Observations, theory and implications of thermal emission from gamma-ray bursts

TL;DR

This paper provides evidence for thermal emission in gamma-ray bursts, explains its origin through photospheric emission models, and discusses implications for jet properties and composition.

Contribution

It demonstrates the existence of thermal components in GRB prompt emission and links them to photospheric emission models, offering new ways to measure jet parameters.

Findings

Thermal flux decays as t^{-2} at late times.

Observed temperature decreases as t^{-α} with α ~ 0.5-0.67.

Thermal emission relates to high-energy non-thermal spectra.

Abstract

Recent analyses show evidence for a thermal emission component that accompanies the non-thermal emission during the prompt phase of GRBs. First, we show the evidence for the existence of this component; Second, we show that this component is naturally explained by considering emission from the photosphere, taking into account high latitude emission from optically thick relativistically expanding plasma. We show that the thermal flux is expected to decay at late times as F_BB ~ t^{-2}, and the observed temperature as T ~ t^{-\alpha}, with \alpha ~ 1/2 - 2/3. These theoretical predictions are in very good agreement with the observations. Finally, we discuss three implications of this interpretation: (a) The relation between thermal emission and high energy, non-thermal spectra observed by Fermi. (b) We show how thermal emission can be used to directly measure the Lorentz factor of the flow and the initial radius of the jet. (c) We show how the lack of detection of the thermal component can be used to constrain the composition of GRB jets.