A Systematic Exploration of Kilonova Candidates from Neutron Star Mergers During the Third Gravitational Wave Observing Run

TL;DR

This paper systematically analyzes 653 optical candidates from the third GW observing run, employing real-time tools and archival data to efficiently vet potential kilonovae and reduce follow-up efforts.

Contribution

It introduces a comprehensive vetting methodology combining real-time data and archival information to identify viable kilonova candidates from GW events.

Findings

65% of candidates eliminated through initial vetting

66 candidates remain viable but lack sufficient classification data

Archival tools reduce follow-up resource strain significantly

Abstract

We present a comprehensive analysis of 653 optical candidate counterparts reported during the third gravitational wave (GW) observing run. Our sample concentrates on candidates from the 15 events (published in GWTC-2, GWTC-3 or not retracted on GraceDB) that had a >1% chance of including a neutron star in order to assess their viability as true kilonovae. In particular, we leverage tools available in real time, including pre-merger detections and cross-matching with catalogs (i.e. point source, variable star, quasar and host galaxy redshift datasets), to eliminate 65% of candidates in our sample. We further employ spectroscopic classifications, late-time detections and light curve behavior analyses, and conclude that 66 candidates remain viable kilonovae. These candidates lack sufficient information to determine their classifications, and the majority would require luminosities greater than that of AT2017gfo. Pre-merger detections in public photometric survey data and comparison of catalogued host galaxy redshifts with the GW event distances are critical to incorporate into vetting procedures, as these tools eliminated >20% and >30% of candidates, respectively. We expect that such tools which leverage archival information will significantly reduce the strain on spectroscopic and photometric follow-up resources in future observing runs. Finally, we discuss the critical role prompt updates from GW astronomers to the EM community play in reducing the number of candidates requiring vetting.