1,286 research outputs found
Resonant relaxation near a massive black hole: the stellar distribution and gravitational wave sources
Resonant relaxation (RR) of orbital angular momenta occurs near massive black
holes (MBHs) where the stellar orbits are nearly Keplerian and so do not
precess significantly. The resulting coherent torques efficiently change the
magnitude of the angular momenta and rotate the orbital inclination in all
directions. As a result, many of the tightly bound stars very near the MBH are
rapidly destroyed by falling into the MBH on low-angular momentum orbits, while
the orbits of the remaining stars are efficiently randomized. We solve
numerically the Fokker-Planck equation in energy for the steady state
distribution of a single mass population with a RR sink term. We find that the
steady state current of stars, which sustains the accelerated drainage close to
the MBH, can be up to ~10 times larger than that due to non-coherent 2-body
relaxation alone. RR mostly affects tightly bound stars, and so it increases
only moderately the total tidal disruption rate, which is dominated by stars
originating from less bound orbits farther away. We show that the event rate of
gravitational wave (GW) emission from inspiraling stars, originating much
closer to the MBH, is dominated by RR dynamics. The GW event rate depends on
the uncertain efficiency of RR. The efficiency indicated by the few available
simulations implies rates ~10 times higher than those predicted by 2-body
relaxation, which would improve the prospects of detecting such events by
future GW detectors, such as LISA. However, a higher, but still plausible RR
efficiency can lead to the drainage of all tightly bound stars and strong
suppression of GW events from inspiraling stars. We apply our results to the
Galactic MBH, and show that the observed dynamical properties of stars there
are consistent with RR.Comment: Accepted to ApJ; Minor revision
Extreme mass ratio inspiral rates: dependence on the massive black hole mass
We study the rate at which stars spiral into a massive black hole (MBH) due
to the emission of gravitational waves (GWs), as a function of the mass M of
the MBH. In the context of our model, it is shown analytically that the rate
approximately depends on the MBH mass as M^{-1/4}. Numerical simulations
confirm this result, and show that for all MBH masses, the event rate is
highest for stellar black holes, followed by white dwarfs, and lowest for
neutron stars. The Laser Interferometer Space Antenna (LISA) is expected to see
hundreds of these extreme mass ratio inspirals per year. Since the event rate
derived here formally diverges as M->0, the model presented here cannot hold
for MBHs of masses that are too low, and we discuss what the limitations of the
model are.Comment: Accepted to CQG, special LISA issu
The effect of mass-segregation on gravitational wave sources near massive black holes
Gravitational waves (GWs) from the inspiral of compact remnants (CRs) into
massive black holes (MBHs) will be observable to cosmological distances. While
a CR spirals in, 2-body scattering by field stars may cause it to fall into the
MBH before reaching a short period orbit that would give an observable signal.
As a result, only CRs very near (~0.01 pc) the MBH can spiral in successfully.
In a multi-mass stellar population, the heaviest objects sink to the center,
where they are more likely to slowly spiral into the MBH without being
swallowed prematurely. We study how mass-segregation modifies the stellar
distribution and the rate of GW events. We find that the inspiral rate per
galaxy for white dwarfs is 30 per Gyr, for neutron stars 6 per Gyr, and for
stellar black holes (SBHs) 250 per Gyr. The high rate for SBHs is due to their
extremely steep density profile, n_{BH}(r)\propto r^{-2}. The GW detection rate
will be dominated by SBHs.Comment: Submitted to ApJ
The impact of realistic models of mass segregation on the event rate of extreme-mass ratio inspirals and cusp re-growth
One of the most interesting sources of gravitational waves (GWs) for LISA is
the inspiral of compact objects on to a massive black hole (MBH), commonly
referred to as an "extreme-mass ratio inspiral" (EMRI). The small object,
typically a stellar black hole (bh), emits significant amounts of GW along each
orbit in the detector bandwidth. The slowly, adiabatic inspiral of these
sources will allow us to map space-time around MBHs in detail, as well as to
test our current conception of gravitation in the strong regime. The event rate
of this kind of source has been addressed many times in the literature and the
numbers reported fluctuate by orders of magnitude. On the other hand, recent
observations of the Galactic center revealed a dearth of giant stars inside the
inner parsec relative to the numbers theoretically expected for a fully relaxed
stellar cusp. The possibility of unrelaxed nuclei (or, equivalently, with no or
only a very shallow cusp) adds substantial uncertainty to the estimates. Having
this timely question in mind, we run a significant number of direct-summation
body simulations with up to half a million particles to calibrate a much
faster orbit-averaged Fokker-Planck code. We then investigate the regime of
strong mass segregation (SMS) for models with two different stellar mass
components. We show that, under quite generic initial conditions, the time
required for the growth of a relaxed, mass segregated stellar cusp is shorter
than a Hubble time for MBHs with
(i.e. nuclei in the range of LISA). SMS has a significant impact boosting the
EMRI rates by a factor of for our fiducial models of Milky Way type
galactic nuclei.Comment: Accepted by CQG, minor changes, a bit expande
Three-Body Dynamics with Gravitational Wave Emission
We present numerical three-body experiments that include the effects of
gravitational radiation reaction by using equations of motion that include the
2.5-order post-Newtonian force terms, which are the leading order terms of
energy loss from gravitational waves. We simulate binary-single interactions
and show that close approach cross sections for three 1 solar mass objects are
unchanged from the purely Newtonian dynamics except for close approaches
smaller than 1.0e-5 times the initial semimajor axis of the binary. We also
present cross sections for mergers resulting from gravitational radiation
during three-body encounters for a range of binary semimajor axes and mass
ratios including those of interest for intermediate-mass black holes (IMBHs).
Building on previous work, we simulate sequences of high-mass-ratio three-body
encounters that include the effects of gravitational radiation. The simulations
show that the binaries merge with extremely high eccentricity such that when
the gravitational waves are detectable by LISA, most of the binaries will have
eccentricities e > 0.9 though all will have circularized by the time they are
detectable by LIGO. We also investigate the implications for the formation and
growth of IMBHs and find that the inclusion of gravitational waves during the
encounter results in roughly half as many black holes ejected from the host
cluster for each black hole accreted onto the growing IMBH.Comment: 34 pages, 14 figures, minor corrections to match version accepted by
Ap
Towards adiabatic waveforms for inspiral into Kerr black holes: I. A new model of the source for the time domain perturbation equation
We revisit the problem of the emission of gravitational waves from a test
mass orbiting and thus perturbing a Kerr black hole. The source term of the
Teukolsky perturbation equation contains a Dirac delta function which
represents a point particle. We present a technique to effectively model the
delta function and its derivatives using as few as four points on a numerical
grid. The source term is then incorporated into a code that evolves the
Teukolsky equation in the time domain as a (2+1) dimensional PDE. The waveforms
and energy fluxes are extracted far from the black hole. Our comparisons with
earlier work show an order of magnitude gain in performance (speed) and
numerical errors less than 1% for a large fraction of parameter space. As a
first application of this code, we analyze the effect of finite extraction
radius on the energy fluxes. This paper is the first in a series whose goal is
to develop adiabatic waveforms describing the inspiral of a small compact body
into a massive Kerr black hole.Comment: 21 pages, 6 figures, accepted by PRD. This version removes the
appendix; that content will be subsumed into future wor
Warping the young stellar disc in the Galactic Centre
We examine influence of the circum-nuclear disc (CND) upon the orbital
evolution of young stars in the Galactic Centre. We show that gravity of the
CND causes precession of the orbits which is highly sensitive upon the
semi-major axis and inclination. We consider such a differential precession
within the context of an ongoing discussion about the origin of the young stars
and suggest a possibility that all of them have originated in a thin disc which
was partially destroyed due to the influence of the CND during the period of
~6Myr.Comment: proc. conf. "The Universe Under the Microscope - Astrophysics at High
Angular Resolution", 21-25 April 2008, Bad Honnef, German
Simulations of Extreme-Mass-Ratio Inspirals Using Pseudospectral Methods
Extreme-mass-ratio inspirals (EMRIs), stellar-mass compact objects (SCOs)
inspiralling into a massive black hole, are one of the main sources of
gravitational waves expected for the Laser Interferometer Space Antenna (LISA).
To extract the EMRI signals from the expected LISA data stream, which will also
contain the instrumental noise as well as other signals, we need very accurate
theoretical templates of the gravitational waves that they produce. In order to
construct those templates we need to account for the gravitational
backreaction, that is, how the gravitational field of the SCO affects its own
trajectory. In general relativity, the backreaction can be described in terms
of a local self-force, and the foundations to compute it have been laid
recently. Due to its complexity, some parts of the calculation of the
self-force have to be performed numerically. Here, we report on an ongoing
effort towards the computation of the self-force based on time-domain
multi-grid pseudospectral methods.Comment: 6 pages, 4 figures, JPCS latex style. Submitted to JPCS (special
issue for the proceedings of the 7th International LISA Symposium
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