  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 eleven events reported in the GWTC-1, in two cases --
GW151012 and GW151226 -- cWB detects an excess of coherent energy after the
merger ($\Delta t \simeq 0.2$ s and $\simeq 0.1$ 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. We
believe that the methodology described in the paper shall be useful in future
searches for compact binary coalescences.
