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

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
e-mail: sandro@iasf-milano.inaf.it
² INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
³ INAF, Osservatorio Astronomico di Cagliari, loc. Poggio dei Pini, strada 54, 09012 Capoterra (CA), Italy
⁴ IUSS, Istituto Universitario di Studi Superiori, viale Lungo Ticino Sforza 56, 27100 Pavia, Italy

Received: 7 September 2011
Accepted: 24 October 2011

Abstract

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 Ṁ_W < 10^-13−10^-12   M_⊙ yr^-1, if indeed they have neutron star companions.