Cosmography with strong lensing of LISA gravitational wave sources

LISA might detect gravitational waves from mergers of massive black hole binaries strongly lensed by intervening galaxies (Sereno et al. 2010). The detection of multiple gravitational lensing events would provide a new tool for cosmography. Constraints on cosmological parameters could be placed by exploiting either lensing statistics of strongly lensed sources or time delay measurements of lensed gravitational wave signals. These lensing methods do not need the measurement of the redshifts of the sources and the identification of their electromagnetic counterparts. They would extend cosmological probes to redshift z <= 10 and are then complementary to other lower or higher redshift tests, such as type Ia supernovae or cosmic microwave background. The accuracy of lensing tests strongly depends on the formation history of the merging binaries, and the related number of total detectable multiple images. Lensing amplification might also help to find the host galaxies. Any measurement of the source redshifts would allow to exploit the distance-redshift test in combination with lensing methods. Time-delay analyses might measure the Hubble parameter H_0 with accuracy of >= 10 km s^{-1}Mpc^{-1}. With prior knowledge of H_0, lensing statistics and time delays might constrain the dark matter density (delta Omega_M >= 0.08, due to parameter degeneracy). Inclusion of our methods with other available orthogonal techniques might significantly reduce the uncertainty contours for Omega_M and the dark energy equation of state.Comment: 10 pages, 10 figures, in press on MNRA

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 $f=1$yr$^{-1}$ drops from $1.3\times10^{-15}$ to $4\times10^{-16}$. 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

Life-cycle assessment of light steel frame buildings : A systematic literature review.

Light Steel Frame structures (LSF) have become one of the main competitors of traditional construction systems. The optimized material use, its lightness, and the timesaving in the construction phase, show the potential of this technology to reduce environmental impacts. The purpose of this study is to review and analyse the current literature on the application of the Life Cycle Assessment (LCA) methodology to LSF buildings and identify related gaps. A systematic literature review has been performed to query Web of Science and Scopus databases, highlighting methods, limitations, trends, and tools used to address LCA applied to LSF buildings. Although many efforts have been made to evaluate LSF buildings in comparison with other construction solutions, a gap persists in performing whole LCA. Considering the potential disassembly and reuse offered by LSF and the recyclability of steel, there is a need for future research focusing beyond the end-of-life stage

Cosmography with strong lensing of LISA gravitational wave sources

Laser Interferometer Space Antenna (LISA) might detect gravitational waves from mergers of massive black hole binaries strongly lensed by intervening galaxies. The detection of multiple gravitational lensing events would provide a new tool for cosmography. Constraints on cosmological parameters could be placed by exploiting either lensing statistics of strongly lensed sources or time-delay measurements of lensed gravitational wave signals. These lensing methods do not need the measurement of the redshifts of the sources and the identification of their electromagnetic counterparts. They would extend cosmological probes to redshift z≲ 10 and are then complementary to other lower or higher redshift tests, such as Type Ia supernovae or cosmic microwave background. The accuracy of lensing tests strongly depends on the formation history of the merging binaries, and the related number of total detectable multiple images. Lensing amplification might also help to find the host galaxies. Any measurement of the source redshifts would allow us to exploit the distance-redshift test in combination with lensing methods. Time-delay analyses might measure the Hubble parameter H0 with an accuracy of ≳10 km s-1 Mpc−1. With prior knowledge of H0, lensing statistics and time delays might constrain the dark matter density (δΩM≳ 0.08, due to parameter degeneracy). Inclusion of our methods with other available orthogonal techniques might significantly reduce the uncertainty contours for ΩM and the dark energy equation of stat

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

Low-frequency gravitational radiation from coalescing massive black hole binaries in hierarchical cosmologies

We compute the expected gravitational wave signal from coalescing massive black hole (MBH) binaries at the center of galaxies in a hierarchical structure formation scenario in which seed holes of intermediate mass form far up in the dark halo merger tree. The merger history of DM halos and MBHs is followed from z=20 to the present in a LCDM cosmology. MBHs get incorporated through halo mergers into larger and larger structures, sink to the center owing to dynamical friction against the DM background, accrete cold material in the merger remnant, and form MBH binary systems. Stellar dynamical interactions cause the hardening of the binary at large separations, while gravitational wave emission takes over at small radii and leads to the final coalescence of the pair. The integrated emission from inspiraling MBH binaries results in a gravitational wave background (GWB). The characteristic strain spectrum has the standard h_c(f)\propto f^{-2/3} behavior only in the range 1E-9<f<1E-6 Hz. At lower frequencies the orbital decay of MBH binaries is driven by the ejection of background stars, and h_c(f) \propto f. At higher frequencies, f>1E-6 Hz, the strain amplitude is shaped by the convolution of last stable circular orbit emission. We discuss the observability of inspiraling MBH binaries by the planned LISA. Over a 3-year observing period LISA should resolve this GWB into discrete sources, detecting ~60 (~250) individual events above a S/N=5 (S/N=1) confidence level. (Abridged)Comment: 11 pages, 8 figues. Revised version accepted to be published in ApJ Discussion on number counts corrected and expande

Gravitational waves from resolvable massive black hole binary systems and observations with Pulsar Timing Arrays

Massive black holes are key components of the assembly and evolution of cosmic structures and a number of surveys are currently on-going or planned to probe the demographics of these objects and to gain insight into the relevant physical processes. Pulsar Timing Arrays (PTAs) currently provide the only means to observe gravitational radiation from massive black hole binary systems with masses >10^7 solar masses. The whole cosmic population produces a stochastic background that could be detectable with upcoming Pulsar Timing Arrays. Sources sufficiently close and/or massive generate gravitational radiation that significantly exceeds the level of the background and could be individually resolved. We consider a wide range of massive black hole binary assembly scenarios, we investigate the distribution of the main physical parameters of the sources, such as masses and redshift, and explore the consequences for Pulsar Timing Arrays observations. Depending on the specific massive black hole population model, we estimate that on average at least one resolvable source produces timing residuals in the range ~5-50 ns. Pulsar Timing Arrays, and in particular the future Square Kilometre Array (SKA), can plausibly detect these unique systems, although the events are likely to be rare. These observations would naturally complement on the high-mass end of the massive black hole distribution function future surveys carried out by the Laser Interferometer Space Antenna (LISA)Comment: 12 pages, 10 figures, accepted for publication in MNRAS. Results revised (differences within a factor of two) after a bug in the code for generating the timing residuals has been fixe

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