Interpreting LIGO/Virgo "Mass-Gap" events as lensed Neutron Star-Black
  Hole binaries
  Gravitational lensing allows the detection of binary black holes (BBH) at
cosmological distances with chirp masses that appear to be enhanced by $1+z$ in
the range $1<z<4$, in good agreement with the reported BBH masses. We propose
this effect also accounts for the puzzling "mass gap" events (MG) newly
reported by LIGO/Virgo, as distant, lensed NSBH events with $1<z<4$. The fitted
mass of the neutron star member becomes $(1+z)\times 1.4M_\odot$, and is
therefore misclassified as a low mass black hole. In this way, we derive a
redshift of $z\simeq 3.5$ and $z\simeq 1.0$ for two newly reported "mass
asymmetric" events GW190412 \& GW190814, by interpreting them as lensed NSBH
events, comprising a stellar mass black hole and neutron star. Over the past
year an additional 31 BBH events and 5 MG events have been reported with high
probability ($>95\%$), from which we infer a factor $\simeq 5$ higher intrinsic
rate of NSBH events than BBH events, reflecting a higher proportion of neutron
stars formed by early star formation. We predict a distinctive locus for lensed
NSBH events in the observed binary mass plane, spanning $1<z<4$ with a narrow
mass ratio, $q \simeq 0.2$, that can be readily tested when the waveform data
are unlocked. All such events may show disrupted NS emission and are worthy of
prompt follow-up as the high lensing magnification means EM detections are not
prohibitive despite the high redshifts that we predict. Such lensed NSBH events
provide an exciting prospect of directly charting the history of coalescing
binaries via the cosmological redshift of their waveforms, determined relative
to the characteristic mass of the neutron star member.
