X1908+075: A Pulsar Orbiting in the Stellar Wind of a Massive Companion

Abstract

We have observed the persistent but optically unidentified X-ray source X1908+075 with the Proportional Counter Array and High-Energy X-Ray Timing Experiment instruments on the Rossi X-Ray Timing Explorer (RXTE). The binary nature of this source was recently established by Wen et al., who found a 4.4 day orbital period in results from the RXTE All-Sky Monitor. We report the discovery of 605 s pulsations in the X-ray flux. The Doppler delay curve is measured and provides a mass function of 6.1 M☉, which is a lower limit to the mass of the binary companion of the neutron star. The degree of attenuation of the low-energy end of the spectrum is found to be a strong function of orbital phase. A simple model of absorption in a stellar wind from the companion star fits the orbital phase dependence reasonably well and limits the orbital inclination angle to the range 38°-72°. These measured parameters lead to an orbital separation of ~60-80 lt-s, a mass for the companion star in the range 9-31 M☉, and an upper limit to the size of the companion of ~22 R☉. From our analysis, we also infer a wind mass-loss rate from the companion star of ≳1.3 × 10-6 M☉ yr-1 and, when the properties of the companion star and the effects of photoionization are considered, likely ≳4 × 10-6 M☉ yr-1. Such a high rate is inconsistent with the allowed masses and radii that we find for a main-sequence or modestly evolved star unless the mass-loss rate is enhanced in the binary system relative to that of an isolated star. We discuss the possibility that the companion might be a Wolf-Rayet star that could evolve to become a black hole in 104 to 105 yr. If so, this would be the first identified progenitor of a neutron star-black hole binary.