract [en]

    The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape of a Band function. This fact is best illustrated by studying GRB090902B. The main gamma-ray spectral component is initially close to a Planck function, which can only be explained by emission from the jet photosphere. Later, the same component evolves into a broader Band function. This burst thus provides observational evidence that the photosphere can give rise to a non-thermal spectrum. We show that such a broadening is most naturally explained by subphotospheric dissipation in the jet. The broadening mainly depends on the strength and location of the dissipation, the magnetic field strength and the relation between the energy densities of thermal photons and electrons. We suggest that the evolution in spectral shape observed in GRB090902B is due to a decrease in the bulk Lorentz factor of the flow, leading to the main dissipation becoming subphotospheric. Such a change in the flow parameters can also explain the correlation observed between the peak energy of the spectrum and low-energy power-law slope, a, a correlation commonly observed in GRBs. We conclude that photospheric emission could indeed be a ubiquitous feature during the prompt phase in GRBs and play a decisive role in creating the diverse spectral shapes and spectral evolutions that are observed.

  • 74. Sokolovsky, K. V.
    et al.
    Schinzel, F. K.
    Tanaka, Y. T.
    Abolmasov, P. K.
    Angelakis, E.
    Bulgarelli, A.
    Carrasco, L.
    Cenko, S. B.
    Cheung, C. C.
    Clubb, K. I.
    D'Ammando, F.
    Escande, L.
    Fegan, S. J.
    Filippenko, A. V.
    Finke, J. D.
    Fuhrmann, L.
    Fukazawa, Y.
    Hays, E.
    Healey, S. E.
    Ikejiri, Y.
    Itoh, R.
    Kawabata, K. S.
    Komatsu, T.
    Kovalev, Yu A.
    Kovalev, Y. Y.
    Krichbaum, T. P.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Lister, M. L.
    Lott, B.
    Max-Moerbeck, W.
    Nestoras, I.
    Pittori, C.
    Pursimo, T.
    Pushkarev, A. B.
    Readhead, A. C. S.
    Recillas, E.
    Richards, J. L.
    Riquelme, D.
    Romani, R. W.
    Sakimoto, K.
    Sasada, M.
    Schmidt, R.
    Shaw, M. S.
    Sievers, A.
    Thompson, D. J.
    Uemura, M.
    Ungerechts, H.
    Vercellone, S.
    Verrecchia, F.
    Yamanaka, M.
    Yoshida, M.
    Zensus, J. A.
    Two active states of the narrow-line gamma-ray-loud AGN GB 1310+4872014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 565, article id A26Article in journal (Refereed)
    Abstract [en]

    Context. Previously unremarkable, the extragalactic radio source GB 1310+487 showed gamma-ray flare on 2009 November 18, reaching a daily flux of similar to 10(-6) photons cm(-2) s(-1) at energies E > 100 MeV and became one of the brightest GeV sources for about two weeks. Its optical spectrum shows strong forbidden-line emission while lacking broad permitted lines, which is not typical for a blazar. Instead, the spectrum resembles those of narrow emission-line galaxies. Aims. We investigate changes in the object's radio-to-GeV spectral energy distribution (SED) during and after the prominent gamma-ray flare with the aim of determining the nature of the object and of constraining the origin of the variable high-energy emission. Methods. The data collected by the Fermi and AGILE satellites at gamma-ray energies; Swift at X-ray and ultraviolet (UV); the Kanata, NOT, and Keck telescopes at optical; OAGH and WISE at infrared (IR); and IRAM 30m, OVRO 40m, Effelsberg 100 m, RATAN-600, and VLBA at radio are analyzed together to trace the SED evolution on timescales of months. Results. The gamma-ray/ radio-loud narrow-line active galactic nucleus (AGN) is located at redshift z = 0.638. It shines through an unrelated foreground galaxy at z = 0.500. The AGN light is probably amplified by gravitational lensing. The AGN SED shows a two-humped structure typical of blazars and gamma-ray-loud narrow-line Seyfert 1 galaxies, with the high-energy (inverse-Compton) emission dominating by more than an order of magnitude over the low-energy (synchrotron) emission during gamma-ray flares. The difference between the two SED humps is smaller during the low-activity state. Fermi observations reveal a strong correlation between the gamma-ray flux and spectral index, with the hardest spectrum observed during the brightest gamma-ray state. The gamma-ray flares occurred before and during a slow rising trend in the radio, but no direct association between gamma-ray and radio flares could be established. Conclusions. If the gamma-ray flux is a mixture of synchrotron self-Compton and external Compton emission, the observed GeV spectral variability may result from varying relative contributions of these two emission components. This explanation fits the observed changes in the overall IR to gamma-ray SED.

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