416 research outputs found
Massive black hole binaries: dynamical evolution and observational signatures
The study of the dynamical evolution of massive black hole pairs in mergers
is crucial in the context of a hierarchical galaxy formation scenario. The
timescales for the formation and the coalescence of black hole binaries are
still poorly constrained, resulting in large uncertainties in the expected rate
of massive black hole binaries detectable in the electromagnetic and
gravitational wave spectra. Here we review the current theoretical
understanding of the black hole pairing in galaxy mergers, with a particular
attention to recent developments and open issues. We conclude with a review of
the expected observational signatures of massive binaries, and of the
candidates discussed in literature to date.Comment: 4 Figures. Accepted for publication in Advances in Astronom
Selection bias in dynamically-measured super-massive black hole samples: consequences for pulsar timing arrays
Supermassive black hole -- host galaxy relations are key to the computation
of the expected gravitational wave background (GWB) in the pulsar timing array
(PTA) frequency band. It has been recently pointed out that standard relations
adopted in GWB computations are in fact biased-high. We show that when this
selection bias is taken into account, the expected GWB in the PTA band is a
factor of about three smaller than previously estimated. Compared to other
scaling relations recently published in the literature, the median amplitude of
the signal at yr drops from to
. Although this solves any potential tension between
theoretical predictions and recent PTA limits without invoking other dynamical
effects (such as stalling, eccentricity or strong coupling with the galactic
environment), it also makes the GWB detection more challenging.Comment: 6 pages 4 figures, submitted to MNRAS letter
Blindly detecting orbital modulations of jets from merging supermassive black holes
In the last few years before merger, supermassive black hole binaries will
rapidly inspiral and precess in a magnetic field imposed by a surrounding
circumbinary disk. Multiple simulations suggest this relative motion will
convert some of the local energy to a Poynting-dominated outflow, with a
luminosity 10^{43} erg/s * (B/10^4 G)^2(M/10^8 Msun)^2 (v/0.4 c)^2, some of
which may emerge as synchrotron emission at frequencies near 1 GHz where
current and planned wide-field radio surveys will operate. On top of a secular
increase in power on the gravitational wave inspiral timescale, orbital motion
will produce significant, detectable modulations, both on orbital periods and
(if black hole spins are not aligned with the binary's total angular momenta)
spin-orbit precession timescales. Because the gravitational wave merger time
increases rapidly with separation, we find vast numbers of these transients are
ubiquitously predicted, unless explicitly ruled out (by low efficiency
) or obscured (by accretion geometry f_{geo}). If the fraction of
Poynting flux converted to radio emission times the fraction of lines of sight
accessible is sufficiently large (f_{geo} \epsilon > 2\times 10^{-4}
for a 1 year orbital period), at least one event is accessible to future blind
surveys at a nominal 10^4 {deg}^2 with 0.5 mJy sensitivity. Our procedure
generalizes to other flux-limited surveys designed to investigate EM signatures
associated with many modulations produced by merging SMBH binaries.Comment: Submitted to ApJ. v1 original submission; v2 minor changes in
response to refere
Fatigue Damage Estimation from Random Vibration Testing: Application to a notched specimen
Vibrations are random in a wide range of applications and they are the main cause of mechanical failure. To prevent such failure, it is necessary to evaluate the fatigue life using test or analysis techniques. For computing the severity of the damage many methods are available in literature, but the estimation damage is just an approximation. The objective of this study is to propose a numerical model, together with experimental validation, in order to estimate fatigue damage caused by random vibrations in metallic materials undergoing uniaxial fatigue testing
Pulsar timing arrays and the challenge of massive black hole binary astrophysics
Pulsar timing arrays (PTAs) are designed to detect gravitational waves (GWs)
at nHz frequencies. The expected dominant signal is given by the superposition
of all waves emitted by the cosmological population of supermassive black hole
(SMBH) binaries. Such superposition creates an incoherent stochastic
background, on top of which particularly bright or nearby sources might be
individually resolved. In this contribution I describe the properties of the
expected GW signal, highlighting its dependence on the overall binary
population, the relation between SMBHs and their hosts, and their coupling with
the stellar and gaseous environment. I describe the status of current PTA
efforts, and prospect of future detection and SMBH binary astrophysics.Comment: 18 pages, 4 figures. To appear in the Proceedings of the 2014 Sant
Cugat Forum on Astrophysics. Astrophysics and Space Science Proceedings, ed.
C.Sopuerta (Berlin: Springer-Verlag
SDSSJ092712.65+294344.0: a candidate massive black hole binary
In this Letter we explore the hypothesis that the quasar
SDSSJ092712.65+294344.0 is hosting a massive black hole binary embedded in a
circumbinary disc. The lightest, secondary black hole is active, and gas
orbiting around it is responsible for the blue-shifted broad emission lines
with velocity off-set of 2650 km/s, relative to the galaxy rest frame. As the
tidal interaction of the binary with the outer disc is expected to excavate a
gap, the blue-shifted narrow emission lines are consistent with being emitted
from the low-density inhomogeneous gas of the hollow region. From the
observations we infer a binary mass ratio q ~ 0.3, a mass for the primary of M1
~ 2 billion Msun and a semi-major axis of 0.34 pc, corresponding to an orbital
period of 370 years. We use the results of cosmological merger trees to
estimate the likely-hood of observing SDSSJ092712.65+294344.0 as recoiling
black hole or as a binary. We find that the binary hypothesis is preferred
being one hundred times more probable than the ejection hypothesis. If
SDSSJ092712.65+294344.0 hosts a binary, it would be the one closest massive
black hole binary system ever discovered.Comment: Accepted for publication in MNRAS Letter
ACTIVE THERMOGRAPHY TECHNIQUE FOR FATIGUE DAMAGE CHARACTERIZATION IN GEARS
Active Thermography (AT) is a Non Destructive Technique (NDT) that may be an efficient
alternative to evaluate possible microstructural alterations inside materials due to damaging
conditions. In this paper, a fatigue damage identification on two different gears (standard and
thin-rim) was conducted by using an AT approach with a Lock-In technique. Both gears were
previously tested under bending fatigue conditions, by loading the teeth at the so called Single
Contact Point by a dedicated equipment. Damaged and undamaged zones were identified, phase
maps and thermal diffusivities were estimated. In this way, a possible fatigue damage
characterisation was pointed out by using the thermal diffusivity variation as damage parameter
From bright binaries to bumpy backgrounds: Mapping realistic gravitational wave skies with pulsar-timing arrays
Within the next several years, pulsar-timing array programs will likely usher in the next era of gravitational-wave astronomy through the detection of a stochastic background of nanohertz-frequency gravitational waves, originating from a cosmological population of inspiraling supermassive binary black holes. While the source positions will likely be isotropic to a good approximation, the gravitational-wave angular power distribution will be anisotropic, with the most massive and/or nearby binaries producing signals that may resound above the background. We study such a realistic angular power distribution, developing fast and accurate sky-mapping strategies to localize pixels and extended regions of excess power while simultaneously modeling the background signal from the less massive and more distant ensemble. We find that power anisotropy will be challenging to discriminate from isotropy for realistic gravitational-wave skies, requiring SNR >10 in order to favor anisotropy with 10:1 posterior odds in our case study. Amongst our techniques, modeling the population signal with multiple point sources in addition to an isotropic background provides the most physically motivated and easily interpreted maps, while spherical-harmonic modeling of the square-root power distribution, P(Ω)^(1/2), performs best in discriminating from overall isotropy. Our techniques are modular and easily incorporated into existing pulsar-timing array analysis pipelines
On the search of electromagnetic cosmological counterparts to coalescences of massive black hole binaries
We explore the nature of possible electromagnetic counterparts of
coalescences of massive black hole binaries at cosmological distances
detectable by the Laser Interferometer Space Antenna (LISA). An electromagnetic
precursor, during the last year of gravitational wave (GW)-driven inspiral, or
an afterglow within few years after coalescence, may highlight the position in
the sky of galaxies hosting LISA sources. We show that observations of
precursors and afterglows are mutually exclusive, depending on the mass of the
primary black hole. Precursors are expected to occur in binaries where the
primary (more massive) black hole is heavier than ~10^7 \Msun. They may
correspond to on-off states of accretion, i.e., to a bright X-ray source
decaying into quiescence before black hole coalescence, and are likely
associated to disturbed galaxies showing signs of ongoing starbursts.
Coalescences of lighter binaries, with masses <5x10^6 \Msun, lack of any
precursor, as gas is expected to be consumed long before the GW-driven orbital
decay. Such events would not be hosted by (massive) galaxies with an associated
starburst, given the slow binary inspiral time compared to the typical time
scale of starbursts. By contrast, coalescence, for such light binaries, is
followed by an electromagnetic afterglow, i.e., an off-on accretion state
rising in <20 yrs. Using a cosmological merger tree algorithm, we show that
future X-ray missions such as XEUS will be able to identify, in 20 yrs
operation, almost all the massive BH binary detectable by LISA, and, in only 5
yrs, all the LISA sources at z>6.Comment: 7 pages, 6 figures, MNRAS, in pres
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