Precession of the isolated neutron star PSR B1828-11

Stairs, Lyne & Shemar have found that the arrival-time residuals from PSR B1828- 11 vary periodically with a period approximate to 500 d. This behaviour can be accounted for by precession of the radio pulsar, an interpretation that is reinforced by the detection of variations in its pulse profile on the same time-scale. Here, we model the period residuals from PSR B1828- 11 in terms of precession of a triaxial rigid body. We include two contributions to the residuals: ( i) the geometric effect, which arises because the times at which the pulsar emission beam points towards the observer varies with precession phase; and (ii) the spin-down contribution, which arises from any dependence of the spin-down torque acting on the pulsar on the angle between its spin ((Omega) over cap) and magnetic ((b) over cap) axes. We use the data to probe numerous properties of the pulsar, most notably its shape, and the dependence of its spin-down torque on (Omega) over cap . (b) over cap, for which we assume the sum of a spin-aligned component ( with a weight 1 - a) and a dipolar component perpendicular to the magnetic beam axis ( weight a), rather than the vacuum dipole torque ( a = 1). We find that a variety of shapes are consistent with the residuals, with a slight statistical preference for a prolate star. Moreover, a range of torque possibilities fit the data equally well, with no strong preference for the vacuum model. In the case of a prolate star, we find evidence for an angle-dependent spin-down torque. Our results show that the combination of geometrical and spin-down effects associated with precession can account for the principal features of the timing behaviour of PSR B1828- 11, without fine tuning of the parameters.