Low-energy Spectra of Gamma-Ray Bursts from Cooling Electrons

Abstract

The low-energy spectra of gamma-ray bursts' (GRBs) prompt emission are closely related to the energy distribution of electrons, which is further regulated by their cooling processes. We develop a numerical code to calculate the evolution of the electron distribution with given initial parameters, in which three cooling processes (i.e., adiabatic, synchrotron and inverse Compton cooling) and the effect of decaying magnetic field are coherently considered. A sequence of results are presented by exploring the plausible parameter space for both the fireball and the Poynting-flux-dominated regime. Different cooling patterns for the electrons can be identified and they are featured by a specific dominant cooling mechanism. Our results show that the hardening of the low-energy spectra can be attributed to the dominance of synchrotron self-Compton cooling within the internal shock model, or to decaying synchrotron cooling within the Poynting-flux-dominated jet scenario. These two mechanisms can be distinguished by observing the hard low-energy spectra of isolated short pulses in some GRBs. The dominance of adiabatic cooling can also lead to hard low-energy spectra when the ejecta is moving at an extreme relativistic speed. The information from the time-resolved low-energy spectra can help to probe the physical characteristics of the GRB ejecta via our numerical results.

“Double-tracking” Characteristics of the Spectral Evolution of GRB 131231A: Synchrotron Origin?

“…The increase of γ e,ch is consistent with the particle-in-cell simulations (e.g., Werner & Uzdensky 2017;Petropoulou & Sironi 2018). Such an increase would enhance synchrotron self-Compton cooling of electrons (see Equation 27 in Geng et al 2018). On the other hand, the increasing flux intensity 5 indicates that the ratio of the radiation energy density to the magnetic energy density is rising, which also supports that the synchrotron self-Compton cooling for electrons is getting more significant.…”

“…The resulting photon spectrum should also experience an evolution from fast-cooling-like to slow-coolinglike. Another way to harden α is to introduce the transition from synchrotron cooling to synchrotron "Double-tracking" Characteristic of the Spectral Evolution of GRB 131231A 7 self-Compton cooling in the Klein-Nishina regime for the electrons (Bošnjak et al 2009;Daigne et al 2011;Geng et al 2018). Since E p increases in the rising phase, the characteristic Lorentz factor γ e,ch of emitting electrons should be increasing with R when the magnetic field is decaying.…”

GRB Polarization: A Unique Probe of GRB Physics

“…Therefore, studying the polarization properties of X-ray flares may provide new insights both for their origin, as well as on the emission and/or dissipation mechanisms that are common with the prompt emission. There are some theoretical predictions for their polarization properties e.g., [251,252], but there is still much room for more detailed and realistic predictions that could be tested against future observations. Their observed similarities to prompt GRB pulses suggests that many of the models for prompt GRB polarization may be generalized to apply also for X-ray flares.…”

Electromagnetic counterparts of compact binary mergers

“…Liang et al 1997;Blinnikov, Kozyreva & Panchenko 1999;Rees & Mészáros 2005;Giannios 2006;Pe'er 2008;Beloborodov 2010;Chhotray & Lazzati 2015;Vurm & Beloborodov 2016;Bhattacharya & Kumar 2020). For the second class of models (studies that consider synchrotron radiation above the photosphere), effects producing a hardening of the low-energy spectral index have been invoked, such as (1) effect of an energy dependent inverse Compton radiation (Klein-Nishina regime) (Derishev, Kocharovsky & Kocharovsky 2001), or self-absorption effect (Lloyd & Petrosian 2000), (2) effects of the magnetic field profile within the emitting region (Uhm & Zhang 2014), (3) the non-uniform small-scale magnetic fields (Medvedev 2000), (4) adiabatic cooling of electrons (Geng et al 2018;Panaitescu 2019), (5) reacceleration and slow or balanced heating of the cooling electrons (Kumar & McMahon 2008;Asano & Terasawa 2009;Beniamini, Barniol Duran & Giannios 2018;Xu, Yang & Zhang 2018) (6) supercritical regime of hadronic cascades, i.e. a rapid energy conversion from protons to pions and muons triggered by the γ -ray photons (Petropoulou et al 2014).…”