# LIGO Document P2000357-v9

- We report the observation of gravitational waves from two compact binary coalescences in LIGO's and Virgo's third observing run with properties consistent with neutron star--black hole (NSBH) binaries. The two events are named GW200105_162426 and GW200115_042309, abbreviated as GW200105 and GW200115; the first was observed by LIGO Livingston and Virgo, and the second by all three LIGO-Virgo detectors. The source of GW200105 has component masses \(8.9^{+1.2}_{-1.5}\,M_\odot\) and \(1.9^{+0.3}_{-0.2}\,M_\odot\), whereas the source of GW200115 has component masses \(5.7^{+1.8}_{-2.1}\,M_\odot\) and \(1.5^{+0.7}_{-0.3}\,M_\odot\) (all measurements quoted at the 90% credible level). The probability that the secondary's mass is below the maximal mass of a neutron star is 89%-96% and 87%-98%, respectively, for GW200105 and GW200115, with the ranges arising from different astrophysical assumptions. The source luminosity distances are \(280^{+110}_{-110}\) Mpc and \(300^{+150}_{-100}\) Mpc, respectively. The magnitude of the primary spin of GW200105 is less than \(0.23\) at the 90% credible level, and its orientation is unconstrained. For GW200115, the primary spin has a negative spin projection onto the orbital angular momentum at 88% probability. We are unable to constrain spin or tidal deformation of the secondary component for either event. We infer a NSBH merger rate density of \(45^{+75}_{-33}\,\mathrm{Gpc}^{-3} \mathrm{yr}^{-1}\) when assuming GW200105 and GW200115 are representative of the NSBH population, or \(130^{+112}_{-69}\,\mathrm{Gpc}^{-3} \mathrm{yr}^{-1}\) under the assumption of a broader distribution of component masses.

- Version uploaded to arXiv, identical to the one in ApJL, except for typesetting.

- Published in ApJ Letters vol. 915 pg. L5.

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