GRB170817A associated with GW170817: multifrequency observations and modeling of prompt gamma-ray emission

TL;DR

This paper reports multifrequency observations of GW170817/GRB 170817A, revealing unique prompt gamma-ray components and proposing a model involving neutron star merger, jet breakout, and shock heating to explain the emission features.

Contribution

It introduces a detailed model for the prompt gamma-ray emission of GRB 170817A, linking it to neutron star merger dynamics and jet-wind interactions, with new observational insights.

Findings

Detection of optical transient in NGC 4993

Identification of two distinct gamma-ray components

Model linking gamma-ray features to jet breakout and shock heating

Abstract

We present our observations of electromagnetic transients associated with GW170817/GRB 170817A using optical telescopes of Chilescope observatory and Big Scanning Antenna (BSA) of Pushchino Radio Astronomy Observatory at 110MHz. The Chilescope observatory detected an optical transient of $\sim19^m$ on the third day in the outskirts of the galaxy NGC 4993; we continued observations following its rapid decrease. We put an upper limit of $1.5\times10^{4}$ Jy on any radio source with a duration of 10-60 s which may be associated with GW170817/GRB 170817A. The prompt gamma-ray emission consists of two distinctive components - a hard short pulse delayed by $\sim2$ seconds with respect to the LIGO signal and softer thermal pulse with $T\sim 10 $ keV lasting for another $\sim2$ seconds. The appearance of a thermal component at the end of the burst is unusual for short GRBs. Both the hard and the soft components do not satisfy the Amati relation, making GRB 170817A distinctively different from other short GRBs. Based on gamma-ray and optical observations, we develop a model for the prompt high-energy emission associated with GRB 170817A. The merger of two neutron stars creates an accretion torus of $\sim10^{-2} M_\odot$, which supplies the black hole with magnetic flux and confines the Blandford-Znajek-powered jet. We associate the hard prompt spike with the quasispherical breakout of the jet from the disk wind. As the jet plows through the wind with subrelativistic velocity, it creates a radiation-dominated shock that heats the wind material to tens of kiloelectron volts, producing the soft thermal component.