The Anatomy of a Long Gamma-Ray Burst: A Simple Classification Scheme for the Emission Mechanism(s)

Abstract

Ultra-relativistic motion and efficient conversion of kinetic energy to radiation are required by gamma-ray burst (GRB) observations, yet they are difficult to simultaneously achieve. Three leading mechanisms have been proposed to explain the observed emission emanating from GRB outflows: radiation from either relativistic internal or external shocks, or thermal emission from a photosphere. Previous works were dedicated to independently treating these three mechanisms and arguing for a sole, unique origin of the prompt emission of GRBs. In contrast, herein, we first explain why all three models are valid mechanisms and that a contribution from each of them is expected in the prompt phase. Additionally, we show that a single parameter, the dimensionless entropy of the GRB outflow, determines which mechanism contributes the most to the emission. More specifically, internal shocks dominate for low values of the dimensionless entropy, external shocks for intermediate values, and finally, photospheric emission for large values. We present a unified framework for the emission mechanisms of GRBs with easily testable predictions for each process.

Science prospects for SPHiNX – A small satellite GRB polarimetry mission

“…Different mechanisms can dominate the GRB prompt phase emission. Depending primarily on the dimen-sionless entropy and the magnetisation of the outflow, the emission can stem from the photosphere, internal shocks, magnetic reconnection, or the external shock [26,27]. The high-energy emission is thus typically attributed to synchrotron radiation [28,29] or emission from the photosphere [30,31,32].…”

In-orbit Background and Sky Survey Simulation Study of POLAR-2/LPD

“…Satellites like Swift (Burrows et al 2005) and Fermi (Atwood et al 2009) have significantly advanced our understanding of GRBs by providing valuable observations of their energy spectra and timing properties, both during the prompt emission phase and the subsequent afterglow. Despite these advancements, several fundamental questions regarding the nature of GRBs remain unanswered, including the properties of the central engine, the mechanisms driving the energetic jets, the composition of the jets, the processes responsible for energy dissipation, the configurations of magnetic fields, and the mechanisms behind particle acceleration and radiation (Rees & Meszaros 1994;Zhang & Mészáros 2002;Narayan & Kumar 2009;Zalamea & Beloborodov 2011;Lundman et al 2012;Kumar & Zhang 2015;Bégué & Burgess 2016).…”

A comprehensive statistical study on gamma-ray bursts

“…Recently, the spectra observations of GRBs show a mixture of thermal and non-thermal spectra (Ryde 2005;Ryde & Pe'er 2009;Guiriec et al 2010;Nappo et al 2017), which implies that there is an interplay between different emission mechanisms. The exist of GRBs subgroups might be the consequence of different emission mechanisms (Bégué & Burgess 2016). Acuner & Ryde (2017) searched the full Fermi /GBM catalogue using Gaussian Mixture Models to cluster bursts according to their low energy photon index of Band model (Band et al 1993) (α band ), high energy photon index of Band model (β band ), spectral peak energy of Band model (E p,band ), fluence (F g ) and duration of 5% to 95% γ-ray fluence (T 90 ), in order to divide bursts into photospheric origin and synchrotron origin.…”