Merger of a Neutron Star with a Black Hole: one-family vs. two-families scenario

TL;DR

This paper compares the kilonova signals from Black Hole-Neutron Star mergers under one-family and two-families compact star scenarios, highlighting differences in expected signals based on equations of state and recent observational constraints.

Contribution

It introduces a comparative analysis of kilonova signals in one-family versus two-families scenarios, incorporating recent NICER data and theoretical models of compact stars.

Findings

Kilonova signals are stronger in the one-family scenario with stiff equations of state.

Two-families scenario predicts very weak or negligible kilonova signals.

Merger signals depend on black hole mass and spin, testable with future detectors.

Abstract

A kilonova 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 kilonova signal is therefore strong, at least if the mass of the Black Hole does not exceed $\sim 10 \mathrm{\,M}_\odot$ and if 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 kilonova 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 kilonova 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 $\chi_\mathrm{BH}\gtrsim 0.2$ or a mass $\mathrm M_\mathrm{BH} \lesssim 4 \mathrm M_\odot$ can be present in the merger.