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 1.4 M _ { \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 .