Exploring Realistic Nanohertz Gravitationalwave Backgrounds
Abstract
Hundreds of millions of supermassive black hole binaries are expected to contribute to the gravitationalwave signal in the nanohertz frequency band. Their signal is often approximated either as an isotropic Gaussian stochastic background with a powerlaw spectrum or as an individual source corresponding to the brightest binary. In reality, the signal is best described as a combination of a stochastic background and a few of the brightest binaries modeled individually. We present a method that uses this approach to efficiently create realistic pulsar timing array data sets using synthetic catalogs of binaries based on the Illustris cosmological hydrodynamic simulation. We explore three different properties of such realistic backgrounds that could help distinguish them from those formed in the early universe: (i) their characteristic strain spectrum, (ii) their statistical isotropy, and (iii) the variance of their spatial correlations. We also investigate how the presence of confusion noise from a stochastic background affects detection prospects of individual binaries. We calculate signaltonoise ratios of the brightest binaries in different realizations for a simulated pulsar timing array based on the NANOGrav 12.5 yr data set extended to a time span of 15 yr. We find that ~6% of the realizations produce systems with signaltonoise ratios larger than 5, suggesting that individual systems might soon be detected (the fraction increases to ~41% at 20 yr). These can be taken as a pessimistic prediction for the upcoming NANOGrav 15 yr data set, since it does not include the effect of potentially improved timing solutions and newly added pulsars.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2022
 DOI:
 10.3847/15384357/aca1b2
 arXiv:
 arXiv:2207.01607
 Bibcode:
 2022ApJ...941..119B
 Keywords:

 Gravitational waves;
 Gravitational wave astronomy;
 Gravitational wave sources;
 Supermassive black holes;
 Millisecond pulsars;
 678;
 675;
 677;
 1663;
 1062;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology
 EPrint:
 18 pages, 16 figures, version matching published paper