Constraining binary neutron star population synthesis models using short gamma-ray burst data

¹ Gran Sasso Science Institute (GSSI), I-67100 L’Aquila, Italy
² INFN, Laboratori Nazionali del Gran Sasso, I-67100 Assergi, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 24 February 2026
Accepted: 14 May 2026

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 observatories raises the question of whether BNS mergers alone can account for the full observed sGRB population. We analysed 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 considered 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 characterised by low local BNS merger rates (R_BNS(0) ≲ 50 Gpc⁻³ yr⁻¹) predict too few observable sGRBs to reproduce the Fermi-GBM population, effectively disfavouring them as sole progenitors. Even when relaxing the assumptions on jet geometry, low-rate models remain viable only for wide jets (θ_c ≥ 15°), in tension with the narrow jet cores (θ_c ≈ 6°) inferred from sGRB afterglow observations. In contrast, models with local merger rates of order R_BNS(0) ≈ 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 binary neutron star population synthesis models with sGRB observations and comparing them with current gravitational wave constraints to assess the role of BNS mergers as sGRB progenitors and to identify any emerging tensions between these populations.