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1. Fresnel models for gravitational wave effects on pulsar timing

   https://doi.org/10.1093/mnras/stab1417  

   McGrath, Casey; Creighton, Jolien (June 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Merging supermassive black hole binaries produce low-frequency gravitational waves, which pulsar timing experiments are searching for. Much of the current theory is developed within the plane-wave formalism, and here we develop the more general Fresnel formalism. We show that Fresnel corrections to gravitational wave timing residual models allow novel measurements to be made, such as direct measurements of the source distance from the timing residual phase and frequency, as well as direct measurements of chirp mass from a monochromatic source. Probing the Fresnel corrections in these models will require future pulsar timing arrays with more distant pulsars across our Galaxy (ideally at the distance of the Magellanic Clouds), timed with precisions less than 100 ns, with distance uncertainties reduced to the order of the gravitational wavelength. We find that sources with chirp mass of order 109 M⊙ and orbital frequency ω0 > 10 nHz are good candidates for probing Fresnel corrections. With these conditions met, the measured source distance uncertainty can be made less than 10 per cent of the distance to the source for sources out to ∼100 Mpc, source sky localization can be reduced to sub-arcminute precision, and source volume localization can be made to less than 1 Mpc3 for sources out to 1-Gpc distances. 

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2. 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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3. The SKAO Pulsar Timing Array

   https://doi.org/10.33232/001c.154243  

   Shannon, Ryan M; Bhat, N_D Ramesh; Chalueau, Aurelien; Chen, Siyuan; Cromartie, H Thankful; Gopukumar, A; Grunthal, Kathrin; Hazboun, Jeffrey S; Iraci, Francesco; Joshi, Bhal Chandra; et al (January 2025, The Open Journal of Astrophysics)

   Pulsar timing arrays (PTAs) are ensembles of millisecond pulsars observed for years to decades. The primary goal of PTAs is to study gravitational-wave astronomy at nanohertz frequencies, with secondary goals of undertaking other fundamental tests of physics and astronomy. Recently, compelling evidence has emerged in established PTA experiments for the presence of a gravitational-wave background. To accelerate a confident detection of such a signal and then study gravitational-wave emitting sources, it is necessary to observe a larger number of millisecond pulsars to greater timing precision. The SKA telescopes, which will be a factor of three to four greater in sensitivity compared to any other southern hemisphere facility, is poised to make such an impact. In this chapter, we motivate an SKAO pulsar timing array (SKAO PTA) experiment. We discuss the classes of gravitational waves present in PTA observations and how an SKAO PTA can detect and study them. We then describe the sources that can produce these signals. We discuss the astrophysical noise sources that must be mitigated to undertake the most sensitive searches. We then describe a realistic PTA experiment implemented with the SKA and place it in context alongside other PTA experiments likely ongoing in the 2030s. We describe the techniques necessary to search for gravitational waves in the SKAO PTA and motivate how very long baseline interferometry can improve the sensitivity of an SKAO PTA. The SKAO PTA will provide a view of the Universe complementary to those of the other large facilities of the 2030s. 

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4. A sensitivity curve approach to tuning a pulsar timing array in the detection era

   https://doi.org/10.1088/1361-6382/adbbab  

   Baier, Jeremy_G; Hazboun, Jeffrey; Romano, Joseph (February 2025, Classical and Quantum Gravity)

   Abstract As pulsar timing arrays (PTAs) transition into the detection era of the stochastic gravitational wave background (GWB), it is important for PTA collaborations to review and possibly revise their observing campaigns. The detection of a “single source” would be a boon for gravitational astrophysics, as such a source would emit gravitational waves for millions of years in the PTA frequency band. Here we present generic methods for studying the effects of various observational strategies, taking advantage of detector sensitivity curves, i.e., noise-averaged, frequency-domain detection statistics. The statistical basis for these methods is presented along with myriad examples of how to tune a detector towards single, deterministic signals or a stochastic background. We demonstrate that trading observations of the worst pulsars for high cadence campaigns on the best pulsars increases sensitivity to single sources at high frequencies while hedging losses in GWB and single source sensitivity at low frequencies. We also find that sky-targeted observing campaigns yield minimal sensitivity improvements compared with other PTA tuning options. Lastly, we show the importance of the uncorrelated half of the GWB, i.e. the pulsar-term, as an increasingly prominent sources of noise and show the impact of this emerging noise source on various PTA configurations. 

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5. 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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