372 research outputs found
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
Testing the binary hypothesis: pulsar timing constraints on supermassive black hole binary candidates
The advent of time domain astronomy is revolutionizing our understanding of
the Universe. Programs such as the Catalina Real-time Transient Survey (CRTS)
or the Palomar Transient Factory (PTF) surveyed millions of objects for several
years, allowing variability studies on large statistical samples. The
inspection of 250k quasars in CRTS resulted in a catalogue of 111
potentially periodic sources, put forward as supermassive black hole binary
(SMBHB) candidates. A similar investigation on PTF data yielded 33 candidates
from a sample of 35k quasars. Working under the SMBHB hypothesis, we
compute the implied SMBHB merger rate and we use it to construct the expected
gravitational wave background (GWB) at nano-Hz frequencies, probed by pulsar
timing arrays (PTAs). After correcting for incompleteness and assuming virial
mass estimates, we find that the GWB implied by the CRTS sample exceeds the
current most stringent PTA upper limits by almost an order of magnitude. After
further correcting for the implicit bias in virial mass measurements, the
implied GWB drops significantly but is still in tension with the most stringent
PTA upper limits. Similar results hold for the PTF sample. Bayesian model
selection shows that the null hypothesis (whereby the candidates are false
positives) is preferred over the binary hypothesis at about and
for the CRTS and PTF samples respectively. Although not decisive,
our analysis highlights the potential of PTAs as astrophysical probes of
individual SMBHB candidates and indicates that the CRTS and PTF samples are
likely contaminated by several false positives.Comment: 14 pages, 11 figures, 3 tables. Resubmitted to the Astrophysical
Journal after some major revision of the results including a proper estimate
of the intrinsic mass of the binary candidate
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
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
Mergers of luminous early-type galaxies in the local universe and gravitational wave background
Supermassive black hole (SMBH) coalescence in galaxy mergers is believed to
be one of the primary sources of very low frequency gravitational waves (GWs).
Significant contribution of the GWs comes from mergers of massive galaxies with
redshifts z<2. Very few previous studies gave the merger rate of massive
galaxies. % We selected a large sample (1209) of close pairs of galaxies with
projected separations 7<r_p<50 kpc from 87,889 luminous early-type galaxies
(M_r<-21.5) from the Sloan Digital Sky Survey Data Release 6. These pairs
constitute a complete volume-limited sample in the local universe (z<0.12).
Using our newly developed technique, 249 mergers have been identified by
searching for interaction features. From them, we found that the merger
fraction of luminous early-type galaxies is 0.8%, and the merger rate in the
local universe is % R_g=(1.0+/-0.4)*10^{-5} Mpc^{-3} Gyr^{-1}} % with an
uncertainty mainly depending on the merging timescale. % We estimated the
masses of SMBHs in the centers of merging galaxies based on their luminosities.
We found that the chirp mass distribution of the SMBH binaries follows a power
law with an index of -3.0+/-0.5 in the range 5*10^8--5*10^{9} M_{\odot}. %
Using the SMBH population in the mergers and assuming that the SMBHs can be
efficiently driven into the GW regime, we investigated the stochastic GW
background in the frequency range 10^{-9}--10^{-7} Hz. We obtained the spectrum
of the GW background of h_c(f)=10^{-15}(f/yr^{-1})^{-2/3}, which is one
magnitude higher than that obtained by Jaffe & Backer in 2003, but consistent
with those calculated from galaxy-formation models.Comment: 27 pages, 9 figures, Corrected typos and reference
Ozone in Lombardy: Years 1998-1999
Photochemical pollutants, especially ozone, have reached very high levels in Lombardy in recent years, with peaks of up to 150 ppb in late spring and summer. Lombardy, lying on the Po Plain, supports a large number of cities and
industries and these, along with heavy traffic, produce copious amounts of primary pollutants such as nitrogen oxides and numerous volatile organic compounds. Furthermore,
the peculiar orography of this region fosters the stagnation of air masses on a basin-scale and the presence of diurnal breezes towards northern areas, along with the evolution of the Mixing Layer, spread the polluted air masses over a large territory. Numerous stations in Lombardy give the concentrations of ozone and of nitrogen oxides. In this paper, ozone measurements carried out at the plain area around Milan and at pre-alpine sites in the spring and summer 1998 and 1999 will be shown and discussed, focusing on the months of May and July. The study of temporal and spatial behaviour of ozone goes hand in hand with the analysis of the Boundary Layerâs evolution. A number of radon stations were operating in Milan and in other sites in Lombardy. Measurements of atmospheric concentrations of radon yield an index of atmospheric stability, of the formation of thermal inversion, of convective turbulence, and of the movement of air masses, and hence they are very relevant to the understanding of the conditions of atmospheric pollutants
The Challenges in Gravitational Wave Astronomy for Space-Based Detectors
The Gravitational Wave (GW) universe contains a wealth of sources which, with
the proper treatment, will open up the universe as never before. By observing
massive black hole binaries to high redshifts, we should begin to explore the
formation process of seed black holes and track galactic evolution to the
present day. Observations of extreme mass ratio inspirals will allow us to
explore galactic centers in the local universe, as well as providing tests of
General Relativity and constraining the value of Hubble's constant. The
detection of compact binaries in our own galaxy may allow us to model stellar
evolution in the Milky Way. Finally, the detection of cosmic (super)strings and
a stochastic background would help us to constrain cosmological models.
However, all of this depends on our ability to not only resolve sources and
carry out parameter estimation, but also on our ability to define an optimal
data analysis strategy. In this presentation, I will examine the challenges
that lie ahead in GW astronomy for the ESA L3 Cosmic Vision mission, eLISA.Comment: 12 pages. Plenary presentation to appear in the Proceedings of the
Sant Cugat Forum on Astrophysics, Sant Cugat, April 22-25, 201
Quality over Quantity: Optimizing pulsar timing array analysis for stochastic and continuous gravitational wave signals
The search for gravitational waves using Pulsar Timing Arrays (PTAs) is acomputationally expensive complex analysis that involves source-specific noisestudies. As more pulsars are added to the arrays, this stage of PTA analysiswill become increasingly challenging. Therefore, optimizing the number ofincluded pulsars is crucial to reduce the computational burden of dataanalysis. Here, we present a suite of methods to rank pulsars for use withinthe scope of PTA analysis. First, we use the maximization of thesignal-to-noise ratio as a proxy to select pulsars. With this method, we targetthe detection of stochastic and continuous gravitational wave signals. Next, wepresent a ranking that minimizes the coupling between spatial correlationsignatures, namely monopolar, dipolar, and Hellings & Downs correlations.Finally, we also explore how to combine these two methods. We test theseapproaches against mock data using frequentist and Bayesian hypothesis testing.For equal-noise pulsars, we find that an optimal selection leads to an increasein the log-Bayes factor two times steeper than a random selection for thehypothesis test of a gravitational wave background versus a common uncorrelatedred noise process. For the same test but for a realistic EPTA dataset, a subsetof 25 pulsars selected out of 40 can provide a log-likelihood ratio that is of the total, implying that an optimally selected subset of pulsars canyield results comparable to those obtained from the whole array. We expectthese selection methods to play a crucial role in future PTA data combinations.<br
The spin-flip phenomenon in supermassive black hole binary mergers
Massive merging black holes will be the primary sources of powerful
gravitational waves at low frequency, and will permit to test general
relativity with candidate galaxies close to a binary black hole merger. In this
paper we identify the typical mass ratio of the two black holes but then show
that the distance when gravitational radiation becomes the dominant dissipative
effect (over dynamical friction) does not depend on the mass ratio. However the
dynamical evolution in the gravitational wave emission regime does. For the
typical range of mass ratios the final stage of the merger is preceded by a
rapid precession and a subsequent spin-flip of the main black hole. This
already occurs in the inspiral phase, therefore can be described analytically
by post-Newtonian techniques. We then identify the radio galaxies with a
super-disk as those in which the rapidly precessing jet produces effectively a
powerful wind, entraining the environmental gas to produce the appearance of a
thick disk. These specific galaxies are thus candidates for a merger of two
black holes to happen in the astronomically near future.Comment: v3: 36 pages, 1 figure; discussion on the validity of the model and
estimates for the angular value of the spin-flip added to Section 5; v4:
minor changes, 2 new references, published versio
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