1,493 research outputs found
Detecting massive black hole binaries and unveiling their cosmic history with gravitational wave observations
Space based gravitational wave astronomy will open a completely new window on
the Universe and massive black holes binaries are expected to be among the
primary actors on this upcoming stage. The New Gravitational-wave Observatory
(NGO) is a space interferometer proposal derived from the former Laser
Interferometer Space Antenna (LISA) concept. We describe here its capabilities
of observing massive black hole binaries throughout the Universe, measuring
their relevant parameters (masses, spins, distance to the observer) to high
precision. The statistical properties of the population of detected systems can
be used to constrain the massive black hole cosmic history, providing deep
insights into the faint, high redshift Universe.Comment: 11 pages, 4 figures, 1 table, proceeding of the 9th LISA Symposiu
Insights on the astrophysics of supermassive black hole binaries from pulsar timing observations
Pulsar timing arrays (PTAs) are designed to detect the predicted
gravitational wave (GW) background produced by a cosmological population of
supermassive black hole (SMBH) binaries. In this contribution I review the
physics of such GW background, highlighting its dependence on the overall
binary population, the relation between SMBHs and their hosts, and their
coupling with the stellar and gaseous environment. The latter is particularly
relevant when it drives the binaries to extreme eccentricities (e>0.9), which
might be the case for stellar-driven systems. This causes a substantial
suppression of the low frequency signal, potentially posing a serious threat to
the effectiveness of PTA observations. A future PTA detection will allow to
directly observe for the first time subparsec SMBH binaries on their way to the
GW driven coalescence, providing important answers of the outstanding questions
related to the physics underlying the formation and evolution of these
spectacular sources.Comment: 14 pages, 2 figures. Invited contribution to the Focus Issue "Pulsar
Timing Array", to appear in Classical and Quantum Gravit
Systematic investigation of the expected gravitational wave signal from supermassive black hole binaries in the pulsar timing band
In this letter we carry out the first systematic investigation of the
expected gravitational wave (GW) background generated by supermassive black
hole (SMBH) binaries in the nHz frequency band accessible to pulsar timing
arrays (PTAs). We take from the literature several estimates of the redshift
dependent galaxy mass function and of the fraction of close galaxy pairs to
derive a wide range of galaxy merger rates. We then exploit empirical black
hole-host relations to populate merging galaxies with SMBHs. The result of our
procedure is a collection of a large number of phenomenological SMBH binary
merger rates consistent with current observational constraints on the galaxy
assembly at z<1.5. For each merger rate we compute the associated GW signal,
eventually producing a large set of estimates of the nHz GW background that we
use to infer confidence intervals of its expected amplitude. When considering
the most recent SMBH-host relations, accounting for ultra-massive black holes
in brightest cluster galaxies, we find that the nominal interval of
the expected GW signal is only a factor of 3-to-10 below current PTA limits,
implying a non negligible chance of detection in the next few years.Comment: 6 pages, 3 figures, submitted to MNRAS lette
Enhanced tidal disruption rates from massive black hole binaries
"Hard" massive black hole (MBH) binaries embedded in steep stellar cusps can
shrink via three-body slingshot interactions. We show that this process will
inevitably be accompanied by a burst of stellar tidal disruptions, at a rate
that can be several orders of magnitude larger than that appropriate for a
single MBH. Our numerical scattering experiments reveal that: 1) a significant
fraction of stars initially bound to the primary hole are scattered into its
tidal disruption loss cone by gravitational interactions with the secondary
hole, an enhancement effect that is more pronounced for very unequal-mass
binaries; 2) about 25% (40%) of all strongly interacting stars are tidally
disrupted by a MBH binary of mass ratio q=1/81 (q=1/243) and eccentricity 0.1;
and 3) two mechanisms dominate the fueling of the tidal disruption loss cone, a
Kozai non-resonant interaction that causes the secular evolution of the stellar
angular momentum in the field of the binary, and the effect of close encounters
with the secondary hole that change the stellar orbital parameters in a chaotic
way. For a hard MBH binary of 10^7 solar masses and mass ratio 0.01, embedded
in an isothermal stellar cusp of velocity dispersion sigma*=100 km/s, the tidal
disruption rate can be as large as 1/yr. This is 4 orders of magnitude higher
than estimated for a single MBH fed by two-body relaxation. When applied to the
case of a putative intermediate-mass black hole inspiraling onto Sgr A*, our
results predict tidal disruption rates ~0.05-0.1/yr.Comment: 5 pages, 3 figures, accepted for publication in the Astrophysical
Journal Letter
A practical guide to the massive black hole cosmic history
I review our current understanding of massive black hole (MBH) formation and
evolution along the cosmic history. After a brief introductory overview of the
relevance of MBHs in the hierarchical structure formation paradigm, I discuss
the main viable channels for seed BH formation at high redshift and for their
subsequent mass growth and spin evolution. The emerging hierarchical picture,
where MBHs grow through merger triggered accretion episodes, acquiring their
mass while shining as quasars, is overall robust, but too simplistic to explain
the diversity observed in MBH phenomenology. I briefly discuss which future
observations will help to shed light on the MBH cosmic history in the near
future, paying particular attention to the upcoming gravitational wave window.Comment: 21 pages, 4 figures, accepted for publication in Advances in
Astronom
Gravitational wave emission from binary supermassive black holes
Massive black hole binaries (MBHBs) are unavoidable outcomes of the
hierarchical structure formation process, and according to the theory of
general relativity are expected to be the loudest gravitational wave (GW)
sources in the Universe. In this article I provide a broad overview of MBHBs as
GW sources. After reviewing the basics of GW emission from binary systems and
of MBHB formation, evolution and dynamics, I describe in some details the
connection between binary properties and the emitted gravitational waveform.
Direct GW observations will provide an unprecedented wealth of information
about the physical nature and the astrophysical properties of these extreme
objects, allowing to reconstruct their cosmic history, dynamics and coupling
with their dense stellar and gas environment. In this context I describe
ongoing and future efforts to make a direct detection with space based
interferometry and pulsar timing arrays, highlighting the invaluable scientific
payouts of such enterprises.Comment: 26 pages, 9 figures, invited article for the focus issue on
astrophysical black holes in Classical and Quantum Gravity, guest editors: D.
Merritt and L. Rezzolla. Submitte
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
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
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