Wider look at the gravitational-wave transients from GWTC-1 using an unmodeled reconstruction method

In this paper, we investigate the morphology of the events from the GWTC-1 catalog of compact binary coalescences as reconstructed by a method based on coherent excess power: we use an open-source version of the coherent WaveBurst (cWB) analysis pipeline, which does not make use of waveform models. The coherent response of the LIGO-Virgo network of detectors is estimated by using loose bounds on the duration and bandwidth of the signal. This pipeline version reproduces the same results that are reported for cWB in recent publications by the LIGO and Virgo collaborations. In particular, the sky localization and waveform reconstruction are in a good agreement with those produced by methods which exploit the detailed theoretical knowledge of the expected waveform for compact binary coalescences. However, in some cases cWB also detects features in excess in well-localized regions of the time-frequency plane. Here we focus on such deviations and present the methods devised to assess their significance. Out of the 11 events reported in the GWTC-1, in two cases - GW151012 and GW151226 - cWB detects an excess of coherent energy after the coalescence (Δt≃0.2 and ≃0.1 s, respectively) with p-values that call for further investigations (0.004 and 0.03, respectively), though they are not sufficient to exclude noise fluctuations. We discuss the morphological properties and plausible interpretations of these features. In case they are genuine, we anticipate that several more such outliers will be uncovered by our methodology in the ongoing advanced LIGO-Virgo observation run (O3).