The Origin of the Prompt Emission for Short GRB 170817A: Photosphere Emission or Synchrotron Emission?

TL;DR

This paper investigates whether the prompt emission of GRB 170817A originated from photosphere emission in a structured jet or from synchrotron radiation in an expanding jet, using theoretical modeling and data fitting.

Contribution

It develops a structured jet photosphere emission model and compares it with a synchrotron emission model to explain the GRB's spectrum, providing insights into the jet physics.

Findings

Photosphere model can reproduce the observed spectrum with a Lorentz factor of ~20.

Synchrotron model fits the data well with a larger Lorentz factor.

Both models are consistent with the observed spectral features.

Abstract

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.