Search For a Counterpart to the Subsolar Mass Gravitational Wave Candidate S251112cm

TL;DR

This paper develops a framework to vet and score candidate electromagnetic counterparts to subsolar mass gravitational wave events, aiming to improve follow-up strategies despite no confirmed counterparts found.

Contribution

The paper introduces a new vetting and scoring framework for electromagnetic counterparts to subsolar mass GW events, enhancing multi-messenger follow-up capabilities.

Findings

Vetted and scored 248 candidates with no likely counterparts.

Demonstrated the framework's ability to distinguish transient types.

Prepared strategies for future SSM event follow-ups.

Abstract

The recent gravitational-wave (GW) alert from a compact object merger involving at least one subsolar mass (SSM) object has prompted questions about their origins. S251112cm is reported by LIGO/Virgo with a false alarm rate of 1 per 6.2 years, nearby luminosity distance $93 \pm 27$ Mpc, probability of containing a SSM object of 100%, and probability of containing a $1-3~M_\odot$ object of just 8%. Such a system likely did not involve the supersolar neutron stars or black holes invoked to explain kilonovae. One must then also invoke hitherto unobserved and speculative models to produce SSM mergers and the resultant electromagnetic (EM) counterparts. We introduce a framework which vets and scores candidate counterparts to SSM GW events to inform follow-up in search of any among the zoo of potential EM transients: kilonovae, kilonovae-within-supernovae, super-kilonovae, or AGN flares from binary black hole mergers. We use a suite of telescopes to perform tiling, galaxy-targeted observations, and photometric/spectroscopic follow-up of promising candidates. In near-real time, we ingest candidates reported by the community, including some of the first observations reported by the Vera C. Rubin Observatory. We vet and score a total of 248 candidates, including 67 from Rubin, but find no likely counterpart. We nonetheless highlight candidates which demonstrate the ability of our framework to distinguish between different transient types and describe strategies to maximize the chances of detecting a counterpart to the next SSM event. Our framework will be implemented in the forthcoming Multimessenger Tool for Rapid Object Vetting and Examination (TROVE).