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  • 1.
    Axelsson, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Engdegård, Olle
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ryde, Felix
    Stockholm University, Faculty of Science, Department of Astronomy.
    Larsson, Stefan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pearce, Mark
    Hjalmarsdotter, Linnea
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiss, Mozsi
    Marini Bettolo, Cecilia
    Arimoto, M
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlson, P
    Fukazawa, Y
    Kamae, T
    Kanai, Y
    Kataoka, J
    Kawai, N
    Klamra, W
    Madejski, G
    Mizuno, T
    Ng, J
    Tajima, H
    Takahashi, T
    Tanaka, T
    Ueno, M
    Varner, G
    Yamamoto, K
    Measuring energy dependent polarization in soft γ-rays using Compton scattering in PoGOLite2007In: Astroparticle Physics, Vol. 28, no 3, p. 327-337Article in journal (Refereed)
    Abstract [en]

    Linear polarization in X- and γ-rays is an important diagnostic of many astrophysical sources, foremost giving information about their geometry, magnetic fields, and radiation mechanisms. However, very few X-ray polarization measurements have been made, and then only mono-energetic detections, whilst several objects are assumed to have energy dependent polarization signatures. In this paper, we investigate whether detection of energy dependent polarization from cosmic sources is possible using the Compton technique, in particular with the proposed PoGOLite balloon-experiment, in the 25–100 keV range. We use Geant4 simulations of a PoGOLite model and input photon spectra based on Cygnus X-1 and accreting magnetic pulsars (100 mCrab). Effective observing times of 6 and 35 h were simulated, corresponding to a standard and a long duration flight, respectively. Both smooth and sharp energy variations of the polarization are investigated and compared to constant polarization signals using chi-square statistics. We can reject constant polarization, with energy, for the Cygnus X-1 spectrum (in the hard state), if the reflected component is assumed to be completely polarized, whereas the distinction cannot be made for weaker polarization. For the accreting pulsar, constant polarization can be rejected in the case of polarization in a narrow energy band with at least 50% polarization, and similarly for a negative step distribution from 30% to 0% polarization.

  • 2.
    Björnsson, C. -I.
    Stockholm University, Faculty of Science, Department of Astronomy.
    JOINING RADIO WITH X-RAYS: A REVISED MODEL FOR SN 1993J2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 813, no 1, article id 43Article in journal (Refereed)
    Abstract [en]

    A joint analysis is done of the radio and X-ray observations of SN 1993J. It is argued that neither synchrotron cooling behind the forward shock nor thermal cooling behind the reverse shock is supported by observations. In order for adiabatic models to be consistent, a reinterpretation of the radius of the spatially resolved very long baseline interferometry-source (VLBI) is needed during the first few hundred days. Instead of reflecting the position of the forward shock, it is then associated with the expansion of the Rayleigh-Taylor unstable region emanating from the contact discontinuity. Although observations imply a constant ratio between the energy densities in magnetic fields and relativistic electrons, they do not appear to scale individually with the thermal energy density behind the forward shock; rather, in adiabatic models, the evolution of the magnetic field strength is best understood as scaling inversely with the supernova radius.

  • 3.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    GRB 060218: THE NATURE OF THE OPTICAL-UV COMPONENT2008In: The Astrophysical Journal, Vol. 672Article in journal (Refereed)
    Abstract [en]

    The optical-UV component in GRB 060218 is assumed to be due to optically thick cyclotron emission. The key aspect of this model is the high temperature of the absorbing electrons. The heat input derives from nuclei accelerated in semirelativistic internal shocks, like in ordinary gamma-ray bursts. Coulomb collisions transfer part of that energy to electrons. Inverse Compton cooling on the X-ray photons leads to electron temperatures around ~100 keV. Such a high brightness temperature for the optical-UV emission implies an emitting area roughly equal to that of the thermal X-ray component. This suggests a model in which the radio, optical-UV, and thermal X-ray emission are closely related. Although the optical-UV and thermal X-ray emission are two separate spectral components, it is argued that they both come from the photosphere of a quasi-spherical, continuous outflow, whose interaction with the circumstellar medium gives rise to the radio emission. The properties of GRB 060218, as measured in the comoving frame, are similar to those of ordinary gamma-ray bursts; i.e., the main difference is the much lower value of the bulk Lorentz factor in GRB 060218. The cyclotron absorption implies a magnetic field in rough equipartition with the matter energy density in the outflow. Hence, the magnetic field could have a dynamically important role, possibly with a magnetar as the central engine.

  • 4.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    GRB 060218: The nature of the optical-UV component2007Report (Other academic)
    Abstract [en]

    The optical-UV component in GRB 060218 is assumed to be due to optically thick cyclotron emission. The key aspect of this model is the high temperature of the absorbing electrons. The heat input derives from nuclei accelerated in semi-relativistic internal shocks, like in ordinary gamma-ray bursts. Coulomb collisions transfer part of that energy to electrons. Inverse Compton cooling on the X-ray photons leads to electron temperatures around 100 keV. Such a high brightness temperature for the optical-UV emission implies an emitting area roughly equal to that of the thermal X-ray component. This suggests a model in which the radio, optical-UV and thermal X-ray emission are closely related: Although the optical-UV and thermal X-ray emission are two separate spectral components, it is argued that they both come from the photosphere of a quasi-spherical, continuous outflow, whose interaction with the circumstellar medium gives rise to the radio emission. The properties of GRB 060218, as measured in the co-moving frame, are similar to those of ordinary gamma-ray burst; i.e., the main difference is the much lower value of the bulk Lorentz factor in GRB 060218. The cyclotron absorption implies a magnetic field in rough equipartition with the matter energy density in the outflow. Hence, the magnetic field could have a dynamically important role, possibly with a magnetar as the central engine.

  • 5.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    INHOMOGENEITIES IN TYPE Ib/c SUPERNOVAE: AN INVERSE COMPTON SCATTERING ORIGIN OF THE X-RAY EMISSION2013In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 769, no 1, p. 65-Article in journal (Refereed)
    Abstract [en]

    Inhomogeneities in a synchrotron source can severely affect the conclusions drawn from observations regarding the source properties. However, their presence is not always easy to establish, since several other effects can give rise to similar observed characteristics. It is argued that the recently observed broadening of the radio spectra and/or light curves in some Type Ib/c supernovae is a direct indication of inhomogeneities. As compared to a homogeneous source, this increases the deduced velocity of the forward shock and the observed correlation between total energy and shock velocity could in part be due to a varying covering factor. The X-ray emission from at least some Type Ib/c supernovae is unlikely to be synchrotron radiation from an electron distribution accelerated in a nonlinear shock. Instead it is shown that the observed correlation during the first few hundred days between the radio, X-ray, and bolometric luminosities indicates that the X-ray emission is inverse Compton scattering of the photospheric photons. Inhomogeneities are consistent with equipartition between electrons and magnetic fields in the optically thin synchrotron emitting regions.

  • 6.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    MULTIPLE INVERSE COMPTON SCATTERINGS AND THE BLAZAR SEQUENCE2010In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 723, no 1, p. 417-424Article in journal (Refereed)
    Abstract [en]

    The high frequency component in blazars is thought to be due to inverse Compton scattered radiation. Recent observations by Fermi-LAT are used to evaluate the details of the scattering process. A comparison is made between the usually assumed single scattering scenario and one in which multiple scatterings are energetically important. In the latter case, most of the radiation is emitted in the Klein-Nishina limit. It is argued that several of the observed correlations defining the blazar sequence are most easily understood in a multiple scattering scenario. Observations indicate also that, in such a scenario, the blazar sequence is primarily governed by the energy density of relativistic electrons rather than that of the seed photons. The pronounced X-ray minimum in the spectral energy distribution often observed in the most luminous blazars is discussed. It is shown how this feature can be accounted for in a multiple scattering scenario by an extension of standard one-zone models.

  • 7.
    Björnsson, Claes-Ingvar
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Keshavarzi, S. T.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Inhomogeneities and the Modeling of Radio Supernovae2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 841, no 1, article id 12Article in journal (Refereed)
    Abstract [en]

    Observations of radio supernovae (SNe) often exhibit characteristics not readily accounted for by a homogeneous, spherically symmetric synchrotron model; e.g., flat-topped spectra/light. curves. It is shown that many of these deviations from the standard model can be attributed to an inhomogeneous source structure. When inhomogeneities are present, the deduced radius of the source and, hence, the shock velocity, is sensitive to the details of the modeling. As the inhomogeneities are likely to result from the same mechanism that amplify the magnetic field, a comparison between observations and the detailed numerical simulations now under way may prove mutually beneficial. It is argued that the radio emission in Type Ib/c SNe has a small volume filling factor and comes from a narrow region associated with the forward shock, while the radio emission region in SN 1993J (Type IIb) is determined by the extent of the Rayleigh-Taylor instability emanating from the contact discontinuity. Attention is also drawn to the similarities between radio SNe and the structural properties of SN remnants.

  • 8.
    Björnsson, Claes-Ingvar
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    HEATING FROM FREE-FREE ABSORPTION AND THE MASS-LOSS RATE OF THE PROGENITOR STARS TO SUPERNOVAE2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 787, no 2, p. 143-Article in journal (Refereed)
    Abstract [en]

    An accurate determination of the mass-loss rate of the progenitor stars to core-collapse supernovae is often limited by uncertainties pertaining to various model assumptions. It is shown that under conditions when the temperature of the circumstellar medium is set by heating due to free-free absorption, observations of the accompanying free-free optical depth allow a direct determination of the mass-loss rate from observed quantities in a rather model-independent way. The temperature is determined self-consistently, which results in a characteristic time dependence of the free-free optical depth. This can be used to distinguish free-free heating from other heating mechanisms. Since the importance of free-free heating is quite model dependent, this also makes possible several consistency checks of the deduced mass-loss rate. It is argued that the free-free absorption observed in SN 1993J is consistent with heating from free-free absorption. The deduced mass-loss rate of the progenitor star is, approximately, 10(-5) M-circle dot yr(-1) for a wind velocity of 10 km s(-1).

  • 9.
    Björnsson, Claes-Ingvar
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sandberg, Andreas
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy.
    The location of the Crab pulsar emission region: restrictions on synchrotron emission models2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 516, p. A65-Article in journal (Refereed)
    Abstract [en]

    Recent observations of the Crab pulsar show no evidence of a spectral break in the infrared regime. It is argued that the observations are consistent with a power-law spectrum in the whole observable infrared-optical range. This is taken as the starting point for evaluating of how self-consistent incoherent synchrotron models fare in a comparison with observations. Inclusion of synchrotron self-absorption proves important as does the restriction on the observed size of the emission region imposed by the relativistic beaming thought to define the pulse profile. It is shown that the observations can be used to derive two independent constraints on the distance from the neutron star to the emission region; in addition to a direct lower limit, an indirect measure is obtained from an upper limit to the magnetic field strength. Both of these limits indicate that the emission region is located at a distance considerably greater than the light cylinder radius. The implications of this result are discussed, and it is emphasized that, for standard incoherent synchrotron models to fit inside the light cylinder, rather special physical conditions need to be invoked.

  • 10.
    Borgonovo, Luis
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Statistical Analysis of BATSE Gamma-Ray Bursts: Self-Similarity and the Amati Relation2006In: The Astrophysical Journal, ISSN 0004-637X, Vol. 652, no 2, p. 1423-1435Article in journal (Refereed)
    Abstract [en]

    The statistical properties of a complete, flux-limited sample of 197 long gamma-ray bursts (GRBs) detected by BATSE are studied. In order to bring forth their main characteristics, care was taken to define a representative set of 10 parameters. A multivariate analysis gives that ~70% of the total variation in parameter values is driven by only three principal components. The variation of the temporal parameters is clearly distinct from that of the spectral ones. A close correlation is found between the half-width of the autocorrelation function (τ) and the emission time (T50) most importantly, this correlation is self-similar in the sense that the mean values and dispersions of both τ and T50 scale with the duration of the burst (T90). It is shown that the Amati relation can be derived from the sample and that the scatter around this relation is correlated with the value of τ. Hence, τ has a role similar to that of the break in the afterglow light curve (tb) in the Ghirlanda-relation. In the standard GRB-scenario, the close relation between a global parameter (tb) and a local one (τ) indicates that some of the jet-properties do not vary much for different lines of sight. Finally, it is argued that the basic temporal and spectral properties are associated with individual pulses, while the overall properties of a burst is determined mainly by the number of pulses.

  • 11. 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. 

  • 12. 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.

  • 13.
    Lundqvist, Natalia
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pires, S.
    Shibanov, Yu. A.
    Zyuzin, D. A.
    Spectral evolution and polarization of variable structures in the pulsar wind nebula of PSR B0540-69.32011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 413, no 1, p. 611-627Article in journal (Refereed)
    Abstract [en]

    We present high spatial resolution optical imaging and polarization observations of the PSR B0540-69.3 and its highly dynamical pulsar wind nebula (PWN) performed with Hubble Space Telescope, and compare them with X-ray data obtained with the Chandra X-ray Observatory. In particular, we have studied the bright region south-west of the pulsar where a bright 'blob' is seen in 1999. In a recent paper by De Luca et al. it was argued that the 'blob' moves away from the pulsar at high speed. We show that it may instead be a result of local energy deposition around 1999, and that the emission from this then faded away rather than moved outward. Polarization data from 2007 show that the polarization properties show dramatic spatial variations at the 1999 blob position arguing for a local process. Several other positions along the pulsar-'blob' orientation show similar changes in polarization, indicating previous recent local energy depositions. In X-rays, the spectrum steepens away from the 'blob' position, faster orthogonal to the pulsar-'blob' direction than along this axis of orientation. This could indicate that the pulsar-'blob' orientation is an axis along where energy in the PWN is mainly injected, and that this is then mediated to the filaments in the PWN by shocks. We highlight this by constructing an [S ii]-to-[O iii]-ratio map, and comparing this to optical continuum and X-ray emission maps. We argue, through modelling, that the high [S ii]/[O iii] ratio is not due to time-dependent photoionization caused by possible rapid X-ray emission variations in the 'blob' region. We have also created a multiwavelength energy spectrum for the 'blob' position showing that one can, to within 2 Sigma, connect the optical and X-ray emission by a single power law. The slope of that power law (defined from <file name=mnr_18159_mu1.gif type=gif/>) would be alpha(nu) = 0.74 +/- 0.03, which is marginally different from the X-ray spectral slope alone with alpha(nu) = 0.65 +/- 0.03. A single power law for most of the PWN is, however, not be possible. We obtain best power-law fits for the X-ray spectrum if we include 'extra' oxygen, in addition to the oxygen column density in the interstellar gas of the Large Magellanic Cloud and the Milky Way. This oxygen is most naturally explained by the oxygen-rich ejecta of the supernova remnant. The oxygen needed likely places the progenitor mass in the 20-25 M(circle dot) range, i.e. in the upper mass range for progenitors of Type IIP supernovae.

  • 14. Perez-Torres, M. A.
    et al.
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Beswick, R. J.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Muxlow, T. W. B.
    Paragi, Z.
    Ryder, S.
    Alberdi, A.
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Marcaide, J. M.
    Marti-Vidal, I.
    Ros, E.
    Argo, M. K.
    Guirado, J. C.
    CONSTRAINTS ON THE PROGENITOR SYSTEM AND THE ENVIRONS OF SN 2014J FROM DEEP RADIO OBSERVATIONS2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 792, no 1, p. 38-Article in journal (Refereed)
    Abstract [en]

    We report deep EVN and eMERLIN observations of the Type Ia SN 2014J in the nearby galaxy M82. Our observations represent, together with JVLA observations of SNe 2011fe and 2014J, the most sensitive radio studies of Type Ia SNe ever. By combining data and a proper modeling of the radio emission, we constrain the mass-loss rate from the progenitor system of SN 2014J to (M) over dot less than or similar to 7.0 x 10(-10) M yr(-1) (for a wind speed of 100 km s(-1)). If the medium around the supernova is uniform, then n(ISM) less than or similar to 1.3 cm(-3), which is the most stringent limit for the (uniform) density around a Type Ia SN. Our deep upper limits favor a double-degenerate (DD) scenario-involving two WD stars-for the progenitor system of SN 2014J, as such systems have less circumstellar gas than our upper limits. By contrast, most single-degenerate (SD) scenarios, i.e., the wide family of progenitor systems where a red giant, main-sequence, or sub-giant star donates mass to an exploding WD, are ruled out by our observations. (While completing our work, we noticed that a paper by Margutti et al. was submitted to The Astrophysical Journal. From a non-detection of X-ray emission from SN 2014J, the authors obtain limits of (M) over dot less than or similar to 1.2 x 10(-9) M-circle dot yr(-1) (for a wind speed of 100 km s(-1)) and n(ISM) less than or similar to 3.5 cm(-3), for the rho proportional to r(-2) wind and constant density cases, respectively. As these limits are less constraining than ours, the findings by Margutti et al. do not alter our conclusions. The X-ray results are, however, important to rule out free-free and synchrotron self-absorption as a reason for the radio non-detections.) Our estimates on the limits on the gas density surrounding SN2011fe, using the flux density limits from Chomiuk et al., agree well with their results. Although we discuss the possibilities of an SD scenario passing observational tests, as well as uncertainties in the modeling of the radio emission, the evidence from SNe 2011fe and 2014J points in the direction of a DD scenario for both.

  • 15.
    Ryde, Felix
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kaneko, Yuki
    Mészáros, Peter
    Preece, Robert
    Battelino, Milan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Gamma-Ray Burst Spectral Correlations: Photospheric and Injection Effects2006In: The Astrophysical Journal, ISSN 0004-637X, Vol. 652, no 2, p. 1400-1415Article in journal (Refereed)
    Abstract [en]

    We present a physical framework that can account for most of the observed spectral properties of prompt gamma-ray burst emission. This includes the variety of spectral shapes and shape evolutions, and spectral correlations between flux and spectral peaks within bursts, described by Borgonovo & Ryde, and among bursts described by Amati and Ghirlanda. In our proposed model the spectral peak is given by the photospheric emission from a relativistic outflow for which the horizon length is much smaller that the radial width. The observed duration of the thermal flash is given by the radial light-crossing time. This then gives the typical emission site at ~1011 cm with a Lorentz factor of ~300. This emission is accompanied by nonthermal emission from dissipation locations outside the photosphere. The relative strengths of these two components depend on injection effects at the central engine, leading to varying relative locations of the saturation and photospheric radii. The total emission can then reproduce the observed variety. The spectral correlations are found by assuming that the amount of energy dissipated depends nonlinearly on the averaged particle density. Besides the spectral correlations, this also gives a description of how the relative strength of the thermal component varies with temperature within a burst.

  • 16. Sandin, Christer
    et al.
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Lundqvist, Natallia
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Shibanov, Yuri A.
    Properties of the three-dimensional structure in the central region of the supernova remnant SNR 0540−69.32013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 432, no 4, p. 2854-2868Article in journal (Refereed)
    Abstract [en]

    We present and discuss new visual wavelength-range observations of the inner regions of the supernova remnant SNR 0540−69.3 that is located in the Large Magellanic Cloud (LMC). These observations provide us with more spatial and spectral information than were previously available for this object. We use these data to create a detailed three-dimensional model of the remnant, assuming linear expansion of the ejecta. With the observations and the model, we study the general three-dimensional structure of the remnant, and the influence of an active region in the remnant – a ‘blob’ – that we address in previous papers. We used the fibre-fed integral-field Visual Multi-Object Spectrograph at the Very Large Telescope of the European Southern Observatory. The observations provide us with three-dimensional data in [O iii] λ5007 and [S ii] λλ6717, 6731 at a 0.33 arcsec × 0.33 arcsec spatial sampling and a velocity resolution of about 35 km s− 1. We decomposed the two, partially overlapping, sulphur lines and used them to calculate electron densities across the remnant at a high signal-to-noise ratio. In our study, we recover results of previous studies, but we are more importantly able to obtain more detailed information than before. Our analysis reveals a structure that stretches from the position of the ‘blob’, and into the plane of the sky at a position angle of PA ≃ 60°. Assuming a remnant age of 1000 yr and the usual LMC distance, the structure has an inclination angle of about 65° to the line of sight. The position angle is close to the symmetry axis with present and past activity in the visual and the X-ray wavelength ranges. We speculate that the pulsar is positioned along this activity axis, where it has a velocity along the line of sight of a few hundred  km s− 1. The ‘blob’ is most likely a region of shock activity, as it is mainly bright in [S ii]; future observations of [O ii] λλ3726, 3729 would be useful to test whether the S/O abundance ratio is higher than average for that location in the remnant. The striking resemblance in X-rays between the pulsar wind nebula (PWN) of SNR 0540−69.3 and the Crab, in combination with our findings in this paper, suggests that the symmetry axis is part of a torus in the PWN. This is in agreement with the original suggestion by Gotthelf & Wang.

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