1,035 research outputs found
On gravitational-wave echoes from neutron-star binary coalescences
A tentative detection of gravitational-wave echoes in the post-merger signal
of GW170817 has been recently claimed at significance level. It has
been speculated that the signal might provide evidence for near-horizon quantum
structures in the remnant exotic object. We point out that if the remnant
object is an ultracompact neutron star, echoes are expected for objects with
radius only slightly smaller than that of an ordinary neutron star. The
reported echoes at ~72 Hz are compatible with a toy model of incompressible
star with mass approximately and radius close to the
Buchdahl limit, R~9GM/(4c^2). If confirmed, low-frequency gravitational-wave
echoes would be in tension with all current neutron-star models and would have
dramatic implications for nuclear physics and gravity.Comment: v3: 4 pages, 2 figures, extended discussion, results unchanged.
Version accepted in CQG Letter
Low latency search for Gravitational waves from BH-NS binaries in coincidence with Short Gamma Ray Bursts
We propose a procedure to be used in the search for gravitational waves from
black hole-neutron star coalescing binaries, in coincidence with short
gamma-ray bursts. It is based on two recently proposed semi-analytic fits, one
reproducing the mass of the remnant disk surrounding the black hole which forms
after the merging as a function of some binary parameters, the second relating
the neutron star compactness, i.e. the ratio of mass and radius, with its tidal
deformability. Using a Fisher matrix analysis and the two fits, we assign a
probability that the emitted gravitational signal is associated to the
formation of an accreting disk massive enough to supply the energy needed to
power a short gamma ray burst. This information can be used in low-latency data
analysis to restrict the parameter space searching for gravitational wave
signals in coincidence with short gamma-ray bursts, and to gain information on
the dynamics of the coalescing system and on the internal structure of the
components. In addition, when the binary parameters will be measured with high
accuracy, it will be possible to use this information to trigger the search for
off-axis gamma-ray bursts afterglows.Comment: 5 pages, 1 figure, changes in the introduction and in the concluding
remarks. Accepted for publication in Phys. Rev.
Gravitational waves emitted by solar-type stars excited by orbiting planets
The possibility of exciting the g-modes of a solar-type star as a consequence
of the gravitational interaction with a close companion (a planet or a brown
dwarf) is studied by a perturbative approach. The amplitude of the emitted
gravitational wave is computed and compared with the quadrupole emission of the
system, showing that in some cases it can be considerably larger. The effects
of radiation reaction are considered to evaluate the timescale of the emission
process, and a Roche lobe analysis is used to establish the region where the
companion can orbit without being disrupted by tidal interactions with the
star.Comment: 19 pages, 1 figure, submitted to Phys. Rev. D. Typo in formula (5.4)
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A hybrid approach to black hole perturbations from extended matter sources
We present a new method for the calculation of black hole perturbations
induced by extended sources in which the solution of the nonlinear
hydrodynamics equations is coupled to a perturbative method based on
Regge-Wheeler/Zerilli and Bardeen-Press-Teukolsky equations when these are
solved in the frequency domain. In contrast to alternative methods in the time
domain which may be unstable for rotating black-hole spacetimes, this approach
is expected to be stable as long as an accurate evolution of the matter sources
is possible. Hence, it could be used under generic conditions and also with
sources coming from three-dimensional numerical relativity codes. As an
application of this method we compute the gravitational radiation from an
oscillating high-density torus orbiting around a Schwarzschild black hole and
show that our method is remarkably accurate, capturing both the basic
quadrupolar emission of the torus and the excited emission of the black hole.Comment: 12 pages, 4 figures. Phys. Rev. D, in pres
Constraining the equation of state of nuclear matter with gravitational wave observations: Tidal deformability and tidal disruption
We study how to extract information on the neutron star equation of state
from the gravitational wave signal emitted during the coalescence of a binary
system composed of two neutron stars or a neutron star and a black hole. We use
post-Newtonian templates which include the tidal deformability parameter and,
when tidal disruption occurs before merger, a frequency cut-off. Assuming that
this signal is detected by Advanced LIGO/Virgo or ET, we evaluate the
uncertainties on these parameters using different data analysis strategies
based on the Fisher matrix approach, and on recently obtained analytical fits
of the relevant quantities. We find that the tidal deformability is more
effective than the stellar compactness to discriminate among different possible
equations of state.Comment: 13 pages, 4 figures, 4 tables. Minor changes to match the version
appearing on Phys. Rev.
Stochastic background of gravitational waves generated by a cosmological population of young, rapidly rotating neutron stars
We estimate the spectral properties of the stochastic background of
gravitational radiation emitted by a cosmological population of hot, young,
rapidly rotating neutron stars. Their formation rate as a function of redshift
is deduced from an observation-based determination of the star formation
history in the Universe, and the gravitational energy is assumed to be radiated
during the spin-down phase associated to the newly discovered r-mode
instability. We calculate the overall signal produced by the ensemble of such
neutron stars, assuming various cosmological backgrounds. We find that the
spectral strain amplitude has a maximum , at frequencies Hz, while the corresponding
closure density, , has a maximum amplitude plateau of in the frequency range Hz. We compare
our results with a preliminary analysis done by Owen et al. (1998), and discuss
the detectability of this background.Comment: 8 pages, 9 figures, accepted for publication in MNRA
Exotic Compact Objects and How to Quench their Ergoregion Instability
Gravitational-wave astronomy can give us access to the structure of black
holes, potentially probing microscopic or even Planckian corrections at the
horizon scale, as those predicted by some quantum-gravity models of exotic
compact objects. A generic feature of these models is the replacement of the
horizon by a reflective surface. Objects with these properties are prone to the
so-called ergoregion instability when they spin sufficiently fast. We
investigate in detail a simple model consisting of scalar perturbations of a
Kerr geometry with a reflective surface near the horizon. The instability
depends on the spin, on the compactness, and on the reflectivity at the
surface. The instability time scale increases only logarithmically in the
black-hole limit and, for a perfectly reflecting object, this is not enough to
prevent the instability from occurring on dynamical time scales. However, we
find that an absorption rate at the surface as small as 0.4% (reflectivity
coefficient as large as ) is sufficient to quench the
instability completely. Our results suggest that exotic compact objects are not
necessarily ruled out by the ergoregion instability.Comment: v3: 14 pages, 9 figures; further clarifications added, new appendix
on the superspinar case, results unchanged. Accepted in Phys. Rev.
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