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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE,General Relativity and Quantum Cosmology, gr-qc
Abstract:
Neutron star$-$black hole (NS$-$BH) coalescences are widely believed to be
promising gravitational wave sources in the era of advanced detectors of
LIGO/Virgo but such binaries have never been directly detected yet. Evidence
for NS$-$BH coalescences have been suggested in short and hybrid GRB
observations, which are examined critically. Based on the suggested connection
between the observed macronovae/kilonovae events and NS$-$BH coalescences, we
get a fiducial lower limit of NS$-$BH coalescence rate density ${\cal R}_{\rm
nsbh} \approx 18.8^{+12.5}_{-8.6} ~{\rm Gpc^{-3}~ yr^{-1}~ (\theta_j/0.1~{\rm
rad})^{-2}}$, where $\theta_{\rm j}$ is the typical half-opening angle of the
GRB ejecta. The real value of ${\cal R}_{\rm nsbh}$ is likely at least $\sim
{\rm a~few}$ times larger, depending upon the equation of state of NS material
and the properties of the NS$-$BH system, such as the mass and spin
distribution of the black hole. If the link between macronovae/kilonovae and
NS$-$BH coalescence is valid, one can expect that at design sensitivity the
aLIGO/AdVirgo network will detect NS$-$BH coalescence signals at a rate of at
least a dozen per year, and to consequently place constraints on certain
physical properties of NS$-$BH systems.
