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 $N$ pulsars there are $N-2$ 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