Mon. Not. R. Astron. Soc., 441, 2684-2690 (2014/July-1)
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.
MEREGHETTI S., LA PALOMBARA N., ESPOSITO P., GASTALDELLO F., TIENGO A., HEBER U., GEIER S. and WILMS J.
Abstract (from CDS):
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.
© 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society (2014)
stars: individual: CD -30 11223, PG 1232-136 - subdwarfs - stars: winds, outflows - X-rays: binaries - X-rays: galaxies: clusters - X-rays: individual: XMMU J141057.7-305132
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