PSR J1740-3052 - a pulsar with a massive companion

We report on the discovery of a binary pulsar, PSR J1740-3052, during the Parkes multibeam pulsar survey. The pulsar has a spin period of 570 ms and is in a 231-day, highly eccentric orbit with a companion whose mass exceeds 11 M_sun. An accurate position for the pulsar was obtained using the Australia Telescope Compact Array. Timing observations at Jodrell Bank and Parkes show that the pulsar is young, having a characteristic age of 350 kyr, and unrecycled. Dual-frequency radio observations of the pulsar near periastron passage suggest that the dispersion measure and polarisation vary slightly but significantly during the orbit. These variations suggest a non-degenerate companion with a magnetized stellar wind. No significant X-ray emission was found in an archival ASCA observation two weeks after periastron passage. Near-infrared 2.2-um observations at the Siding Spring Observatory 2.3-m telescope and the 3.9-m Anglo-Australian Telescope reveal a late-type star coincident with the pulsar position. However, we do not believe that this star is the pulsar's companion, because a typical star of this mass and spectral type would extend beyond the pulsar's orbit. Furthermore, the measured advance of periastron of the pulsar is not significantly larger than that expected for a compact companion in general relativity, arguing for a main-sequence companion with radius only a few times that of the sun. Finally, the small dispersion measure variations are also consistent with a main-sequence companion, but inconsistent with the large mass-loss rates expected for a late-type supergiant. Although we cannot conclusively rule out a black-hole companion, we believe the true companion is most likely a main-sequence B star hidden by the light of the unassociated late-type star.