On the free-precession candidate PSR B1828-11: Evidence for increasing deformation

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 ($|ε_{\mathrm{p}}|{\sim}10^{-8}$) of this pulsar. We perform a Bayesian model comparison against the 'base' precession model (with constant $ε_{\mathrm{p}}$) 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 $\dotε_{\mathrm{p}}{\sim}10^{-18}\,\mathrm{s}^{-1}$ and odds of $10^{75}$ compared to the base-model, and (ii) $N$ discrete positive jumps in $ε_{\mathrm{p}}$ with very similar odds to the linear $ε_{\mathrm{p}}$-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.

14 pages, 12 figures. Updated to incorporate referee comments, including new appendix discussing differences in contributions from the two data sets employed. Accepted by MNRAS. (Identical to v2 aside from this comment line)