2004/04/30 by Alan M. Levine, A. M. Levine, S. Rappaport +4
Physics and Astronomy · #Astrophysical Phenomena and Observations #Gamma-ray bursts and supernovae #Pulsars and Gravitational Waves Research #astro-ph
paper · pdf · doi:10.1086/425567
published as Astrophys.J. 617 (2004) 1284-1295 · 16 pages, 11 Postscript figures (some different from those in original version), heavily revised second version includes extended stellar wind model analysis, accepted by ApJ
arxiv created 2004/09/03 · openalex publication_date 2004/12/18 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
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 10 4 to 10 5 yr. If so, this would be the first identified progenitor of a neutron star-black hole binary.