Transient radio emission from the PSR B1259-63 system near periastron

Observations of the pulsar-Be-star binary system PSR B1259-63 around the 19 94 and 1997 periastron encounters, using the Australia Telescope Compact Ar ray at four frequencies between 1.4 and 8.4 GHz, and the Molonglo Observato ry Synthesis Telescope at 843 MHz, show a relatively strong and rapidly var ying but unpulsed radio source. The source appears 22 d before periastron a nd lasts for more than 100 d. Over most of the event the spectral index is similar to -0.5, indicating an optically thin synchrotron source. The peak flux of similar to 50 mJy at 1 GHz occurs about 20 d after periastron. Ther e are strong similarities in the light curves of the 1994 and 1997 periastr on passages. The radio light curve shows day-to-day variability and varies by a factor of more than 100 over the observation interval. This is in cont rast to the X-ray emission which varies by only a factor of 2 close to peri astron. The emission is interpreted as synchrotron emission from relativist ic electrons accelerated in shocks at the interface between the pulsar wind and the Be-star emission-line disc.