Radio emission from the high-mass X-ray binary BP Cru: first detection

TL;DR

This study reports the first detection of radio emission from the high-mass X-ray binary BP Cru, revealing potential jet activity and orbital modulation of the radio signals, which enhances understanding of emission mechanisms in such systems.

Contribution

It provides the first observational evidence of radio emission from BP Cru, suggesting the presence of a jet and orbital phase-dependent variability in a high-mass X-ray binary.

Findings

Detected radio emission at 4.8 and 8.6 GHz on multiple occasions.

Radio spectral index varies with X-ray flux and orbital phase.

Radio emission likely originates from both the donor star and a possible jet.

Abstract

BP Cru is a well known high-mass X-ray binary composed 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 other bands than X-rays and optical has been reported to date in the literature, though massive X-ray binaries containing black holes can have radio emission from a jet. In order to assess the presence of a radio jet, we searched for radio emission towards BP Cru using the Australia Compact Array Telescope during a survey for radio emission from Be/X-ray transients. We probed the 41.5d orbit of BP Cru with the Australia Telescope Compact Array not only close to periastron but also close to apastron. 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. We propose that the radio emission of BP Cru probably arises from two components: a persistent component, coming from the mass donor Wray 977, and a periodic component connected to the accretion onto the neutron star, possibly coming from a (weak and short lived) jet.