233 research outputs found
Measuring the parameters of massive black hole binary systems with Pulsar Timing Array observations of gravitational waves
The observation of massive black hole binaries (MBHBs) with Pulsar Timing
Arrays (PTAs) is one of the goals of gravitational wave astronomy in the coming
years. Massive (>10^8 solar masses) and low-redshift (< 1.5) sources are
expected to be individually resolved by up-coming PTAs, and our ability to use
them as astrophysical probes will depend on the accuracy with which their
parameters can be measured. In this paper we estimate the precision of such
measurements using the Fisher-information-matrix formalism. We restrict to
"monochromatic" sources. In this approximation, the system is described by
seven parameters and we determine their expected statistical errors as a
function of the number of pulsars in the array, the array sky coverage, and the
signal-to-noise ratio (SNR) of the signal. At fixed SNR, the gravitational wave
astronomy capability of a PTA is achieved with ~20 pulsars; adding more pulsars
(up to 1000) to the array reduces the source error-box in the sky \Delta\Omega
by a factor ~5 and has negligible consequences on the statistical errors on the
other parameters. \Delta\Omega improves as 1/SNR^2 and the other parameters as
1/SNR. For a fiducial PTA of 100 pulsars uniformly distributed in the sky and a
coherent SNR = 10, we find \Delta\Omega~40 deg^2, a fractional error on the
signal amplitude of ~30% (which constraints only very poorly the chirp mass -
luminosity distance combination M_c^{5/3}/D_L), and the source inclination and
polarization angles are recovered at the ~0.3 rad level. The ongoing Parkes PTA
is particularly sensitive to systems located in the southern hemisphere, where
at SNR = 10 the source position can be determined with \Delta\Omega ~10 deg^2,
but has poorer performance for sources in the northern hemisphere. (Abridged)Comment: 20 pages, 12 figures, 2 color figures, submitted to Phys. Rev.
Missing black holes in brightest cluster galaxies as evidence for the occurrence of superkicks in nature
We investigate the consequences of superkicks on the population of
supermassive black holes (SMBHs) in the Universe residing in brightest cluster
galaxies (BCGs). There is strong observational evidence that BCGs grew
prominently at late times (up to a factor 2-4 in mass from z=1), mainly through
mergers with satellite galaxies from the cluster, and they are known to host
the most massive SMBHs ever observed. Those SMBHs are also expected to grow
hierarchically, experiencing a series of mergers with other SMBHs brought in by
merging satellites. Because of the net linear momentum taken away from the
asymmetric gravitational wave emission, the remnant SMBH experiences a kick in
the opposite direction. Kicks may be as large as ~5000 Km/s ("superkicks"),
pushing the SMBHs out in the cluster outskirts for a time comparable to
galaxy-evolution timescales. We predict, under a number of plausible
assumptions, that superkicks can efficiently eject SMBHs from BCGs, bringing
their occupation fraction down to a likely range 0.9<f<0.99 in the local
Universe. Future thirty-meter-class telescopes like ELT and TMT will be capable
of measuring SMBHs in hundreds of BCGs up to z=0.2, testing the occurrence of
superkicks in nature and the strong-gravity regime of SMBH mergers.Comment: 19 pages, 11 figures, accepted for publication in MNRA
Linking gravitational waves and X-ray phenomena with joint LISA and Athena observations
The evolution of cosmic structures, the formation and growth of the first
black holes and the connection to their baryonic environment are key unsolved
problems in astrophysics. The X-ray Athena mission and the gravitational-wave
Laser Interferometer Space Antenna (LISA) offer independent and complementary
angles on these problems. We show that up to 10 black hole binaries in the mass
range 10^5 - 10^8 Msun discovered by LISA at redshift <~ 3.5 could be detected
by Athena in an exposure time up to 100 ks, if prompt X-ray emission of ~ 1% -
10% of the Eddington luminosity is present. Likewise, if any LISA-detected
extreme mass ratio inspirals occur in accretion disks, Athena can detect
associated electromagnetic emission out to redshift ~ 1. Finally, warned by
LISA, Athena can point in advance and stare at stellar-mass binary black hole
mergers at redshift <~ 0.1. These science opportunities emphasise the vast
discovery space of simultaneous observations from the two observatories, which
would be missed if they were operated in different epochs.Comment: Published in Nature Astronom
Migration of massive black hole binaries in self--gravitating accretion discs: Retrograde versus prograde
We study the interplay between mass transfer, accretion and gravitational
torques onto a black hole binary migrating in a self-gravitating, retrograde
circumbinary disc. A direct comparison with an identical prograde disc shows
that: (i) because of the absence of resonances, the cavity size is a factor
a(1+e) smaller for retrograde discs; (ii) nonetheless the shrinkage of a
circular binary semi--major axis, a, is identical in both cases; (iii) a
circular binary in a retrograde disc remains circular while eccentric binaries
grow more eccentric. For non-circular binaries, we measure the orbital decay
rates and the eccentricity growth rates to be exponential as long as the binary
orbits in the plane of its disc. Additionally, for these co-planar systems, we
find that interaction (~ non--zero torque) stems only from the cavity edge plus
a(1+e) in the disc, i.e. for dynamical purposes, the disc can be treated as a
annulus of small radial extent. We find that simple 'dust' models in which the
binary- disc interaction is purely gravitational can account for all main
numerical results, both for prograde and retrograde discs. Furthermore, we
discuss the possibility of an instability occurring for highly eccentric
binaries causing it to leave the disc plane, secularly tilt and converge to a
prograde system. Our results suggest that there are two stable configurations
for binaries in self-gravitating discs: the special circular retrograde case
and an eccentric (e~ 0.6) prograde configuration as a stable attractor.Comment: 14 pages, 2 Tabes, 11 Figures, submitted to MNRAS, comments welcom
Origin and Implications of high eccentricities in massive black hole binaries at sub-pc scales
We outline the eccentricity evolution of sub-parsec massive black hole
binaries (MBHBs) forming in galaxy mergers. In both stellar and gaseous
environments, MBHBs are expected to grow large orbital eccentricities before
they enter the gravitational wave (GW) observational domain. We re--visit the
predicted eccentricities detectable by space based laser interferometers (as
the proposed ELISA/NGO) for both environments. Close to coalescence, many MBHBs
will still maintain detectable eccentricities, spanning a broad range from
<10^{-5} up to <~ 0.5. Stellar and gas driven dynamics lead to distinct
distributions, with the latter favoring larger eccentricities. At larger binary
separations, when emitted GWs will be observed by pulsar timing arrays (PTAs),
the expected eccentricities are usually quite large, in the range 0.01-0.7,
which poses an important issue for signal modelling and detection algorithms.
In this window, large eccentricities also have implications on proposed
electromagnetic counterparts to the GW signal, which we briefly review.Comment: AMALDI9 proceedings, submitted to CQG ; 10 Pages 2 Figure
The astrophysical science case for a decihertz gravitational-wave detector
We discuss the astrophysical science case for a decihertz gravitational-wave
mission. We focus on unique opportunities for scientific discovery in this
frequency range, including probes of type IA supernova progenitors, mergers in
the presence of third bodies, intermediate mass black holes, seeds of massive
black holes, improved sky localization, and tracking the population of merging
compact binaries
The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays
Massive black hole binary systems, with masses in the range ~10^4-10^10
\msun, are among the primary sources of gravitational waves in the frequency
window ~10^-9 Hz - 0.1 Hz. Pulsar Timing Arrays (PTAs) and the Laser
Interferometer Space Antenna (LISA) are the observational means by which we
will be able to observe gravitational radiation from these systems. We carry
out a systematic study of the generation of the stochastic gravitational-wave
background from the cosmic population of massive black hole binaries. We
consider a wide variety of assembly scenarios and we estimate the range of
signal strength in the frequency band accessible to PTAs. We show that, taking
into account the uncertainties surrounding the actual key model parameters, the
amplitude lies in the interval h_c(f = 10^-8 Hz)~5x10^-16 - 8x10^-15. The most
optimistic predictions place the signal level at a factor of ~3 below the
current sensitivity of Pulsar Timing Arrays, but within the detection range of
the complete Parkes PTA for a wide variety of models, and of the future
Square-Kilometer-Array PTA for all the models considered here. We also show
that at frequencies >10^-8 Hz the frequency dependency of the generated
background follows a power-law significantly steeper than f^-2/3, that has been
considered so far. Finally we show that LISA observations of individual
resolvable massive black hole binaries are complementary and orthogonal to PTA
observations of a stochastic background from the whole population in the
Universe. In fact, the detection of gravitational radiation in both frequency
windows will enable us to fully characterise the cosmic history of massive
black holes.Comment: 21 pages, 14 figures, minor revisions, accepted for publication in
MNRA
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