Radio observations of PSR B1259-63 through the 2004 periastron passage

We report here on extensive radio observations of the binary system containing PSR B1259-63 and the Be star SS 2883, made around the time of the 2004 periastron. This is the fourth periastron to have been observed in detail. As in previous observations, changes in the dispersion and rotation measures of the pulsar are detected over a period spanning 200 d. We show that the eclipse of the pulsed emission lasts from 16 d prior to periastron to 15 d after periastron and is consistent from one periastron to the next. We demonstrate that the timing solution proposed by Wang, Johnston & Manchester provides a good fit through the 2004 periastron. The light curve of the transient unpulsed radio emission is broadly similar from one periastron to the next. For this periastron, however, the light curve is strongly peaked post-periastron with rather little enhancement prior to periastron, in contrast to the 2000 periastron where the peak flux densities were more equal. These observations remain consistent with the interpretation that the pulsar passes through the dense circumstellar disc of the Be star just before and just after periastron. The observed differences from one periastron to the next can be ascribed to variations in the local disc density and magnetic field structure at the time the pulsar enters the disc.