How Special Is GRB 170817A?

TL;DR

This paper investigates the uniqueness of GRB 170817A by comparing it with other underluminous short gamma-ray bursts, discussing their possible origins, and suggesting that similar events are likely to be detected in future gravitational-wave observations.

Contribution

The study provides a comparative analysis of GRB 170817A with other nearby underluminous GRBs, proposing a potential new class of neutron star merger events and their diverse electromagnetic signatures.

Findings

GRB 130603B/macronova may be an on-axis analog of GRB 170817A.

GRB 170817A's dim afterglow suggests a low-density circumburst medium and structured outflow.

Some nearby short GRBs could originate from neutron star mergers, but their optical counterparts vary significantly.

Abstract

GRB 170817A is the first short gamma-ray burst (GRB) with direct detection of the gravitational-wave radiation and also the spectroscopically identified macronova emission (i.e., AT 2017gfo). The prompt emission of this burst, however, is underluminous in comparison with the other short GRBs with known redshift. In this work, we examine whether GRB 170817A is indeed unique. We firstly show that GRB 130603B/macronova may be the on-axis "analogs" of GRB 170817A/AT 2017gfo, and the extremely dim { but long-lasting} afterglow emission of GRB 170817A may suggest a low number density ($\sim 10^{-5}~{\rm cm^{-3}}$) of its circumburst medium { and a structured outflow}. We then discuss whether GRB 070923, GRB 080121, GRB 090417A, GRB 111005A, and GRB 170817A form a new group of very nearby underluminous GRBs originated from neutron star mergers. If the short events GRB 070923, GRB 080121, and GRB 090417A are indeed at a redshift of $\sim 0.076,~0.046,~0.088$, respectively, their isotropic energies of the prompt emission are $\sim 10^{47}$ erg and thus comparable to the other two events. The non-detection of optical counterparts of GRB 070923, GRB 080121, GRB 090417A, and GRB 111005A, however, strongly suggests that the macronovae from neutron star mergers are significantly diverse in luminosities or, alternatively, there is the other origin channel (for instance, the white dwarf and black hole mergers). We finally suggest that GW170817/GRB 170817A are likely not alone and similar events will be detected by the upgraded/upcoming gravitational-wave detectors and the electromagnetic monitors.