Speaker
Description
The photospheric emission in the prompt emission is the natural prediction
of the original fireball model for gamma-ray burst (GRB) due to the large
optical depth ($\tau >1$) at the base of the outflow, which is supported by
the quasi-thermal components found in several Fermi GRBs. However,
the main origin of the most prompt emission spectrum (photosphere or
synchrotron) is still under hot debate. To explore this problem, the shape
of the observed photosphere emission spectrum from a pure hot fireball or a
Poynting-flux-dominated outflow has been investigated. In this work, we
further study that for the hybrid outflow containing a thermal component and
a magnetic component with moderate magnetization ($\sigma _{0}=L_{P}/L_{%
\text{Th}}\sim 2-9$), by invoking the probability photosphere model. It is
interesting to find that the high-energy spectrum is a power-law rather than
an exponential cutoff, compatible with the observed Band function in large
amounts of GRBs. Also, the distribution of the low-energy indices
(corresponding to the peak-flux spectra) is found to be quite consistent
with the statistical result for the peak-flux spectra of GRBs best-fitted by
the Band function, with the similar angular profiles of the dimensionless
entropy $\eta $ as those of the unmagnetized jet considered in our previous
works. Finally, the observed distribution of the high-energy indices can be
well understood after considering the different magnetic acceleration (due
to magnetic reconnection and kink instability) and the angular profiles of $%
\eta $ with the narrower core.
