Searching for Neutron Star Mergers in the Absence of Gravitational Waves with Optical Afterglow Emission

TL;DR

This paper explores how optical afterglow observations, especially with LSST, can improve detection of neutron star mergers independently of gravitational wave signals, by analyzing afterglow properties and distinguishing features.

Contribution

It introduces a method to enhance neutron star merger detection rates using bright short gamma-ray burst afterglows in optical surveys like LSST, without relying on gravitational wave triggers.

Findings

On-axis afterglows can increase detection rates from 29 to 91 per year.

Color analysis helps distinguish neutron star merger counterparts with and without kilonova emission.

Detectable events are likely near peak brightness, enabling rapid follow-up.

Abstract

With the forth observing run of the LIGO-Virgo-KAGRA gravitational-wave network, which enabled the discovery of the kilonova (KN) counterpart to GW170817, ending with no new confirmed neutron star mergers, the intrinsic rate of these events must be even lower than previously estimated. As a result, building a sample of KNe will remain challenging even with continued GW observations, motivating complementary discovery strategies that do not rely on gravitational-wave triggers. In this work, we consider how leveraging bright short gamma-ray burst afterglows can aid in the discovery on KNe with the Rubin Observatory's upcoming Legacy Survey of Space and Time (LSST), whose unprecedented depth will make such detections feasible. We find that nearly on-axis (${\theta}_{\rm view} \leq 30{\deg}$) afterglows can enhance KN detection rates in the LSST $g$-band from $29^{+51}_{-21} \ \rm yr^{-1}$ to $91^{+160}_{-65} \ \rm yr^{-1}$. We further show how the colors of the observed events can be used to distinguish between neutron star merger counterparts with and without KN emission. This study demonstrates how critical multi-wavelength and multi-survey observations are for these rare events, especially without context from gravitational waves. Fortunately, detectable events will likely be discovered near peak with LSST, allowing for rapid follow-up and confirmation. We discuss key uncertainties in our study, particularly volume rate of merger events, and the degeneracy between the empirically determined explosion energy and ambient medium density.