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

Abstract

We observe that the periodic variations in spin-down rate and beamwidth 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 Pfp. We investigate how a precession model can account for such a decrease in Pfp, in terms of an increase over time in the absolute biaxial deformation (|єp| ∼ 10−8) of this pulsar. We perform a Bayesian model comparison against the 'base' precession model (with constant єp) developed in Ashton et al., 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 єp∼10−18 s−1 and odds of 1075 compared to the base model, and (ii) N discrete positive jumps in єp with very similar odds to the linear єp 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.