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Free keywords:
Astrophysics, Instrumentation and Methods for Astrophysics, astro-ph.IM, Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE, Astrophysics, Solar and Stellar Astrophysics, astro-ph.SR,General Relativity and Quantum Cosmology, gr-qc
Abstract:
We analyse the stochastic properties of the 49 pulsars that comprise the
first International Pulsar Timing Array (IPTA) data release. We use Bayesian
methodology, performing model selection to determine the optimal description of
the stochastic signals present in each pulsar. In addition to spin-noise and
dispersion-measure (DM) variations, these models can include timing noise
unique to a single observing system, or frequency band. We show the improved
radio-frequency coverage and presence of overlapping data from different
observing systems in the IPTA data set enables us to separate both system and
band-dependent effects with much greater efficacy than in the individual PTA
data sets. For example, we show that PSR J1643$-$1224 has, in addition to DM
variations, significant band-dependent noise that is coherent between PTAs
which we interpret as coming from time-variable scattering or refraction in the
ionised interstellar medium. Failing to model these different contributions
appropriately can dramatically alter the astrophysical interpretation of the
stochastic signals observed in the residuals. In some cases, the spectral
exponent of the spin noise signal can vary from 1.6 to 4 depending upon the
model, which has direct implications for the long-term sensitivity of the
pulsar to a stochastic gravitational-wave (GW) background. By using a more
appropriate model, however, we can greatly improve a pulsar's sensitivity to
GWs. For example, including system and band-dependent signals in the PSR
J0437$-$4715 data set improves the upper limit on a fiducial GW background by
$\sim 60\%$ compared to a model that includes DM variations and spin-noise
only.
