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

TL;DR

This paper develops a numerical model to study how different electron cooling processes influence the low-energy spectra of gamma-ray bursts, providing insights into the physical conditions of GRB jets.

Contribution

It introduces a comprehensive numerical code that considers multiple cooling mechanisms and magnetic field decay to analyze electron energy evolution in GRBs.

Findings

Synchrotron self-Compton cooling can cause spectral hardening.

Decaying synchrotron cooling explains hard spectra in Poynting-flux-dominated jets.

Adiabatic cooling can produce hard spectra at high relativistic speeds.

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.