Constraints on the winds of hot subdwarf stars from X-ray observations of two sdB binaries with compact companions: CD -30 11223 and PG 1232-136

摘要

Little observational data are available on the weak stellar winds of hotsubdwarf stars of B spectral type (sdB). Close binary systems composed of ansdB star and a compact object (white dwarf, neutron star or black hole) couldbe detected as accretion-powered X-ray sources. The study of their X-rayemission can probe the properties of line-driven winds of sdB stars that cannot be derived directly from spectroscopy because of the low luminosity ofthese stars. Here we report on the first sensitive X-ray observations of twosdB binaries with compact companions. CD -30 11223 is the sdB binary with theshortest known orbital period (1.2 h) and its companion is certainly a whitedwarf. PG 1232-136 is an sdB binary considered the best candidate to host ablack hole companion. We observed these stars with XMM-Newton in August 2013for 50 ks and in July 2009 for 36 ks, respectively. None of them was detectedand we derived luminosity upper limits of about 1.5x10^29 erg/s for CD -3011223 5x10^29 erg/s for PG 1232-136. The corresponding mass loss rate for PG1232-136 is poorly constrained, owing to the unknown efficiency for black holeaccretion. On the other hand, in the case of CD -30 11223 we could derive,under reasonable assumptions, an upper limit of about 3x10^-13 solar masses/yron the wind mass loss rate from the sdB star. This is one of the fewobservational constraints on the weak winds expected in this class of low masshot stars. We also report the results on the X-ray emission from a cluster ofgalaxies serendipitously discovered in the field of CD -30 11223.