PHOTOSPHERE EMISSION FROM A HYBRID RELATIVISTIC OUTFLOW WITH ARBITRARY DIMENSIONLESS ENTROPY AND MAGNETIZATION IN GRBs

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 η) and a cold Poyntingflux component (defined by magnetization σ 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 (η, σ 0 ); and a "top-down" approach to diagnose central engine parameters (η and σ 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 η and σ 0 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 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 (η, σ 0 ) at the central engine, instead of invoking a varying engine base size r 0 as proposed by previous authors. 6-7 orders of magnitude in energy, allowing a close investigation of various spectral components in the GRB prompt emission spectra. The first bright LAT GRB, GRB 080916C, showed nearly featureless Band spectra in 5 different time bins over 6-7 orders of magnitude in energy (Abdo et al.

Lorentz factor — Beaming corrected energy/luminosity correlations and GRB central engine models

“…The Γ min value is significantly lower than the observations even for the fastest spinning BH. This suggests that the energy dissipation region is far above the slow acceleration radius (Gao & Zhang 2015), which is consistent with having significant magnetic dissipation at a large emission radius, as is expected in the ICMART model (Zhang & Yan 2011), and the requirement to interpret GRB spectra within the fast-cooling synchrotron radiation model (Uhm & Zhang 2014).…”

“…2 )) and Γ max = µ 0 , which correspond to the beginning and the end of the slow acceleration phase in a hybrid outflow, respectively (see Gao & Zhang 2015 for a detailed discussion of the acceleration dynamics of an arbitrarily magnetized relativistic or hybrid jet).…”

Multi-messenger Light Curves from Gamma-Ray Bursts in the Internal Shock Model

“…For example, they can explain the high polarization observed in some GRBs (Gotz et al 2009;Yonetoku et al 2012) and the absence of bright thermal emission (Zhang & Pe'er 2009;Gao & Zhang 2015).…”

Energy dissipation below gamma-ray bursts photosphere: Evidence and spectral fits

“…First, the distance to the GRB needs to be known; and second, the observed thermal component should not be strongly distorted, e.g., by sub-photospheric dissipation. A third condition is that the magnetization parameter is small, so that the magnetic field is dynamically unimportant 19,20 . If this is not the case, the dynamical variables are coupled (see 20 for full treatment).…”