Can a black hole–neutron star merger explain GW170817, AT2017gfo, and GRB170817A?

Abstract

The discovery of the compact binary coalescence in both gravitational waves and electromagnetic radiation marks a breakthrough in the field of multi-messenger astronomy and has improved our knowledge in a number of research areas. However, an open question is the exact origin of the observables and if one can confirm reliably that GW170817 and its electromagnetic counterparts resulted from a binary neutron star merger. To answer the question if the observation of GW170817, GRB170817A, and AT2017gfo could be explained by the merger of a neutron star with a black hole, we perform a joint multi-messenger analysis of the gravitational waves, the short gamma-ray burst, and the kilonova. Assuming a black-hole neutron star system, we derive multi-messenger constraints for the tidal deformability of the NS of $\Lambda > 425$ and for the mass ratio of $q < 2.03$ at 90\% confidence, with peaks in the likelihood near $\Lambda = 830$ and $q = 1.0$. Overall, we find that a black hole-neutron star merger could explain the observed signatures, however, our analysis shows that a binary neutron star origin of GW170817 seems more plausible.