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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE,General Relativity and Quantum Cosmology, gr-qc
Abstract:
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.
