New binary black hole mergers in the second observing run of Advanced LIGO and Advanced Virgo

Abstract

We report the detection of new binary black hole merger events in the publicly available data from the second observing run of Advanced LIGO and Advanced Virgo (O2). The mergers were discovered using the new search pipeline described in Venumadhav et al. [Phys. Rev. D 100, 023011 (2019)] and are above the detection thresholds as defined in Abbott et al. (LIGO Scientific and Virgo Collaborations) [Phys. Rev. X 9, 031040 (2019).]. Three of the mergers (GW170121, GW170304, GW170727) have inferred probabilities of being of astrophysical origin ${p}_{\text{astro}}>0.98$. The remaining three (GW170425, GW170202, GW170403) are less certain, with ${p}_{\text{astro}}$ ranging from 0.5 to 0.8. The newly found mergers largely share the statistical properties of previously reported events, with the exception of GW170403, the least secure event, which has a highly negative effective spin parameter ${\ensuremath{\chi}}_{\mathrm{eff}}$. The most secure new event, GW170121 (${p}_{\text{astro}}>0.99$), is also notable due to its inferred negative value of ${\ensuremath{\chi}}_{\mathrm{eff}}$, which is inconsistent with being positive at the $\ensuremath{\approx}95.8%$ confidence level. The new mergers nearly double the sample of gravitational wave events reported from O2 and present a substantial opportunity to explore the statistics of the binary black hole population in the Universe. The number of detected events is not surprising since we estimate that the detection volume of our pipeline may be larger than that of other pipelines by as much as a factor of 2 (with significant uncertainties in the estimate). The increase in volume is larger when the constituent detectors of the network have very different sensitivities, as is likely to be the case in current and future runs.