Short gamma-ray burst progenitors have short delay times

TL;DR

This study analyzes short gamma-ray burst (SGRB) populations to determine their delay times from progenitor formation, finding shorter average delays of 10-800 Myr, contrasting with previous long-delay estimates due to better selection effect treatment.

Contribution

The paper introduces a hierarchical Bayesian analysis of SGRB delay times, revealing shorter delays and correcting prior overestimations caused by selection bias misinterpretation.

Findings

Average delay times between 10 and 800 Myr.

Minimum delay time less than 350 Myr.

Previous studies overestimated delays due to selection effects.

Abstract

Short gamma-ray bursts (SGRBs) are thought to be primarily associated with binary neutron star (BNS) mergers. The SGRB population can therefore be scrutinized to look for signatures of the delay time between the formation of the progenitor massive star binary and the eventual merger, which could produce an evolution of the cosmic rate density of such events whose shape departs from that of the cosmic star formation history (CSFH). To that purpose, we study a large sample of SGRBs within a hierarchical Bayesian framework, with a particular focus on the delay time distribution (DTD) of the population. Following previous studies, we model the DTD either as a power-law with a minimum time delay or as a log-normal function. We consider two models for the intrinsic SGRB luminosity distribution: an empirical luminosity function (ELF) with a doubly broken power-law shape, and one based on a quasi-universal structured jet (QUSJ) model. Regardless of the chosen parametrization, we find average time delays $10\lesssim \langle \tau_\mathrm{d}\mathrm\rangle/\mathrm{Myr}\lesssim 800$ and a minimum delay time $\tau_\mathrm{d,min}\lesssim 350\,\mathrm{Myr}$, in contrast with previous studies that found long delay times of few Gyr. We demonstrate that the cause of the longer inferred time delays in past studies most likely resides in an incorrect treatment of selection effects.