Analysis of a subsolar-mass compact binary candidate from the second observing run of Advanced LIGO

We perform an exhaustive follow-up analysis of a subsolar-mass (SSM) gravitational wave (GW) candidate reported by Phukon et al. from the second observing run of Advanced LIGO. This candidate has a reported signal-to-noise ratio (SNR) of 8.6 and false alarm rate of 0.41 yr which are too low to claim a clear gravitational-wave origin. When improving on the search by using more accurate waveforms, extending the frequency range from 45 Hz down to 20 Hz, and removing a prominent blip glitch, we find that the posterior distribution of the network SNR lies mostly below the search value, with the 90% confidence interval being 7.94−1.05+0.70. Assuming that the origin of the signal is a compact binary coalescence (CBC), the secondary component is m2=0.76−0.14+0.50M⊙, with m2<1M⊙ at 84% confidence level, suggesting an unexpectedly light neutron star or a black hole of primordial or exotic origin. The primary mass would be m1=4.71−2.18+1.57M⊙, likely in the hypothesized lower mass gap and the luminosity distance is measured to be DL= 124−48+82Mpc. We then probe the CBC origin hypothesis by performing the signal coherence tests, obtaining a log Bayes factor of 4.96±0.13 for the coherent vs. incoherent hypothesis. We demonstrate the capability of performing a parameter estimation follow-up on real data for an SSM candidate with moderate SNR. The improved sensitivity of O4 and subsequent LIGO-Virgo-KAGRA observing runs could make it possible to observe similar signals, if present, with a higher SNR and more precise measurement of the parameters of the binary.