GRB 170817A Associated with GW170817: Multi-frequency Observations and Modeling of Prompt Gamma-Ray Emission

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 110 MHz. The Chilescope observatory detected an optical transient of ∼ 19 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 × 10 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 ∼ 2 seconds with respect to the LIGO signal and softer thermal pulse with T ∼ 10 keV lasting for another ∼ 2 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 ∼ 10 −2 M , which supplies the black hole with magnetic flux and confines the BlandfordZnajek-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.

The Ep,i–Eiso correlation: type I gamma-ray bursts and the new classification method

“…It makes the problem of correct blind classification still very actual, especially in the new era of gravitational wave astronomy. The one and only confirmed gamma-ray burst for the moment, associated with gravitational waves, GRB 170817, is placed in the middle of the overlap region (Abbott et al 2017a,b;Goldstein et al 2017;Pozanenko et al 2018).…”

“…One more evident criterion can be added, the detection of kilonova emission for type I bursts (e.g. Jin et al 2016;Yang et al 2015;Tanaka 2016;Pozanenko et al 2018;Pandey et al 2019).…”

The 999thSwiftgamma-ray burst: Some like it thermal

“…From the residual, we noticed that a peaked component should be added to improve the fit. For this reason we refit the spectrum Pozanenko et al 2015) and the fit of the composite XRT-BAT-GBM spectrum (red solid line). The grey shaded area represents the 1σ scatter from the best fit.…”

The properties of prompt emission in short gamma-ray bursts with extended emission observed by Fermi/GBM

“…The first component is a short hard spike whose spectrum can be fit by a CPL with peak energy Ep = 230 +310 −80 keV. The preferred fit for the spectrum of the second component is a blackbody model with kT = 11.3 +3.8 −2.4 keV (Goldstein et al 2017;Zhang et al 2018), but a nonthermal origin with a CPL model fit of Ep = 43 +9 −7 keV cannot be ruled out (Zhang et al 2018;Pozanenko et al 2018). In this section, we compare the properties of GRB 170817A to the short GRBs with EE in our sample.…”

“…More interestingly, the short GRB 170817A associated with the gravitational wave event (GW 170817) from double a NS merger was recently detected by Fermi/GBM (Abbott et al 2017;Goldstein et al 2017), and was found to have a prompt emission that is composed of a hard spike of 0.8 s and a weak tail of up to 2.3 s (Goldstein et al 2017;Zhang et al 2018;Pozanenko et al 2018). An interesting question is whether or not GRB 170817A differs in its spectral properties from other typical short GRBs with EE.…”