The discovery of the compact binary coalescence in both gravitational waves and electromagnetic radiation marks a breakthrough in the field of multimessenger 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 multimessenger analysis of the gravitational waves, the short gamma-ray burst, and the kilonova. Assuming a black hole–neutron star system, we derive multimessenger constraints for the tidal deformability of the neutron star of $\mathrm{\Lambda }>425$ and for the mass ratio of $q<2.03$ at 90% confidence, with peaks in the likelihood near $\mathrm{\Lambda }=830$ 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.

• Received 17 January 2019

DOI:https://doi.org/10.1103/PhysRevD.100.043011