Search for X-ray emission from subdwarf B stars with compact companion candidates

Stellar evolutionary models predict that most early-type subdwarf stars in close binary systems have white dwarf companions. More massive companions, such as neutron stars or black holes, are also expected in some cases. Compact stars in these systems can be detected by means of X-rays powered by either accretion of the subdwarf stellar wind or surface thermal emission. Using the Swift satellite, we carried out a systematic search for X-ray emission from twelve subdwarf B stars that, based on optical studies, appear to have degenerate companions. None of our targets were detected, but the derived upper limits provide one of the few observational constraints on the stellar winds of early-type subdwarfs. Our results constrain the mass loss rates of some of these subdwarf B stars to values. M W < 10(-13) -10(-12) M-circle dot yr(-1), if indeed they have neutron star companions.