An X-ray variable millisecond pulsar in the globular cluster 47 tucanae: Closing the link to low-mass X-ray binaries

We report the discovery of peculiar X-ray spectral variability in the binary radio millisecond pulsar PSR J0024 - 7204W in the globular cluster 47 Tuc. The observed emission consists of a dominant nonthermal component, which is eclipsed for a portion of the orbit, and a thermal component, which appears to be persistent. We propose that the nonthermal X-rays originate in a relativistic intrabinary shock, formed due to interaction between the relativistic particle wind from the pulsar and matter from the main-sequence companion star, while the thermal photons are from the heated magnetic polar caps of the millisecond pulsar. At optical wavelengths, the emission exhibits large-amplitude variations at the orbital period, which can be attributed to heating of one side of the tidally locked secondary star by the pulsar wind. The observed X-ray and optical properties of PSR J0024 - 7204W are remarkably similar to those of the low-mass X-ray binary and X-ray millisecond pulsar SAX J1808.4 - 3658 in quiescence. This supports the conjecture that the nonthermal X-ray emission and optical modulations seen in the SAX J1808.4 - 3658 system in a quiescent state are due to interaction between the wind from a reactivated rotation-powered pulsar and matter from the companion star. The striking similarities between the two systems provide support for the long-sought connection between millisecond radio pulsars and accreting neutron star systems.