Combining (post-)Newtonian ideas with quasi-equilibrium (QE) sequence analysis for black hole-neutron star (BHNS) gravitational wave events

In this paper we present quasi equilibrium models of black hole-neutron star (BHNS) binaries with mass and spin values compatible with parameter estimates derived from gravitational radiation events GW200105 and GW200115, events consistent with the merger of BHNSs. Using the FUKA initial data framework, we determine the location of ISCO (Innermost Stable Circular Orbit) and radius of mass shedding. In most of the cases studied here the innermost stable orbit is located at larger separations. This is consistent with the fact that for those two events no electromagnetic counterparts have been observed since it is believed that the NS will enter into the plunge phase in a short time once the separation of the components of the binary will be smaller of ISCO. In analogy with classical binaries, we have associated to these QE sequences a Newtonian and Post Newtonian Roche Lobe analysis to verify whether the NS is filling its Roche Lobe before approaching the ISCO. For selected configurations explored here, the location of ISCO and of the orbit at which mass shedding occurs are at separation smaller than the last converged solution of our sequences. Our analysis shows that in such cases the neutron star is filling its Roche lobe suggesting that mass transfer might occur well before encountering the last stable orbit, and this should happen in catastrophic way in order to prevent any electromagnetic emissions. If this is the case, we are suggestion a third fate of the neutron star beyond the plunge or tidal disruption ones.