Observation of gravitational waves from two neutron star-black hole coalescences

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_⊙ and 1.9 +0.3 −0.2 M_⊙, whereas the source of GW200115 has component masses 5.7 +1.8 −2.1 M_⊙ and 1.5 +0.7 −0.3 M_⊙ (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 Gpc⁻³yr⁻¹ when assuming GW200105 and GW200115 are representative of the NSBH population, or 130 +112 −69 Gpc⁻³yr⁻¹ under the assumption of a broader distribution of component masses.