Detecting Gravitational Waves With Disparate Detector Responses: Two New Binary Black Hole Mergers

TL;DR

The paper presents a novel method for detecting gravitational wave events that are primarily observed in a single detector, leading to the discovery of two new binary black hole mergers during LIGO-Virgo's second observing run, expanding detection capabilities.

Contribution

A new technique for identifying gravitational wave signals mainly seen in one detector, enabling detection of events previously difficult to confirm.

Findings

Discovered two new binary black hole mergers in O2 data.

Estimated false alarm rates of one in 19 and 11 years for the two events.

Provided mass and spin measurements for GW170817A, informing black hole mass gap studies.

Abstract

We introduce a new technique to search for gravitational wave events from compact binary mergers that produce a clear signal only in a single gravitational wave detector, and marginal signals in other detectors. Such a situation can arise when the detectors in a network have different sensitivities, or when sources have unfavorable sky locations or orientations. We start with a short list of loud single-detector triggers from regions of parameter space that are empirically unaffected by glitches (after applying signal-quality vetoes). For each of these triggers, we compute evidence for astrophysical origin from the rest of the detector network by coherently combining the likelihoods from all detectors and marginalizing over extrinsic geometric parameters. We report the discovery of two new binary black hole (BBH) mergers in the second observing run of Advanced LIGO and Virgo (O2), in addition to the ones that were reported in Abbott et al. (2018) and Venumadhav et al. (2019). We estimate that the two events have false alarm rates of one in 19 years (60 O2) and one in 11 years (36 O2). One of the events, GW170817A, has primary and secondary masses $m_1^{\rm src} = 56_{-10}^{+16} \, M_\odot$ and $m_2^{\rm src} = 40_{-11}^{+10} \, M_\odot$ in the source frame. The existence of GW170817A should be very informative about the theoretically predicted upper mass gap for stellar mass black holes. Its effective spin parameter is measured to be $\chi_{\rm eff} = 0.5 \pm 0.2$, which is consistent with the tendency of the heavier detected BBH systems to have large and positive effective spin parameters. The other event, GWC170402, will be discussed thoroughly in future work.