Photosphere emission from a hybrid relativistic outflow with arbitrary dimensionless entropy and magnetization in GRBs

TL;DR

This paper develops a comprehensive theory of photosphere emission in hybrid relativistic outflows of GRBs, linking observed spectra to central engine parameters and considering magnetic dissipation effects.

Contribution

It introduces a dual approach to predict and diagnose GRB photosphere emission based on outflow parameters, incorporating magnetic dissipation and applying to real GRB data.

Findings

Variety of GRB spectra can be explained by varying outflow parameters.

Most GRBs require significant magnetization at the photosphere.

Magnetic reconnection likely powers non-thermal emission.

Abstract

In view of the recent Fermi observations of GRB prompt emission spectra, we develop a theory of photosphere emission of a hybrid relativistic outflow with a hot fireball component (defined by dimensionless entropy $\eta$) and a cold Poynting-flux component (defined by magnetization $\sigma_0$ at the central engine). We consider the scenarios both without and with sub-photospheric magnetic dissipations. Based on a simplified toy model of jet dynamics, we develop two approaches: a "bottom-up" approach to predict the temperature (for a non-dissipative photosphere) and luminosity of the photosphere emission and its relative brightness for a given pair of $(\eta,\sigma_0)$; and a "top-down" approach to diagnose central engine parameters ($\eta$ and $\sigma_0$) based on the observed quasi-thermal photosphere emission properties. We show that a variety of observed GRB prompt emission spectra with different degrees of photosphere thermal emission can be reproduced by varying $\eta$ and $\sigma_0$ within the non-dissipative photosphere scenario. In order to reproduce the observed spectra, the outflows of most GRBs need to have a significant $\sigma$, both at the central engine, and at the photosphere. The $\sigma$ value at $10^{15}$ cm from the central engine (a possible non-thermal emission site) is usually also greater than unity, so that internal-collision-induced magnetic reconnection and turbulence (ICMART) may be the mechanism to power the non-thermal emission. We apply our top-down approach to GRB 110721A, and find that the temporal evolution behavior of its blackbody component can be well interpreted with a time-varying $(\eta,\sigma_0)$ at the central engine, instead of invoking a varying engine base size $r_0$ as proposed by previous authors.