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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE,Astrophysics, Cosmology and Extragalactic Astrophysics, astro-ph.CO, Astrophysics, Galaxy Astrophysics, astro-ph.GA
Abstract:
We present the results of the search for an isotropic stochastic
gravitational wave background (GWB) at nanohertz frequencies using the second
data release of the European Pulsar Timing Array (EPTA) for 25 millisecond
pulsars and a combination with the first data release of the Indian Pulsar
Timing Array (InPTA). We analysed (i) the full 24.7-year EPTA data set, (ii)
its 10.3-year subset based on modern observing systems, (iii) the combination
of the full data set with the first data release of the InPTA for ten commonly
timed millisecond pulsars, and (iv) the combination of the 10.3-year subset
with the InPTA data. These combinations allowed us to probe the contributions
of instrumental noise and interstellar propagation effects. With the full data
set, we find marginal evidence for a GWB, with a Bayes factor of four and a
false alarm probability of $4\%$. With the 10.3-year subset, we report evidence
for a GWB, with a Bayes factor of $60$ and a false alarm probability of about
$0.1\%$ ($\gtrsim 3\sigma$ significance). The addition of the InPTA data yields
results that are broadly consistent with the EPTA-only data sets, with the
benefit of better noise modelling. Analyses were performed with different data
processing pipelines to test the consistency of the results from independent
software packages. The inferred spectrum from the latest EPTA data from new
generation observing systems is rather uncertain and in mild tension with the
common signal measured in the full data set. However, if the spectral index is
fixed at 13/3, the two data sets give a similar amplitude of
($2.5\pm0.7)\times10^{-15}$ at a reference frequency of $1\,{\rm yr}^{-1}$. By
continuing our detection efforts as part of the International Pulsar Timing
Array (IPTA), we expect to be able to improve the measurement of spatial
correlations and better characterise this signal in the coming years.
