ASCA Observation of MS 1603.6+2600 (=UW Coronae Borealis): a Dipping Low-Mass X-ray Binary in the Outer Halo?

Abstract

MS 1603.6+2600 is a high-latitude X-ray binary with a 111 min orbital period, thought to be either an unusual cataclysmic variable or an unusual low-mass X-ray binary. In an ASCA observation in 1997 August, we find a burst, whose light curve suggests a Type I (thermonuclear flash) origin. We also find an orbital X-ray modulation in MS 1603.6+2600, which is likely to be periodic dips, presumably due to azimuthal structure in the accretion disk. Both are consistent with this system being a normal low-mass X-ray binary harboring a neutron star, but at a great distance. We tentatively suggest that MS 1603.6+2600 is located in the outer halo of the Milky Way, perhaps associated with the globular cluster Palomar 14, 11 degrees away from MS 1603.6+2600 on the sky at an estimated distance of 73.8 kpc.