X 1908+075: an X-ray Binary with a 4.4 Day Period

X 1908+075 is an optically unidentified and highly absorbed X-ray source that appears in early surveys such as Uhuru, OSO-7, Ariel V, HEAO-1, and the EXOSAT Galactic Plane Survey. These surveys measured a source intensity in the range of 2–12 mCrab at 2–10 keV, and the position was localized to ∼ 0.5. We use the Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) to confirm our expectation that a particular Einstein IPC detection (1E 1908.4+0730) provides the correct position for X 1908+075. The analysis of the coded mask shadows from the ASM for the position of 1E 1908.4+0730 yields a persistent intensity ∼ 8 mCrab (1.5–12 keV) over a 3 year interval beginning in 1996 February. Furthermore, we detect a period of 4.400 ± 0.001 days with a false alarm probability < 10−7. The folded light curve is roughly sinusoidal, with an amplitude that is 22% of the mean flux. The X-ray period may be attributed to the scattering and absorption of X-rays through a stellar wind combined with the orbital motion in a binary system. We suggest that X 1908+075 is an X-ray binary with a high mass companion star. Subject headings: binaries: general — stars: individual (X 1908+075) — X-rays: stars