The GW signal from BH-NS mergers is somewhat “cleaner” than that from NS-NS mergers, since the
disruption of the NS and its accretion by the BH rapidly terminate the GW emission. In general, 3PN
estimates model the signal well until tidal effects become important. The more compact the NS, the higher
the dimensionless “cutoff frequency” at which the GW energy spectrum plummets, with direct
plunges in which the NS is swallowed whole typically yielding excess power near the frequency maximum
from the final pre-merger burst. For increasingly prograde BH spins, there is more excess power over the
3PN prediction at lower frequencies, but also a lower cutoff frequency marking the plunge (see the
discussion in [284]). From an observational standpoint, the deviations from point-mass form become more
visible for a higher mass BH-NS system, because frequencies scale characteristically like the inverse of
the total mass. The distinction is particularly important for Advanced LIGO, as systems with
typically yield cutoff frequencies within the advanced LIGO band at source distances of
, while for lower-mass systems the cutoff occurs at or just above the upper end of the
frequency band. This is significantly different than the situation for NS-NS mergers, in which
the characteristic frequencies corresponding to the merger itself typically fall at frequencies
above the advanced LIGO high-frequency sensitivity limit, and those corresponding to remnant
oscillations in the range 2 – 4 kHz, which will prove a challenge even for third-generation GW
detectors.
Disk masses for BH-NS mergers were found to be extremely small in the first calculations, all performed
using non-spinning BHs [290, 289, 91], but have since been corrected to larger values once more
sophisticated grid-based schemes and atmosphere treatments were added to those codes. More recent results
indicate disk masses for reasonable physical parameters can be as large as , for highly-spinning
(
) mergers [108
], with values of
characterizing non-spinning
models with mass ratios
[153]. Mass loss into a disk is suppressed by misaligned
spins, especially for highly-inclined BHs, so the aligned cases should currently be interpreted as
upper limits for the disk mass when alignment is varied [108]. Overall, disk masses for BH-NS
merger remnants are comparable to those from NS-NS merger remnants, and may not be clearly
distinguishable from them based solely on the emission properties of the disk. For BH-NS mergers with
mass ratios
and prograde spins of dimensionless magnitude
, the disk
parameters found after a run performed with the inclusion of a finite-temperature NS EOS [84]
indicated that the neutrino luminosity from the disk might be as high as 1053 erg/s. While
NS-NS merger simulations have led to predictions of neutrino luminosities a few times larger
than this, the result does indicate that BH-NS mergers are also plausible SGRB progenitor
candidates, possibly with lower characteristic luminosities than bursts resulting from NS-NS
mergers.
The role of magnetic fields in BH-NS mergers has only been investigated recently [66, 92], in
simulations that apply an initially poloidal magnetic field to the NSs in the binary. Magnetic fields were
found to have very little effect on the resulting GW signal and the mass accretion rate for the BH
for physically reasonable magnetic field strengths, with visible divergences appearing only for
[92], which is not particularly surprising. Just as in NS-NS mergers, magnetic fields play very
little role during inspiral, and unlike the case of NS-NS mergers there is no opportunity to boost fields
at a vortex sheet that forms when the binary makes contact, nor in a HMNS via differential
rotation. While the MRI may be important in determining the thermal evolution and mass
accretion rate in a post-merger disk, such effects will likely be observable primarily on longer
timescales.
Just as full-GR NS-NS simulations do not indicate that such mergers are likely sources of the r-process elements we observe in the universe, BH-NS simulations in full GR make the same prediction: no detectable mass loss from the system whatsoever, at least in the calculations performed to date. The picture may change when even larger prograde spins are modeled, since this should lead to maximal disk production, or if more detailed microphysical treatments indicate that a significant wind can be generated from either a HMNS or BH disk and unbind astrophysically interesting amounts of material, but neither has been seen in the numerical results to date.
As is seen in NS-NS mergers, the pericenter distance plays a critical role in the evolution
of eccentric BH-NS mergers as well. Large disk masses containing up to , with an
unbound fraction of roughly
, can occur when the periastron separation is located
just outside the classical ISCO, with GW signals taking on the characteristic zoom-whirl form
predicted for elliptical orbits [294]. In between pericenter passages, radial oscillations of the
neutron star produce GW emission at frequencies corresponding to the f-mode for the NS as
well [88].
http://www.livingreviews.org/lrr-2012-8 |
Living Rev. Relativity 15, (2012), 8
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