Compact Binary Progenitors of Short Gamma-Ray Bursts

TL;DR

This paper links observations and simulations of compact binary mergers to constrain the properties of progenitors for short gamma-ray bursts, suggesting most originate from high-mass neutron star mergers with implications for gravitational wave detection.

Contribution

It provides a novel method to infer binary progenitor properties of SGRBs by comparing observed energies with simulation-based torus mass estimates.

Findings

Most tori have masses less than 0.01 solar masses.

High-mass binary NS mergers are favored as SGRB progenitors.

Implications for gravitational wave signals and black hole spins.

Abstract

In recent years, detailed observations and accurate numerical simulations have provided support to the idea that mergers of compact binaries containing either two neutron stars (NSs) or an NS and a black hole (BH) may constitute the central engine of short gamma-ray bursts (SGRBs). The merger of such compact binaries is expected to lead to the production of a spinning BH surrounded by an accreting torus. Several mechanisms can extract energy from this system and power the SGRBs. Here we connect observations and numerical simulations of compact binary mergers, and use the current sample of SGRBs with measured energies to constrain the mass of their powering tori. By comparing the masses of the tori with the results of fully general-relativistic simulations, we are able to infer the properties of the binary progenitors which yield SGRBs. By assuming a constant efficiency in converting torus mass into jet energy, epsilon_{jet}=10%, we find that most of the tori have masses smaller than 0.01M_{sun}, favoring "high-mass" binary NSs mergers, i.e., binaries with total masses >~1.5 the maximum mass of an isolated NS. This has important consequences for the gravitational-wave signals that may be detected in association with SGRBs, since "high-mass" systems do not form a long-lived hypermassive NS after the merger. While NS-BH systems cannot be excluded to be the engine of at least some of the SGRBs, the BH would need to have an initial spin of ~0.9, or higher.