Constraining Binary Neutron Star Populations using Short Gamma-Ray Burst Observations

TL;DR

This study uses population synthesis models and gamma-ray burst data to constrain the rate of binary neutron star mergers and their role as short gamma-ray burst progenitors, highlighting the importance of multi-messenger observations.

Contribution

It provides the first comprehensive analysis combining BNS merger models with sGRB observations to constrain merger rates and jet geometries.

Findings

Low BNS merger rate models predict too few sGRBs, disfavoring them as sole progenitors.

High merger rate models with realistic jet geometries reproduce observed sGRB populations.

Narrow jet cores inferred from afterglows are inconsistent with some low-rate models.

Abstract

The landmark multi-messenger observations of the binary neutron star (BNS) merger GW170817 provided firm evidence that such mergers can produce short gamma-ray bursts (sGRBs). However, the limited number of BNS detections by current gravitational-wave (GW) observatories raises the question of whether BNS mergers alone can account for the full observed sGRB population. We analyze a comprehensive set of 64 BNS population synthesis models with a Monte Carlo-based framework to reproduce the properties of sGRBs detected by Fermi-GBM over the past 16 years. We consider three jet geometry scenarios: a universal structured jet calibrated to GW170817, a universal top-hat jet, and a non-universal top-hat jet with distributions of core opening angles. Our results show that models characterized by low local BNS merger rates ($R_{BNS}(0) \lesssim 50$ Gpc$^{-3}$ yr$^{-1}$) predict too few observable sGRBs to reproduce the Fermi-GBM population, effectively disfavoring them as sole progenitors. Even when relaxing assumptions on jet geometry, low-rate models remain viable only for wide jets ($\theta_c \ge 15^\circ$), in tension with the narrow jet cores ($\theta_c \approx 6^\circ$) inferred from sGRB afterglow observations. In contrast, models with local merger rates of order $R_{BNS}(0) \approx 100$ Gpc$^{-3}$ yr$^{-1}$ successfully reproduce the observed sGRB population, assuming a plausible fraction of BNS mergers launch relativistic jets and realistic jet geometries. This analysis highlights the power of combining GW observations of BNS mergers with electromagnetic observations of sGRBs to place robust constraints on the BNS merger population and to assess their role as progenitors of sGRBs.