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1. Multi-messenger Approaches to Supermassive Black Hole Binary Detection and Parameter Estimation. II. Optimal Strategies for a Pulsar Timing Array

   https://doi.org/10.3847/1538-4357/acb492  

   Liu, Tingting; Cohen, Tyler; McGrath, Casey; Demorest, Paul B.; Vigeland, Sarah J. (March 2023, The Astrophysical Journal)

   Abstract Pulsar timing arrays (PTAs) are Galactic-scale gravitational wave (GW) detectors consisting of precisely timed pulsars distributed across the sky. Within the decade, PTAs are expected to detect nanohertz GWs emitted by close-separation supermassive black hole binaries (SMBHBs), thereby opening up the low-frequency end of the GW spectrum for science. Individual SMBHBs which power active galactic nuclei are also promising multi-messenger sources; they may be identified via theoretically predicted electromagnetic (EM) signatures and be followed up by PTAs for GW observations. In this work, we study the detection and parameter estimation prospects of a PTA which targets EM-selected SMBHBs. Adopting a simulated Galactic millisecond pulsar population, we envisage three different pulsar timing campaigns which observe three mock sources at different sky locations. We find that an all-sky PTA which times the best pulsars is an optimal and feasible approach to observe EM-selected SMBHBs and measure their source parameters to high precision (i.e., comparable to or better than conventional EM measurements). We discuss the implications of our findings in the context of future PTA experiments with the planned Deep Synoptic Array-2000 and the multi-messenger studies of SMBHBs such as the well-known binary candidate OJ 287. 

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2. The NANOGrav 15 yr Data Set: Looking for Signs of Discreteness in the Gravitational-wave Background

   https://doi.org/10.3847/1538-4357/ad93d5  

   Agazie, Gabriella; Anumarlapudi, Akash; Archibald, Anne_M; Arzoumanian, Zaven; Baier, Jeremy_George; Baker, Paul_T; Bécsy, Bence; Blecha, Laura; Brazier, Adam; Brook, Paul_R; et al (December 2024, The Astrophysical Journal)

   Abstract The cosmic merger history of supermassive black hole binaries (SMBHBs) is expected to produce a low-frequency gravitational wave background (GWB). Here we investigate how signs of the discrete nature of this GWB can manifest in pulsar timing arrays (PTAs) through excursions from, and breaks in, the expected $f_{\mathrm{GW}}^{-2/3}$power law of the GWB strain spectrum. To do this, we create a semianalytic SMBHB population model, fit to North American Nanohertz Observatory for Gravitational Waves (NANOGrav’s) 15 yr GWB amplitude, and with 1000 realizations, we study the populations’ characteristic strain and residual spectra. Comparing our models to the NANOGrav 15 yr spectrum, we find two interesting excursions from the power law. The first, at 2 nHz, is below our GWB realizations with ap-value significancep= 0.05–0.06 (≈1.8σ–1.9σ). The second, at 16 nHz, is above our GWB realizations withp= 0.04–0.15 (≈1.4σ–2.1σ). We explore the properties of a loud SMBHB that could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by 3 orders of magnitude, from ∼10⁶to ∼10³, between 2 and 20 nHz. This causes a break in the strain spectrum as the stochasticity of the background breaks down at ${26}_{-19}^{+28}\mathrm{nHz}$, consistent with predictions pre-dating GWB measurements. The diminished GWB signal from SMBHBs at frequencies above the 26 nHz break opens a window for PTAs to detect continuous GWs from individual SMBHBs or GWs from the early Universe. 

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3. Seeing the gravitational wave universe

   https://doi.org/10.1126/science.abq1187  

   Mingarelli, Chiara M.; Casey-Clyde, J. Andrew (November 2022, Science)

   Gravitational waves are ripples in the fabric of spacetime that are caused by events such as the merging of black holes. In principle, many types of events occur that could create gravitational waves with frequencies ranging from as high as a few kilohertz to as low as a few nanohertz. Sources of gravitational waves in the nanohertz frequency range include cosmic strings, quantum fluctuations from the early Universe, and, notably, supermassive black hole binaries (SMBHBs). Some gravitational wave sources are so numerous that they are all expected to contribute to a gravitational wave background (GWB). This GWB has been the target of pulsar timing arrays (PTAs) for decades. 

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4. A Quasar-based Supermassive Black Hole Binary Population Model: Implications for the Gravitational Wave Background

   https://doi.org/10.3847/1538-4357/ac32de  

   Casey-Clyde, J. Andrew; Mingarelli, Chiara M. F.; Greene, Jenny E.; Pardo, Kris; Nañez, Morgan; Goulding, Andy D. (January 2022, The Astrophysical Journal)

   Abstract The nanohertz gravitational wave background (GWB) is believed to be dominated by GW emission from supermassive black hole binaries (SMBHBs). Observations of several dual-active galactic nuclei (AGN) strongly suggest a link between AGN and SMBHBs, given that these dual-AGN systems will eventually form bound binary pairs. Here we develop an exploratory SMBHB population model based on empirically constrained quasar populations, allowing us to decompose the GWB amplitude into an underlying distribution of SMBH masses, SMBHB number density, and volume enclosing the GWB. Our approach also allows us to self-consistently predict the number of local SMBHB systems from the GWB amplitude. Interestingly, we find the local number density of SMBHBs implied by the common-process signal in the NANOGrav 12.5-yr data set to be roughly five times larger than previously predicted by other models. We also find that at most ∼25% of SMBHBs can be associated with quasars. Furthermore, our quasar-based approach predicts ≳95% of the GWB signal comes fromz≲ 2.5, and that SMBHBs contributing to the GWB have masses ≳10⁸M_⊙. We also explore how different empirical galaxy–black hole scaling relations affect the local number density of GW sources, and find that relations predicting more massive black holes decrease the local number density of SMBHBs. Overall, our results point to the important role that a measurement of the GWB will play in directly constraining the cosmic population of SMBHBs, as well as their connections to quasars and galaxy mergers. 

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5. Forecasting Pulsar Timing Array Sensitivity to Anisotropy in the Stochastic Gravitational Wave Background

   https://doi.org/10.3847/1538-4357/ac9836  

   Pol, Nihan; Taylor, Stephen R.; Romano, Joseph D. (December 2022, The Astrophysical Journal)

   Abstract Statistical anisotropy in the nanohertz-frequency gravitational wave background (GWB) is expected to be detected by pulsar timing arrays (PTAs) in the near future. By developing a frequentist statistical framework that intrinsically restricts the GWB power to be positive, we establish scaling relations for multipole-dependent anisotropy decision thresholds that are a function of the noise properties, timing baselines, and cadences of the pulsars in a PTA. We verify that (i) a larger number of pulsars, and (ii) factors that lead to lower uncertainty on spatial cross-correlation measurements between pulsars, lead to a higher overall GWB signal-to-noise ratio, and lower anisotropy decision thresholds with which to reject the null hypothesis of isotropy. Using conservative simulations of realistic NANOGrav data sets, we predict that an anisotropic GWB with angular power C l =1 > 0.3 C l =0 may be sufficient to produce tension with isotropy at the p = 3 × 10 −3 (∼3 σ ) level in near-future NANOGrav data with a 20 yr baseline. We present ready-to-use scaling relationships that can map these thresholds to any number of pulsars, configuration of pulsar noise properties, or sky coverage. We discuss how PTAs can improve the detection prospects for anisotropy, as well as how our methods can be adapted for more versatile searches. 

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