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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
We observe that the periodic variations in spin-down rate and beam-width of
the radio pulsar PSR B1828-11 are getting faster. In the context of a free
precession model, this corresponds to a decrease in the precession period
$P_{\mathrm{fp}}$. We investigate how a precession model can account for such a
decrease in $P_{\mathrm{fp}}$, in terms of an increase over time in the
absolute biaxial deformation ($|\epsilon_{\mathrm{p}}|{\sim}10^{-8}$) of this
pulsar. We perform a Bayesian model comparison against the 'base' precession
model (with constant $\epsilon_{\mathrm{p}}$) developed in Ashton et al (2016),
and we obtain decisive odds in favour of a time-varying deformation. We study
two types of time-variation: (i) a linear drift with a posterior estimate of
$\dot{\epsilon}_{\mathrm{p}}{\sim}10^{-18}\,\mathrm{s}^{-1}$ and odds of
$10^{75}$ compared to the base-model, and (ii) $N$ discrete positive jumps in
$\epsilon_{\mathrm{p}}$ with very similar odds to the linear
$\epsilon_{\mathrm{p}}$-drift model. The physical mechanism explaining this
behaviour is unclear, but the observation could provide a crucial probe of the
interior physics of neutron stars. We also place an upper bound on the rate at
which the precessional motion is damped, and translate this into a bound on a
dissipative mutual friction-type coupling between the star's crust and core.
