TIME-RESOLVED ANALYSIS OFFERMIGAMMA-RAY BURSTS WITH FAST- AND SLOW-COOLED SYNCHROTRON PHOTON MODELS

Abstract

Time-resolved spectroscopy is performed on eight bright, long gamma-ray bursts (GRBs) dominated by single emission pulses that were observed with the Fermi Gamma-ray Space Telescope. Fitting the prompt radiation of GRBs by empirical spectral forms such as the Band function leads to ambiguous conclusions about the physical model for the prompt radiation. Moreover, the Band function is often inadequate to fit the data. The GRB spectrum is therefore modeled with two emission components consisting of optically thin non-thermal synchrotron radiation from relativistic electrons and, when significant, thermal emission from a jet photosphere, which is represented by a blackbody spectrum. To produce an acceptable fit, the addition of a blackbody component is required in 5 out of the 8 cases. We also find that the low-energy spectral index α is consistent with a synchrotron component with α = −0.81 ± 0.1. This value lies between the limiting values of α = −2/3 and α = −3/2 for electrons in the slow and fast-cooling regimes, respectively, suggesting ongoing acceleration at the emission site. The blackbody component can be more significant when using a physical synchrotron model instead of the Band function, illustrating that the Band function does not serve as a good proxy for a non-thermal synchrotron emission component. The temperature and characteristic emission-region size of the blackbody component are found to, respectively, decrease and increase as power laws with time during the prompt phase. In addition, we find that the blackbody and non-thermal components have separate temporal behaviors as far as their respective flux and spectral evolutions.

The rest-frame Golenetskii correlation via a hierarchical Bayesian analysis

“…The choice of spectral model also plays an crucial role in determining the slope and normalization of GCs. The results of Burgess et al (2014); Iyyani et al (2015) where a physical synchrotron model was used to fit the spectra of the same GRBs used in this study produced GCs with different slopes than what is found here with empirical spectral models. This is due to the different curvature of these models around the νFν peak which results in different E peak 's from the spectral fits.…”

“…It has been claimed that the use of multi-component spectra tighten the observed GC in some GRBs (Guiriec et al 2015). I therefore use the time-resolved luminosity and E peak from Band function (Band et al 1993) fits of the sample of GRBs analyzed in Burgess et al (2014). These GRBs are single pulsed in their lightcurves and were analyzed with physical synchrotron models as well as the empirical Band function.…”

“…A key feature of gamma-ray bursts (GRBs) is the observed relation within a burst between the luminosity and the νFν peak energy (E rest peak ) (Golenetskii et al 1983;Kargatis 1994;Borgonovo & Ryde 2001;Ryde et al 2006;Ghirlanda et al 2010;Lu et al 2010;Zhang & Chen 2012;Guiriec et al 2013;Burgess et al 2014). This luminosity-E rest peak relation was first discovered by Golenetskii et al (1983) and has been found in several GRBs regardless of their lightcurve shape.…”

“…Borgonovo & Ryde (2001); Ryde et al (2006) used single-and multi-component spectral models composed of thermal and non-thermal components to analyze several GRBs and obtain their GCs. It was found that the GCs of these bursts exhibit a wide range of γ (see also Burgess et al 2014) and it is difficult to assign a common physical setting to all GRBs: it is known that photospheric emission can take on several forms due to the differences in viewing angle (Lundman et al 2014) as well as subphotospheric dissipation (e.g. Pe'er et al 2005;Beloborodov 2010) or jet composition (Giannios 2006;.…”

Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A

“…The observed emission can be entirely from the photosphere wherein the broadness is contributed by the continuous or localised subphotospheric dissipation of the kinetic energy or Poynting flux (Beloborodov 2010(Beloborodov , 2013Giannios 2012;Pe'er, Mészáros & Rees 2005;Iyyani et al 2015), or may be due to the geometrical effects of the jet emission and its structure (Lundman, Pe'er & Ryde 2013;Beloborodov 2011;Basak & Rao 2015a). Or, the photospheric emission can be a sub-dominant component (Burgess et al 2014;Axelsson et al 2012;Iyyani et al 2013;Guiriec et al 2011Guiriec et al , 2013. All the models have certain pros and cons and confront issues in justifying the constraints obtained from observations.…”