Merger of a Neutron Star with a Black Hole: One-family versus Two-families Scenario

Abstract

A kilonova (KN) signal is generally expected after a black hole–neutron star merger. The strength of the signal is related to the equation of state of neutron star matter, and it increases with the stiffness of the latter. The recent results obtained by NICER from the analyses of PSR J0740+6620 suggest a rather stiff equation of state, and the expected KN signal is therefore strong, at least if the mass of the black hole does not exceed ∼10 M ⊙ , the adimensional spin parameter is not too small, and the orbit is prograde. We compare the predictions obtained by considering equations of state of neutron star matter satisfying the most recent observations and assuming that only one family of compact stars exists with the results predicted in the two-families scenario. In the latter a soft hadronic equation of state produces very compact stellar objects, while a rather stiff quark matter equation of state produces massive strange quark stars, satisfying NICER results. The expected KN signal in the two-families scenario is very weak: in particular, the hadronic star–black hole merger produces a much weaker signal than in the one-family scenario because the hadronic equation of state is very soft. Moreover, according to the only existing simulation, the strange quark star–black hole merger does not produce a KN signal because the amount of mass ejected is negligible. These predictions will be easily tested with the new generation of detectors if black holes with an adimensional spin parameter χ BH ≳ 0.2 or a mass M BH ≲ 4 M ⊙ can be present in the merger.