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Abstract

We report a degeneracy between the gravitational-wave signals from
quasi-circular precessing black-hole mergers and those from extremely eccentric
mergers, namely head-on collisions. Performing model selection on numerically
simulated signals of head-on collisions using models for quasi-circular
binaries we find that, for signal-to-noise ratios of 15 and 25, typical of
Advanced LIGO observations, head-on mergers with respective total masses of
$M\in (125,300)M_\odot$ and $M\in (200,440)M_\odot$ would be identified as
precessing quasi-circular intermediate-mass black hole binaries, located at a
much larger distance. Ruling out the head-on scenario would require to perform
model selection using currently nonexistent waveform models for head-on
collisions, together with the application of astrophysically motivated priors
on the (rare) occurrence of those events. We show that in situations where
standard parameter inference of compact binaries may report component masses
inside (outside) the pair-instability supernova gap, the true object may be a
head-on merger with masses outside (inside) this gap. We briefly discuss the
potential implications of these findings for the recent gravitational-wave
detection GW190521, which we analyse in detail in [Phys. Rev. Lett. 126,
081101].