Dr Christopher BerryI am excited for the upcoming news from pulsar timing arrays on their search for #GravitationalWaves. Results from International Pulsar Timing Array teams will be shared at 0:00 UTC. I don't know what these will be. However… 1/🧵
In 2015 this paper by
Taylor et al. predicted that the first detection of gravitational waves was only a few years away, with an 80% chance of a detection by ~2025
https://arxiv.org/abs/1511.05564
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Are we there yet? Time to detection of nanohertz gravitational waves based on pulsar-timing array limits
Decade-long timing observations of arrays of millisecond pulsars have placed highly constraining upper limits on the amplitude of the nanohertz gravitational-wave stochastic signal from the mergers of supermassive black-hole binaries ($\sim 10^{-15}$ strain at $f = 1/\mathrm{yr}$). These limits suggest that binary merger rates have been overestimated, or that environmental influences from nuclear gas or stars accelerate orbital decay, reducing the gravitational-wave signal at the lowest, most sensitive frequencies. This prompts the question whether nanohertz gravitational waves are likely to be detected in the near future. In this letter, we answer this question quantitatively using simple statistical estimates, deriving the range of true signal amplitudes that are compatible with current upper limits, and computing expected detection probabilities as a function of observation time. We conclude that small arrays consisting of the pulsars with the least timing noise, which yield the tightest upper limits, have discouraging prospects of making a detection in the next two decades. By contrast, we find large arrays are crucial to detection because the quadrupolar spatial correlations induced by gravitational waves can be well sampled by many pulsar pairs. Indeed, timing programs which monitor a large and expanding set of pulsars have an $\sim 80\%$ probability of detecting gravitational waves within the next ten years, under assumptions on merger rates and environmental influences ranging from optimistic to conservative. Even in the extreme case where $90\%$ of binaries stall before merger and environmental coupling effects diminish low-frequency gravitational-wave power, detection is delayed by at most a few years.
Recent NANOGrav results include an excess of noise. You only need one pulsar with no noise to confirm a non-detection of gravitational waves. However, when there is something extra, you need to look at the angular correlation across many to confirm a signal. This is where the pulsar timing arrays come in
If the latest data allow the teams to see the correlation, we might have a detection!
Coincidentally, a detection checklist was put out earlier this year
https://arxiv.org/abs/2304.04767
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The International Pulsar Timing Array checklist for the detection of nanohertz gravitational waves
Pulsar timing arrays (PTAs) provide a way to detect gravitational waves at nanohertz frequencies. In this band, the most likely signals are stochastic, with a power spectrum that rises steeply at lower frequencies. Indeed, the observation of a common red noise process in pulsar-timing data suggests that the first credible detection of nanohertz-frequency gravitational waves could take place within the next few years. The detection process is complicated by the nature of the signals and the noise: the first observational claims will be statistical inferences drawn at the threshold of detectability. To demonstrate that gravitational waves are creating some of the noise in the pulsar-timing data sets, observations must exhibit the Hellings and Downs curve -- the angular correlation function associated with gravitational waves -- as well as demonstrating that there are no other reasonable explanations. To ensure that detection claims are credible, the International Pulsar Timing Array (IPTA) has a formal process to vet results prior to publication. This includes internal sharing of data and processing pipelines between different PTAs, enabling independent cross-checks and validation of results. To oversee and validate any detection claim, the IPTA has also created an eight-member Detection Committee (DC) which includes four independent external members. IPTA members will only publish their results after a formal review process has concluded. This document is the initial DC checklist, describing some of the conditions that should be fulfilled by a credible detection. At the present time none of the PTAs have a detection claim; therefore this document serves as a road map for the future.
What would a first detection from pulsar timing arrays be? The most likely source are supermassive black hole binaries. This paper by Rosado et al. suggest a background on unresolved sources is more probable than individual resolved ones
https://arxiv.org/abs/1503.04803
What could we learn from the detection of a gravitational-wave background? This paper by Middleton et al. gives hints
https://arxiv.org/abs/1707.00623
We would likely measure the total merger rate of supermassive black hole binaries
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Expected properties of the first gravitational wave signal detected with pulsar timing arrays
In this paper we attempt to investigate the nature of the first gravitational wave (GW) signal to be detected by pulsar timing arrays (PTAs): will it be an individual, resolved supermassive black hole binary (SBHB), or a stochastic background made by the superposition of GWs produced by an ensemble of SBHBs? To address this issue, we analyse a broad set of simulations of the cosmological population of SBHBs, that cover the entire parameter space allowed by current electromagnetic observations in an unbiased way. For each simulation, we construct the expected GW signal and identify the loudest individual sources. We then employ appropriate detection statistics to evaluate the relative probability of detecting each type of source as a function of time for a variety of PTAs; we consider the current International PTA, and speculate into the era of the Square Kilometre Array. The main properties of the first detectable individual SBHBs are also investigated. Contrary to previous work, we cast our results in terms of the detection probability (DP), since the commonly adopted criterion based on a signal-to-noise ratio threshold is statistic-dependent and may result in misleading conclusions for the statistics adopted here. Our results confirm quantitatively that a stochastic signal is more likely to be detected first (with between 75 to 93 per cent probability, depending on the array), but the DP of single-sources is not negligible. Our framework is very flexible and can be easily extended to more realistic arrays and to signal models including environmental coupling and SBHB eccentricity.
scabux@cplberry A livestream will be available here: https://nanograv.org/news/2023Announcement
Looking forward to it!
