489 research outputs found
Deep Surveys of Massive Black Holes with LISA
Massive black hole binary systems are among the most interesting sources for
the Laser Interferometer Space Antenna (LISA); gravitational radiation emitted
during the last year of in-spiral could be detectable with a very large
signal-to-noise ratio for sources at cosmological distance. Here we discuss the
impact of LISA for astronomy and cosmology; we review our present understanding
of the relevant issues, and highlight open problems that deserve further
investigations.Comment: 10 pages, 2 figures, Third Amaldi Conference on Gravitational Wave
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.
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
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
Null stream analysis of Pulsar Timing Array data: localisation of resolvable gravitational wave sources
Super-massive black hole binaries are expected to produce a GW signal in the
nano-Hertz frequency band which may be detected by PTAs in the coming years.
The signal is composed of both stochastic and individually resolvable
components. Here we develop a generic Bayesian method for the analysis of
resolvable sources based on the construction of `null-streams' which cancel the
part of the signal held in common for each pulsar (the Earth-term). For an
array of pulsars there are independent null-streams that cancel the
GW signal from a particular sky location. This method is applied to the
localisation of quasi-circular binaries undergoing adiabatic inspiral. We carry
out a systematic investigation of the scaling of the localisation accuracy with
signal strength and number of pulsars in the PTA. Additionally, we find that
source sky localisation with the International PTA data release one is vastly
superior than what is achieved by its constituent regional PTAs.Comment: 13 pages, 7 figures, 1 appendix. Edited Figures 5, 6, 7 due to a bug
in the plotting script (results unchanged). Additional edit to fix a type in
equation
On tests of general relativity with binary radio pulsars
The timing of radio pulsars in binary systems provides a superb testing
ground of general relativity. Here we propose a Bayesian approach to carry out
these tests, and a relevant efficient numerical implementation, that has
several conceptual and practical advantages with respect to traditional methods
based on least-square-fits that have been used so far: (i) it accounts for the
actual structure of the likelihood function - and it is not predicated on the
Laplace approximation which is implicitly built in least-square fits that can
potentially bias the inference - (ii) it provides the ratio of the evidences of
any two models under consideration as the statistical quantity to compare
different theories, and (iii) it allows us to put joint constraints from the
monitoring of multiple systems, that can be expressed in terms of ratio of
evidences or probability intervals of global (thus not system-dependent)
parameters of the theory, if any exists. Our proposed approach optimally
exploits the progress in timing of radio pulsars and the increase in the number
of observed systems. We demonstrate the power of this framework using simulated
data sets that are representative of current observations.Comment: Accepted for publication on MNRAS Letter
Searching for continuous gravitational wave sources in binary systems
We consider the problem of searching for continuous gravitational wave
sources orbiting a companion object. This issue is of particular interest
because the LMXB's, and among them Sco X-1, might be marginally detectable with
2 years coherent observation time by the Earth-based laser interferometers
expected to come on line by 2002, and clearly observable by the second
generation of detectors. Moreover, several radio pulsars, which could be deemed
to be CW sources, are found to orbit a companion star or planet, and the
LIGO/VIRGO/GEO network plans to continuously monitor such systems. We estimate
the computational costs for a search launched over the additional five
parameters describing generic elliptical orbits using match filtering
techniques. These techniques provide the optimal signal-to-noise ratio and also
a very clear and transparent theoretical framework. We provide ready-to-use
analytical expressions for the number of templates required to carry out the
searches in the astrophysically relevant regions of the parameter space, and
how the computational cost scales with the ranges of the parameters. We also
determine the critical accuracy to which a particular parameter must be known,
so that no search is needed for it. In order to disentangle the computational
burden involved in the orbital motion of the CW source, from the other source
parameters (position in the sky and spin-down), and reduce the complexity of
the analysis, we assume that the source is monochromatic and its location in
the sky is exactly known. The orbital elements, on the other hand, are either
assumed to be completely unknown or only partly known. We apply our theoretical
analysis to Sco X-1 and the neutron stars with binary companions which are
listed in the radio pulsar catalogue.Comment: 31 pages, LaTeX, 6 eps figures, submitted to PR
Markov chain Monte Carlo searches for Galactic binaries in Mock LISA Data Challenge 1B data sets
We are developing a Bayesian approach based on Markov chain Monte Carlo
techniques to search for and extract information about white dwarf binary
systems with the Laser Interferometer Space Antenna (LISA). Here we present
results obtained by applying an initial implementation of this method to some
of the data sets released in Round 1B of the Mock LISA Data Challenges. For
Challenges 1B.1.1a and 1b the signals were recovered with parameters lying
within the 95.5% posterior probability interval and the correlation between the
true and recovered waveform is in excess of 99%. Results were not submitted for
Challenge 1B.1.1c due to some convergence problems of the algorithms, despite
this, the signal was detected in a search over a 2 mHz band.Comment: 11 pages, 5 figures, 12th GWDAW (Gravitational Wave Data Analysis
Workshop). Accepted for publication in CQ
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