Gravitational waves from resolvable massive black hole binary systems and observations with Pulsar Timing Arrays (0809.3412v2)

Abstract: 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)

• The paper demonstrates that under specific MBHB formation scenarios, PTAs can resolve individual systems with timing residuals between 5–50 ns.
• It uses Monte Carlo simulations and galaxy merger catalogs from the Millennium Simulation to model black hole binary demographics.
• The findings highlight the potential for future arrays like the SKA to improve gravitational wave detection and deepen our cosmological insights.

Gravitational Waves from Massive Black Hole Binary Systems: Observational Prospects with Pulsar Timing Arrays

The paper by Sesana et al. explores the compelling arena of gravitational wave (GW) detection, particularly focusing on massive black hole binary systems (MBHBs) and the utilization of Pulsar Timing Arrays (PTAs) as a means of observation. The authors investigate various scenarios of MBHB assembly, aiming to understand the demographic distribution of these cosmic entities and the potential for resolving individual systems through the timing residuals observed in pulsars.

Observational Challenges and Opportunities

PTAs, which use radio pulsar signals to detect GWs at low frequencies, present a unique opportunity to observe massive black holes, particularly those forming binary systems. These systems are anticipated to emit continuous gravitational waves in the nanohertz regime, an area where PTAs have significant sensitivity. The paper suggests that sources which are nearby and sufficiently massive can generate GW signals that stand out against the stochastic background, thus allowing their individual resolution.

The stochastic background, produced by the cumulative effect of MBHBs across cosmic distances, poses both a challenge and an opportunity for PTAs. Detecting this background could provide a wealth of information about the population of massive black holes, while resolving individual signals offers insight into specific systems that contribute significantly above this background.

Results and Implications

The paper’s analysis indicates that, depending on the MBHB formation models considered, PTAs can expect to resolve approximately one resolvable system producing timing residuals between ∼5-50 ns. This finding is particularly relevant for future advances such as the Square Kilometer Array (SKA), which promises enhanced sensitivity and the ability to detect rarer and more distant systems than currently possible.

The results underscore the importance of chirp mass and redshift. High chirp mass systems (>5×10⁸) are among the most likely candidates for individual resolution, and most resolved systems are anticipated to be located at higher redshifts (0.2 < z < 1.5), providing a window into the universe at cosmological distances.

Statistical Modeling and Uncertainties

The authors employ a detailed Monte-Carlo simulation approach to explore a diverse landscape of MBHB formation scenarios, utilizing data from galaxy merger catalogs derived from the Millennium Simulation. These models are pivotal in addressing some of the unresolved questions pertaining to cosmic MBH and galaxy evolution and contribute to our understanding of the various dynamical processes governing MBHB formation and mergers.

Given the substantial uncertainties inherent in these model parameters, the spread in predictions underscores the need for caution in interpreting observational prospects. Despite these challenges, the exploration highlights the transformative potential of PTAs in constraining MBHB demographics and advancing our understanding of their evolution.

Future Directions

The ability of PTAs to detect both the collective stochastic GW background and individual MBHB signals opens exciting vistas for future research. Beyond resolving individual systems, PTAs may significantly enhance our understanding of high-mass ends of black hole distribution functions, the physics of galaxy mergers, and our comprehension of the dynamical processes underpinning MBH pair formation.

The insights gleaned from this paper form a foundational basis for continued theoretical and observational pursuit of gravitational waves from MBHBs, complementing future missions like the Laser Interferometer Space Antenna (LISA). These endeavors promise to enrich our cosmological grasp and our understanding of fundamental astrophysical dynamics.