Observations and Modeling of Relativistic Spin Precession in PSR J1141–6545

Observations of the binary pulsar PSR J1141-6545 using the Parkes radio telescope over 9.3 yr show clear time variations in pulse width, shape, and polarization. We interpret these variations in terms of relativistic precession of the pulsar spin axis about the total angular momentum vector of the system changing our view of the emission beam. Over those nine years, the pulse width at the 50% level has changed by more than a factor of 3, reaching a maximum value of nearly 13 deg. in early 2007. Large variations have also been observed in the 1400 MHz mean flux density; this reached a peak of {approx}20 mJy in mid-2002 but over the past several years has been relatively steady at {approx}3 mJy. The pulse polarization has been monitored since 2004 April using digital filterbank systems and also shows large and systematic variations in both linear and circular polarization. Position angle (P.A.) variations, both across the pulse profile and over the data span, are complex, with major differences between the central and outer parts of the pulse profile. We interpret the outer parts as representing the underlying magnetic field and fit the rotating-vector model to these regions. Modeling of the observed P.A. variations by relativistic precession of the pulsar spin axis shows that the spin-orbit misalignment angle is about 110 deg. and that the precessional phase has passed through 180 deg. during the course of our observations. At the start of our observations, the line-of-sight impact parameter was about 4 deg. in magnitude and it reached a minimum very close to 0 deg. around early 2007, consistent with the observed pulse width variations. We have therefore mapped approximately one half of the emission beam, at least out to a radius of about 4 deg., showing that it is very asymmetric with respect to the magnetic axis. The derived precessional parameters imply that the pre-supernova (pre-SN) star had a mass of about 2 M{sub sun} and that the SN recoil kick velocity was relatively small, between 100 and 250 km s{sup -1}, depending on the assumed systemic velocity. With the reversal in the rate of change of the impact parameter, we predict that over the next decade we will see a reversed 'replay' of the variations observed in the past decade.