Short gamma-ray bursts within 200 Mpc

TL;DR

This study systematically searches for short gamma-ray bursts within 200 Mpc using 14 years of Swift data, finding potential nearby events and constraining their properties, with implications for neutron star mergers and kilonovae.

Contribution

It provides the first systematic search for local short GRBs, identifies candidate nearby events, and constrains their properties and rates, informing models of neutron star mergers.

Findings

Potential nearby short GRB candidates within 200 Mpc.

Constraints on kilonova ejecta mass and orientation.

Estimated all-sky rate of detectable short GRBs within 200 Mpc.

Abstract

We present a systematic search for short-duration gamma-ray bursts (GRBs) in the local Universe based on 14 years of observations with the Neil Gehrels Swift Observatory. We cross-correlate the GRB positions with the GLADE catalogue of nearby galaxies, and find no event at a distance $\lesssim$100 Mpc and four plausible candidates in the range 100 Mpc$\lesssim$$D$$\lesssim$200 Mpc. Although affected by low statistics, this number is higher than the one expected for chance alignments to random galaxies, and possibly suggests a physical association between these bursts and nearby galaxies. By assuming a local origin, we use these events to constrain the range of properties for X-ray counterparts of neutron star mergers. Optical upper limits place tight constraints on the onset of a blue kilonova, and imply either low masses ($\lesssim10^{-3}\,M_{\odot}$) of lanthanide-poor ejecta or unfavorable orientations ($\theta_{obs}\gtrsim$30 deg). Finally, we derive that the all-sky rate of detectable short GRBs within 200 Mpc is $1.3^{+1.7}_{-0.8}$ yr$^{-1}$ (68\% confidence interval), and discuss the implications for the GRB outflow structure. If these candidates are instead of cosmological origin, we set a upper limit of $\lesssim$2.0 yr$^{-1}$ (90\% confidence interval) to the rate of nearby events detectable with operating gamma-ray observatories, such as Swift and Fermi.