Single sources in the low-frequency gravitational wave sky: properties and time to detection by pulsar timing arrays

We calculate the properties, occurrence rates and detection prospects of individually resolvable 'single sources' in the low-frequency gravitational wave (GW) spectrum. Our simulations use the population of galaxies and massive black hole binaries from the Illustris cosmological hydrodynamic simulations, coupled to comprehensive semi-analytic models of the binary merger process. Using mock pulsar timing arrays (PTA) with, for the first time, varying red-noise models, we calculate plausible detection prospects for GW single sources and the stochasticGWbackground (GWB). Contrary to previous results,we find that single sources are at least as detectable as theGWbackground. Using mock PTA, we find that these 'foreground' sources (also 'deterministic'/'continuous') are likely to be detected with~20 yr total observing baselines. Detection prospects, and indeed the overall properties of single sources, are only moderately sensitive to binary evolution parameters - namely eccentricity and environmental coupling, which can lead to differences of ~5 yr in times to detection. Red noise has a stronger effect, roughly doubling the time to detection of the foreground between a white-noise only model (~10-15 yr) and severe red noise (~20-30 yr). The effect of red noise on the GWB is even stronger, suggesting that single source detections may be more robust. We find that typical signal-to-noise ratios for the foreground peak near f = 0.1 yr-1, and are much less sensitive to the continued addition of new pulsars to PTA.