非表示:
キーワード:
Astrophysics, Cosmology and Extragalactic Astrophysics, astro-ph.CO, Astrophysics, Instrumentation and Methods for Astrophysics, astro-ph.IM
要旨:
We present new limits on an isotropic stochastic gravitational-wave
background (GWB) using a six pulsar dataset spanning 18 yr of observations from
the 2015 European Pulsar Timing Array data release (Desvignes et al. in prep.).
Performing a Bayesian analysis, we fit simultaneously for the intrinsic noise
parameters for each pulsar in this dataset, along with common correlated
signals including clock, and Solar System ephemeris errors to obtain a robust
95$\%$ upper limit on the dimensionless strain amplitude $A$ of the background
of $A<3.0\times 10^{-15}$ at a reference frequency of $1\mathrm{yr^{-1}}$ and a
spectral index of $13/3$, corresponding to a background from inspiralling
super-massive black hole binaries, constraining the GW energy density to
$\Omega_\mathrm{gw}(f)h^2 < 1.1\times10^{-9}$ at 2.8 nHz. We show that
performing such an analysis when fixing the intrinsic noise parameters for the
individual pulsars leads to an erroneously stringent upper limit, by a factor
$\sim 1.7$. We obtain a difference in the logarithm of the Bayesian evidence
between models that include either a correlated background, or uncorrelated
common red noise of $-1.0 \pm 0.5$, indicating no support for the presence of a
correlated GWB in this dataset. We discuss the implications of our analysis for
the astrophysics of supermassive black hole binaries, and present 95$\%$ upper
limits on the string tension, $G\mu/c^2$, characterising a background produced
by a cosmic string network for a set of possible scenarios, and for a
stochastic relic GWB. For a Nambu-Goto field theory cosmic string network, we
set a limit $G\mu/c^2<1.3\times10^{-7}$, identical to that set by the Planck
Collaboration, combining Planck and high-$\ell$ Cosmic Microwave Background
data from other experiments. (Abridged)
