Probing dark matter with X-ray binaries

Low-mass X-ray binaries (LMXBs), which occur in old stellar populations, have velocities exceeding those of their parent distribution by at least 20 km/s. This makes them ideal probes for dark matter, in particular in dwarf spheroidals (dSph), where the LMXBs should penetrate well outside the visible galaxy. We argue that the most likely explanation of the observation of LMXBs in the Sculptor dSph by Maccarone et al (2005) is the presence of a dark matter halo of >~ 10^9 Msun, corresponding to a total-mass to light ratio of >~ 600 (M/L_V)_sun. In this case there should be an extended halo of LMXBs which may be observable.