Emission from accelerating jets in gamma-ray bursts: Radiation dominated flows with increasing mass outflow rates

TL;DR

This paper models the narrowest gamma-ray burst spectra from radiation-dominated, accelerating jets, and reanalyzes specific Fermi observations to reveal jet dynamics and transitions between acceleration and coasting phases.

Contribution

It provides an analytical spectrum model for radiation-dominated, accelerating flows and applies it to real GRB data to infer jet properties and phase transitions.

Findings

Spectra are consistent with photospheres near the saturation radius.

Transition from acceleration to coasting phase correlates with spectral and temperature changes.

Jets are initially radiation dominated, as evidenced by spectral analysis.

Abstract

We study the narrowest spectra expected from gamma-ray bursts. We present an analytical function for the spectrum that is emitted from the photosphere of a radiation-dominated flow that is under acceleration. This is the narrowest possible spectrum and it differs from a Planck function. We also present numerical spectra from photospheres occurring during the transition into the coasting phase of the flow. Using these spectral models, we reanalyse Fermi observations of GRB100507 and GRB101219, which both have been reported to have very narrow spectra. The bursts can be fitted by the spectral models: For GRB101219 the spectrum is consistent with the photosphere occurring below or close to the saturation radius, while for GRB100507 the photosphere position relative to the saturation radius can be determined as a function of time. In the latter case, we find that the photosphere initially occurs in the acceleration phase and thereafter transitions into the coasting phase. We also find that this transition occurs at the same time as the change in observed cooling behaviour: the temperature is close to constant before the break and decays after. We argue that such a transition can be explained by an increasing mass outflow rate. Both analysed bursts thus give strong evidence that the jets are (initially) radiation dominated.