2014MNRAS.441.2684M

Little observational data are available on the weak stellar winds of hot subdwarf stars of B spectral type (sdB). Close binary systems composed of an sdB star and a compact object (white dwarf, neutron star or black hole) could be detected as accretion-powered X-ray sources. The study of their X-ray emission can probe the properties of line-driven winds of sdB stars that cannot be derived directly from spectroscopy because of the low luminosity of these stars. Here we report on the first sensitive X-ray observations of two sdB binaries with compact companions. CD -30° 11223 is the sdB binary with the shortest known orbital period (1.2 h) and its companion is certainly a white dwarf. PG 1232-136 is an sdB binary considered the best candidate to host a black hole companion. We observed these stars with XMM-Newton in 2013 August for 50 ks and in 2009 July for 36 ks, respectively. None of them was detected and we derived luminosity upper limits of ∼ 1.5x10²⁹ erg/s for CD -30° 11223 and ∼ 5x10²⁹ erg/s for PG 1232-136. The corresponding mass-loss rate for PG 1232-136 is poorly constrained, owing to the unknown efficiency for black hole accretion. On the other hand, in the case of CD -30° 11223 we could derive, under reasonable assumptions, an upper limit of ∼ 3x10^–13M_☉/yr on the wind mass-loss rate from the sdB star. This is one of the few observational constraints on the weak winds expected in this class of low-mass hot stars. We also report the results on the X-ray emission from a cluster of galaxies serendipitously discovered in the field of CD -30° 11223.