http://arxiv.org/abs/1409.3584
Recent broad-band observations of GRBs with the Fermi satellite call for a “hybrid” central engine, with both a hot “fireball” component and a cold “Poynting flux” component. We develop a theory of photosphere emission from such a hybrid relativistic outflow with an arbitrary dimensionless entropy $\eta$ and magnetization $\sigma_0$ at the central engine. We develop two approaches: a “bottom-up” approach to predict the temperature and luminosity of the photosphere emission and its relative brightness with respect to the non-thermal emission component from an optically thin region; and a “top-down” approach to diagnose central engine parameters based on the observed photosphere emission properties. For both approaches, we consider two possibilities: one is that the Poynting flux does not suffer significant dissipation beneath the photosphere, while the other is that there is such dissipation. From our bottom-up approach, we show that a variety of observed GRB prompt emission spectra with different degrees of photosphere emission can be reproduced. 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 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 the time-resolved spectral data of GRB 110721A, and find that the temporal behavior of the blackbody component in its time-resolved spectrum can be well interpreted with a time-varying $(\eta,\sigma_0)$ at the central engine, instead of invoking a varying engine base size $r_0$. (abridged)
H. Gao and B. Zhang
Mon, 15 Sep 14
44/47
Comments: 5 figures, 2 tables
You must be logged in to post a comment.