The spectrum of Gamma-ray Burst: a clue

TL;DR

This paper investigates the thermal radiation efficiency of baryonic outflows in gamma-ray bursts (GRBs), challenges the standard internal shock model, and explores magnetic outflow models and particle acceleration mechanisms to explain observed spectra.

Contribution

It provides numerical calculations of thermal radiation efficiency considering outflow acceleration and evaluates the viability of magnetic outflow models against observational data.

Findings

Thermal radiation is too strong in some bright GRBs, conflicting with internal shock model predictions.

Magnetic outflow models struggle to reproduce the typical GRB spectrum.

Particle acceleration during magnetic dissipation may explain the low energy spectral puzzle.

Abstract

In this work we numerically calculate the thermal radiation efficiency of the baryonic outflow. The possible outflow acceleration in the transparent stage, which lowers thermal radiation efficiency, has been taken into account. In the standard internal shock model for the prompt emission, the fast shells should move with a typical Lorentz factor $\gtrsim 5 \Gamma_{\rm i}$ otherwise the GRB efficiency will be in disagreement with the observations, where $\Gamma_{\rm i}$ is the bulk Lorentz factor of the shocked/emitting region. The photosphere radius of these fast shells is small and the thermal radiation is too strong to be effectively outshone by the internal shock emission. This is particularly the case for some extremely bright events having $\Gamma_{\rm i} \sim 10^{3}$, like GRBs 080319B and 080916C. The absence of a distinct thermal component in the spectrum of most GRBs challenges the standard internal shock model and may suggest a non-baryonic (magnetic) outflow component. Though the magnetic outflow model seems favored by more and more data, it can hardly reproduce the typical GRB spectrum. In the photosphere-gradual magnetic dissipation scenario, the spectrum cuts off at $\sim 1$ GeV, too low to account for the observations of GRBs 080916C. In the sudden magnetic energy dissipation model, the low energy spectrum is expected to be $F_\nu \propto \nu^{-1/2}$, too soft to be consistent with the data $F_\nu \propto \nu^{0}$. We speculate that the low energy spectrum puzzle could be unveiled by the mechanism that particles, in the magnetic dissipation process, are repeatedly accelerated.