Analysis of the subsolar-mass black hole candidate SSM200308 from the second part of the third observing run of Advanced LIGO-Virgo

We present a follow-up study of a subsolar black hole candidate identified in the second part of the third observing run of the LIGO-Virgo-KAGRA collaboration. The candidate was identified by the GstLAL search pipeline in the Hanford and Livingston LIGO detectors with a network signal-to-noise ratio of 8.90 and a false-alarm-rate of 1 per 5 years. It is the most significant of the three candidates found below the O3b subsolar mass false-alarm rate threshold of 2 per year, but still not significant enough above the background to claim a clear gravitational wave origin. A Bayesian parameter estimation of this candidate, denoted SSM200308, reveals that if the signal originates from a compact binary coalescence, the component masses are $m_1=0.62_{-0.20}^{+0.46}{M}_{\odot }$ and $m_2=0.27_{-0.10}^{+0.12}{M}_{\odot }$ (90% credible intervals) with at least one component being firmly subsolar, below the minimum mass of a neutron star. This discards the hypothesis that the signal comes from a standard binary neutron star. The signal coherence test between the two LIGO detectors is consistent with, but does not necessarily imply, a compact object coalescence origin.