Radio emission from the high-mass X-ray binary BP Crucis

Context. BP Cru is a well known high-mass X-ray binary consisting of a late B hypergiant (Wray 977) and a neutron star, also observed as the X-ray pulsar GX 301-2. No information about emission from BP Cru in bands other than X-rays and optical has been reported. A massive X-ray binaries containing black holes can produce radio emission from a jet. Aims. To assess the presence of a radio jet, we searched for radio emission from BP Cru using the Australia Compact Array Telescope as part of a survey for radio emission from Be/X-ray transients. Methods. We probed the 41.5 d orbit of BP Cru with the Australia Telescope Compact Array, close to both periastron and apastron. Results. BP Cru was clearly detected in our data on 4, possibly 6, of 12 occasions at 4.8 and 8.6 GHz. Our data suggest that the spectral index of the radio emission is modulated either by the X-ray flux or the orbital phase of the system. Conclusions. We propose that the radio emission of BP Cru probably originates in two components: a persistent component, related to the mass donor Wray 977, and a periodic component, connected to accretion onto the neutron star, possibly originating from a (weak and short lived) jet.