Is GW190425 Consistent with Being a Neutron Star–Black Hole Merger?

Abstract

GW190425 is the second neutron star merger event detected by the Advanced LIGO/Virgo detectors. If interpreted as a double neutron star merger, the total gravitational mass is substantially larger than that of the binary systems identified in the Galaxy. In this work we analyze the gravitational-wave data within the neutron star−black hole merger scenario. For the black hole, we yield a mass of 2.40 +0.36 −0.32 M and an aligned spin of 0.141 +0.067 −0.064 . As for the neutron star we find a mass of 1.15 +0.15 −0.13 M and the dimensionless tidal deformability of 1.4 +3.8 −1.2 × 10 3 . These parameter ranges are for 90% credibility. The inferred masses of the neutron star and the black hole are not in tension with current observations and we suggest that GW190425 is a viable candidate of a neutron star−black hole merger event. Benefitting from the continual enhancement of the sensitivities of the advanced gravitational detectors and the increase of the number of the observatories, similar events are anticipated to be much more precisely measured in the future and the presence of black holes below the so-called mass gap will be unambiguously clarified. If confirmed, the mergers of neutron stars with (quickly rotating) low-mass black holes are likely important production sites of the heaviest r-process elements.

Coalescence of black hole–neutron star binaries

“…Tidal disruption do not occur for these parameters, and thus neither mass ejection nor electromagnetic emission is expected to occur. We also note that some other gravitational-wave events reported in LIGO-Virgo O3 are also consistent with black hole-neutron star binaries under generous assumptions on the mass of compact objects (Abbott et al 2020a, c), 1 partly because no electromagnetic counterpart was detected (Kyutoku et al 2020;Han et al 2020;Kawaguchi et al 2020a).…”

Neutron-star tidal deformability and equation-of-state constraints

“…The fact that only a single tidal parameter can be extracted from neutron star coalescences [99] and uncertainties about the maximum possible neutron star mass suggests that it is difficult to unambiguously distinguish between neutron star-black hole and binary neutron star coalescences [175]. If one further entertains the possibility of ∼ 1.35M black holes, both the gravitational and electromagnetic signal from GW170817 are consistent with a neutron star-black hole coalescence [176,177] and similar for GW190425 [178,179]. This possibility has led to increased interest in equal-mass neutron star-black hole coalescences and their potential counterparts [180].…”

Interpreting binary neutron star mergers: describing the binary neutron star dynamics, modelling gravitational waveforms, and analyzing detections

“…However the constraints extracted were found to be consistent with those obtained from GW170817. Due to the high masses, the possibility that this event was gravitationally lensed [417] or that one or two components have been BHs can not be ruled out, e.g., [253,310]. Incorporating knowledge about the EOS and the fact that no EM counterpart has been observed can be used to further constrain the properties of GW190425 [135,221,391]…”