The first gravitational-wave event from the merger of a binary neutron star system ( GW170817 ) was detected recently .
The associated short gamma-ray burst ( GRB 170817A ) has a low isotropic luminosity ( \sim 10 ^ { 47 } erg s ^ { -1 } ) and a peak energy E _ { p } \sim 145 keV during the initial main emission between -0.3 and 0.4 s. The origin of this short GRB is still under debate , but a plausible interpretation is that it is due to the off-axis emission from a structured jet .
We consider two possibilities .
First , since the best-fit spectral model for the main pulse of GRB 170817A is a cutoff power law with a hard low-energy photon index ( \alpha = -0.62 _ { -0.54 } ^ { +0.49 } ) , we consider an off-axis photosphere model .
We develop a theory of photosphere emission in a structured jet and find that such a model can reproduce a low-energy photon index that is softer than a blackbody through enhancing high-latitude emission .
The model can naturally account for the observed spectrum .
The best-fit Lorentz factor along the line of sight is \sim 20 , which demands that there is a significant delay between the merger and jet launching .
Alternatively , we consider that the emission is produced via synchrotron radiation in an optically thin region in an expanding jet with decreasing magnetic fields .
This model does not require a delay of jet launching but demands a larger bulk Lorentz factor along the line of sight .
We perform Markov Chain Monte Carlo fitting to the data within the framework of both models and obtain good fitting results in both cases .