Abstract
In view of the recent Fermi observations of gamma-ray burst (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 η) and a cold Poynting-flux component (defined by magnetization σ0 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 (η, σ0) and a "top-down" approach to diagnose central engine parameters (η and σ0) 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 η and σ0 within the non-dissipative photosphere scenario. In order to reproduce the observed spectra, the outflows of most GRBs need to have a significant σ, both at the central engine and at the photosphere. The σ value at 1015 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 (η, σ0) at the central engine, instead of invoking a varying engine base size r 0 as proposed by previous authors.
- Publication:
- The Astrophysical Journal, Volume 801, Issue 2, article id. 103,
16 pp. (2015). - Pub Date:
- March 2015
- DOI:
- 10.1088/0004-637X/801/2/103
- arXiv:
- arXiv:1409.3584
- Bibcode:
- 2015ApJ...801..103G
- Keywords:
- gamma-ray burst: general;Astrophysics - High Energy Astrophysical Phenomena