MS 1603.6+2600, AN UNUSUAL X-RAY SELECTED BINARY-SYSTEM AT HIGH GALACTIC LATITUDE

We describe the discovery of an eclipsing binary system at Galactic latitude 47°, found as a serendipitous X-ray source in the Einstein Extended Medium Sensitivity Survey. The object has X-ray flux 1.1 × 10-12 ergs cm-2 s-1 (0.3-3.5 keV) and mean magnitude R ∼ 19.4. An orbital period of 111 minutes is found. We discuss whether the system has a white dwarf or neutron star primary, in the end preferring the neutron star primary model. If the system has either optical or X-ray luminosities typical of low mass X-ray binaries (LMXB), it must be at a very large distance (30-80 kpc). Blueshifted He I absorption is seen, indicating cool outflowing material, similar to that seen in the LMXB AC 211 in the globular cluster M15.