1.3 Why is the merger of BH-NS binaries important?
The merger of BH-NS binaries (more specifically, tidal disruption of a NS by a BH) is physically and
astrophysically an important phenomenon, and deserves a detailed study, because of at least three reasons
as follows.
- Gravitational waves emitted during tidal disruption of a NS will bring us invaluable information
about the radius and the EOS of the NS, because the orbital frequency at tidal disruption
depends strongly on the compactness of the NS (
), e.g., [107
, 109
]. The masses
of the NS and the BH will be determined by the data analysis for gravitational waves emitted
in the inspiral phase (see, e.g., [49] and also references cited in [96, 180]). If the NS radius
could be determined or constrained from the observation of gravitational waves emitted during
tidal disruption, the resultant relation between the mass and the radius of the observed NS may
be used for constraining the EOS of the high-density nuclear matter [127
, 220
, 70
]. Therefore,
the gravitational-wave observation for BH-NS binaries will provide a new tool for exploring
high-density nuclear matter, which is totally independent of standard nuclear experiments. The
issue here is to theoretically clarify the dependence of gravitational waveforms on the NS EOS
quantitatively; theoretical templates for a variety of possible EOS have to be prepared.
- A tidally-disrupted NS may form a disk or torus of mass larger than
with the density
1011 g/cm3 and the temperature
10 MeV around the remnant BH, if tidal disruption
occurs outside the ISCO. A system consisting of a spinning BH surrounded by a massive, hot, and
dense torus has been proposed as one of the likely sources for the central engine of a
GRB [143, 232, 161, 142
]. For the merger of BH-NS binaries, the resulting disk is likely to be
compact and its mass is relatively small, of order
. Since the lifetime of such a disk is likely to
be short (
1 s), BH-NS binaries are proposed to be the progenitor of SGRB. Specifically, the
merger of a low-mass BH and/or a rapidly-spinning BH, and its companion NS can be a
candidate for the source of the central engine. According to the observational results
by the Swift and HETE-2 satellites [142
], the total energy of the SGRB is larger than
1048 ergs, and typically 1049 – 1050 ergs. This value may be explained if 1 – 10% of
the thermal energy generated in a compact disk around a BH is converted to SGRB:
Here,
is the efficiency,
is the typical inner radius of the disk, and
is the
mass accretion rate. The issue to be resolved is whether or not the mass and thermal
energy of the torus formed after the merger are large enough for driving an SGRB of such
huge total luminosity. The latest observations have also discovered that a longterm X-ray
flare of duration
103 – 104 s is often associated with an SGRB [142
]. Another more
challenging issue is whether or not such an activity may be explained in the BH-NS merger
scenario.
- Material ejected from a tidally-disrupted NS may be important for understanding the observed
abundances of the heavy elements that are formed by rapid neutron capture in the r-process [118].
One crucial question is whether it is possible for a fraction of material to escape from the system,
because the situation is not well prepared for the mass ejection. Tidal disruption of a NS occurs
typically at an orbital separation of
. For a test particle of mass
in a
circular orbit around the BH with
, the total energy is approximately
. For a free nucleon,
. Thus, the issues are, specifically,
to answer whether it is possible to give about 50 MeV energy to each nucleon and, if possible, to
clarify what the relevant process is.