| 1. |
- Perez-Torres, M. A., et al.
(författare)
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CONSTRAINTS ON THE PROGENITOR SYSTEM AND THE ENVIRONS OF SN 2014J FROM DEEP RADIO OBSERVATIONS
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 792:1, s. 38-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Axelsson, Magnus, et al.
(författare)
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Measuring energy dependent polarization in soft γ-rays using Compton scattering in PoGOLite
- 2007
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Ingår i: Astroparticle Physics. - : Elsevier BV. - 0927-6505. ; 28:3, s. 327-337
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Björnsson, Claes-Ingvar
(författare)
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Circular Polarization in Compact Radio Sources : Constraints on Particle Acceleration and Electron-Positron Pairs
- 2019
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 873:1
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Tidskriftsartikel (refereegranskat)abstract
- It is shown that the frequency distribution of the degree of circular polarization for a homogeneous source is sensitive to the properties of the synchrotron emitting plasma. Most of the circular polarization comes from the region around the turnover frequency, where the synchrotron radiation becomes optically thick. However, nearly circular characteristic waves result in circular polarization dominated by frequencies above the turnover frequency, while in the case of nearly linear characteristic waves, it is dominated by frequencies below. Observations argue in favor of nearly circular characteristic waves. This implies a low-energy cutoff in the electron distribution that is substantially below that corresponding to the turnover frequency and simultaneously provides an upper limit to the fraction of electron-positron pairs.
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| 4. |
- Björnsson, Claes-Ingvar
(författare)
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GRB 060218: THE NATURE OF THE OPTICAL-UV COMPONENT
- 2008
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Ingår i: The Astrophysical Journal. ; 672
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Björnsson, Claes-Ingvar
(författare)
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GRB 060218: The nature of the optical-UV component
- 2007
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 6. |
- Björnsson, Claes-Ingvar, et al.
(författare)
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HEATING FROM FREE-FREE ABSORPTION AND THE MASS-LOSS RATE OF THE PROGENITOR STARS TO SUPERNOVAE
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 787:2, s. 143-
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Tidskriftsartikel (refereegranskat)abstract
- 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).
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| 7. |
- Björnsson, Claes-Ingvar
(författare)
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Implications of the Low-frequency Turnover in the Spectrum of Radio Knot C in DG Tau
- 2021
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 923:1
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Tidskriftsartikel (refereegranskat)abstract
- The synchrotron spectrum of radio knot C in the protostellar object DG Tau has a low-frequency turnover. This is used to show that its magnetic field strength is likely to be at least 10 mG, which is roughly two orders of magnitude larger than previously estimated. The earlier, lower value is due to an overestimate of the emission volume together with an omission of the dependence of the minimum magnetic field on the synchrotron spectral index. Since the source is partially resolved, this implies a low volume-filling factor for the synchrotron emission. It is argued that the high pressure needed to account for the observations is due to shocks. In addition, cooling of the thermal gas is probably necessary in order to further enhance the magnetic field strength as well as the density of relativistic electrons. It is suggested that the observed spectral index implies that the energy of the radio-emitting electrons is below that needed to take part in first-order Fermi acceleration. Hence, the radio emission gives insights to the properties of its pre-acceleration phase. Attention is also drawn to the similarities between the properties of radio knot C and the shock-induced radio emission in supernovae.
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| 8. |
- Björnsson, Claes-Ingvar, et al.
(författare)
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Inhomogeneities and the Modeling of Radio Supernovae
- 2017
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 841:1
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Björnsson, Claes-Ingvar
(författare)
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INHOMOGENEITIES IN TYPE Ib/c SUPERNOVAE : AN INVERSE COMPTON SCATTERING ORIGIN OF THE X-RAY EMISSION
- 2013
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 769:1, s. 65-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Björnsson, Claes-Ingvar
(författare)
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Modeling of Radio Supernovae : Including the Effects of Inhomogeneities and Radiative Cooling
- 2024
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 963:2
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Tidskriftsartikel (refereegranskat)abstract
- The presence of inhomogeneities in a spatially unresolved source is often hard to establish. This limits the accuracy with which the source properties can be determined. It is shown how observed features not expected for a homogeneous model can be used to infer the properties of the inhomogeneities in radio supernovae. Furthermore, the observed consequences of radiative cooling can be seriously affected by inhomogeneities. It is shown that the deduced source properties are very sensitive to the observed value of the cooling frequency; even a lower limit is often useful to constrain its characteristics. It is argued that the main synchrotron emission region in SN 2003L has a small volume filling factor, possibly as low as a few percent. On the contrary, deviations from homogeneity are substantially smaller in SN 2002ap. The observed properties of Type Ib/c radio supernovae in general indicate the volume filling factor to remain rather constant with time for individual sources, but those peaking later at radio frequencies have lower filling factors. The conditions in the main synchrotron component in both SN 2003L and SN 2002ap are consistent with equipartition of energy between relativistic electrons and magnetic fields.
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