1,613 research outputs found
The Transition from Inspiral to Plunge for a Compact Body in a Circular Equatorial Orbit Around a Massive, Spinning Black Hole
There are three regimes of gravitational-radiation-reaction-induced inspiral
for a compact body with mass mu, in a circular, equatorial orbit around a Kerr
black hole with mass M>>mu: (i) The "adiabatic inspiral regime", in which the
body gradually descends through a sequence of circular, geodesic orbits. (ii) A
"transition regime", near the innermost stable circular orbit (isco). (iii) The
"plunge regime", in which the body travels on a geodesic from slightly below
the isco into the hole's horizon. This paper gives an analytic treatment of the
transition regime and shows that, with some luck, gravitational waves from the
transition might be measurable by the space-based LISA mission.Comment: 8 Pages and 3 Figures; RevTeX; submitted to Physical Review
Observational Signature of Tidal Disruption of a Star by a Massive Black Hole
We have modeled the time-variable profiles of the Halpha emission line from
the non-axisymmetric disk and debris tail created in the tidal disruption of a
solar-type star by a million solar mass black hole. We find that the line
profiles at these very early stages of the evolution of the post-disruption
debris do not resemble the double peaked profiles expected from a rotating disk
since the debris has not yet settled into such a stable structure. The
predicted line profiles vary on fairly short time scales (of order hours to
days). As a result of the uneven distribution of the debris and the existence
of a ``tidal tail'' (the stream of returning debris), the line profiles depend
sensitively on the orientation of the tail relative to the line of sight. Given
the illuminating UV/X-ray light curve, we also model the Halpha light curve
from the debris.Comment: 2 pages, 1 figure, to appear in the proceedings of "The Interplay
among Black Holes, Stars and ISM in Galactic Nuclei", IAU 222, eds. Th.
Storchi Bergmann, L.C. Ho, and H.R. Schmit
Orbital evolution of a test particle around a black hole: higher-order corrections
We study the orbital evolution of a radiation-damped binary in the extreme
mass ratio limit, and the resulting waveforms, to one order beyond what can be
obtained using the conservation laws approach. The equations of motion are
solved perturbatively in the mass ratio (or the corresponding parameter in the
scalar field toy model), using the self force, for quasi-circular orbits around
a Schwarzschild black hole. This approach is applied for the scalar model.
Higher-order corrections yield a phase shift which, if included, may make
gravitational-wave astronomy potentially highly accurate.Comment: 4 pages, 3 Encapsulated PostScript figure
Stellar Dynamics of Extreme-Mass-Ratio Inspirals
Inspiral of compact stellar remnants into massive black holes (MBHs) is
accompanied by the emission of gravitational waves at frequencies that are
potentially detectable by space-based interferometers. Event rates computed
from statistical (Fokker-Planck, Monte-Carlo) approaches span a wide range due
to uncertaintities about the rate coefficients. Here we present results from
direct integration of the post-Newtonian N-body equations of motion descrbing
dense clusters of compact stars around Schwarzschild MBHs. These simulations
embody an essentially exact (at the post-Newtonian level) treatment of the
interplay between stellar dynamical relaxation, relativistic precession, and
gravitational-wave energy loss. The rate of capture of stars by the MBH is
found to be greatly reduced by relativistic precession, which limits the
ability of torques from the stellar potential to change orbital angular
momenta. Penetration of this "Schwarzschild barrier" does occasionally occur,
resulting in capture of stars onto orbits that gradually inspiral due to
gravitational wave emission; we discuss two mechanisms for barrier penetration
and find evidence for both in the simulations. We derive an approximate formula
for the capture rate, which predicts that captures would be strongly disfavored
from orbits with semi-major axes below a certain value; this prediction, as
well as the predicted rate, are verified in the N-body integrations. We discuss
the implications of our results for the detection of extreme-mass-ratio
inspirals from galactic nuclei with a range of physical properties.Comment: 28 pages, 16 figures. Version 2 is significantly revised to reflect
new insights into J and Q effects, to be published late
A Panchromatic Study of the Globular Cluster NGC 1904. I: The Blue Straggler Population
By combining high-resolution (HST-WFPC2) and wide-field ground based (2.2m
ESO-WFI) and space (GALEX) observations, we have collected a multi-wavelength
photometric data base (ranging from the far UV to the near infrared) of the
galactic globular cluster NGC1904 (M79). The sample covers the entire cluster
extension, from the very central regions up to the tidal radius. In the present
paper such a data set is used to study the BSS population and its radial
distribution. A total number of 39 bright () BSS has been
detected, and they have been found to be highly segregated in the cluster core.
No significant upturn in the BSS frequency has been observed in the outskirts
of NGC 1904, in contrast to other clusters (M 3, 47 Tuc, NGC 6752, M 5) studied
with the same technique. Such evidences, coupled with the large radius of
avoidance estimated for NGC 1904 ( core radii), indicate that
the vast majority of the cluster heavy stars (binaries) has already sunk to the
core. Accordingly, extensive dynamical simulations suggest that BSS formed by
mass transfer activity in primordial binaries evolving in isolation in the
cluster outskirts represent only a negligible (0--10%) fraction of the overall
population.Comment: ApJ accepte
Loss cone: past, present and future
The capture and subsequent in--spiral of compact stellar remnants by central
massive black holes, is one of the more interesting likely sources of
gravitational radiation detectable by LISA. The relevant stellar population
includes stellar mass black holes, and possibly intermediate mass black holes,
generally on initially eccentric orbits. Predicted detectable rates of capture
are highly uncertain, but may be high enough that source confusion is an issue.
Foreground events with relatively high signal-to-noise ratio may provide
important tests of general relativity. I review the rate estimates in the
literature, and the apparent discrepancy between different authors' estimates,
and discuss some of the relevant uncertainties and physical processes. The
white dwarf mergers rate are uncertain by a factor of few; the neutron star
merger rate is completely uncertain and likely to be small; the black hole
merger rate is likely to be dominant for detectable mergers and is uncertain by
at least two orders of magnitude, largely due to unknown physical conditions
and processes. The primary difference in rate estimates is due to different
initial conditions and less directly due to different estimates of key physical
processes, assumed in different model scenarios for in-spiral and capture.Comment: 7 pages, revtex twocolumn, Special LISA Issue Classical and Quantum
Gravity in pres
Gravitational waveforms from a point particle orbiting a Schwarzschild black hole
We numerically solve the inhomogeneous Zerilli-Moncrief and Regge-Wheeler
equations in the time domain. We obtain the gravitational waveforms produced by
a point-particle of mass traveling around a Schwarzschild black hole of
mass M on arbitrary bound and unbound orbits. Fluxes of energy and angular
momentum at infinity and the event horizon are also calculated. Results for
circular orbits, selected cases of eccentric orbits, and parabolic orbits are
presented. The numerical results from the time-domain code indicate that, for
all three types of orbital motion, black hole absorption contributes less than
1% of the total flux, so long as the orbital radius r_p(t) satisfies r_p(t)> 5M
at all times.Comment: revtex4, 24 pages, 23 figures, 3 tables, submitted to PR
Tidal Disruption of a Star By a Black Hole : Observational Signature
We have modeled the time-variable profiles of the Halpha emission line from
the non-axisymmetric disk and debris tail created in the tidal disruption of a
solar-type star by a million solar mass black hole. Two tidal disruption event
simulations were carried out using a three dimensional relativistic
smooth-particle hydrodynamic code, to describe the early evolution of the
debris during the first fifty to ninety days. We have calculated the physical
conditions and radiative processes in the debris using the photoionization code
CLOUDY. We model the emission line profiles in the period immediately after the
accretion rate onto the black hole became significant. We find that the line
profiles at these very early stages of the evolution of the post-disruption
debris do not resemble the double peaked profiles expected from a rotating disk
since the debris has not yet settled into such a stable structure. As a result
of the uneven distribution of the debris and the existence of a ``tidal tail''
(the stream of returning debris), the line profiles depend sensitively on the
orientation of the tail relative to the line of sight. Moreover, the predicted
line profiles vary on fairly short time scales (of order hours to days). Given
the accretion rate onto the black hole we also model the Halpha light curve
from the debris and the evolution of the Halpha line profiles in time.Comment: 20 pages, 9 figures, to appear in ApJ, 1 August 2004 issue; mpeg
simulations of tidal disruption available at
http://www.astro.psu.edu/users/tamarab/tdmovies.htm
Nearly horizon skimming orbits of Kerr black holes
An unusual set of orbits about extreme Kerr black holes resides at the
Boyer-Lindquist radius , the coordinate of the hole's event horizon.
These ``horizon skimming'' orbits have the property that their angular momentum
{\it increases} with inclination angle, opposite to the familiar behavior
one encounters at larger radius. In this paper, I show that this behavior is
characteristic of a larger family of orbits, the ``nearly horizon skimming''
(NHS) orbits. NHS orbits exist in the very strong field of any black hole with
spin a\agt 0.952412M. Their unusual behavior is due to the locking of
particle motion near the event horizon to the hole's spin, and is therefore a
signature of the Kerr metric's extreme strong field. An observational hallmark
of NHS orbits is that a small body spiraling into a Kerr black hole due to
gravitational-wave emission will be driven into orbits of progressively smaller
inclination angle, toward the equator. This is in contrast to the ``normal''
behavior. For circular orbits, the change in inclination is very small, and
unlikely to be of observational importance. I argue that the change in
inclination may be considerably larger when one considers the evolution of
inclined eccentric orbits. If this proves correct, then the gravitational waves
produced by evolution through the NHS regime may constitute a very interesting
and important probe of the strong-field nature of rotating black holes.Comment: 9 pages, 5 figures, accepted for publication in PR
Sequence variants in the PTCH1 gene associate with spine bone mineral density and osteoporotic fractures
Bone mineral density (BMD) is a measure of osteoporosis and is useful in evaluating the risk of fracture. In a genome-wide association study of BMD among 20,100 Icelanders, with follow-up in 10,091 subjects of European and East-Asian descent, we found a new BMD locus that harbours the PTCH1 gene, represented by rs28377268 (freq. 11.4–22.6%) that associates with reduced spine BMD (P=1.0 × 10−11, β=−0.09). We also identified a new spine BMD signal in RSPO3, rs577721086 (freq. 6.8%), that associates with increased spine BMD (P=6.6 × 10−10, β=0.14). Importantly, both variants associate with osteoporotic fractures and affect expression of the PTCH1 and RSPO3 genes that is in line with their influence on BMD and known biological function of these genes. Additional new BMD signals were also found at the AXIN1 and SOST loci and a new lead SNP at the EN1 locus
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