The lack of X-ray pulsations in the extreme helium star BD+37°442 and its possible stellar wind X-ray emission

Abstract

We report the results of a new XMM-Newton observation of the helium-rich hot subdwarf BD+37°442 carried out in 2016 February. The possible periodicity at 19 s seen in a 2011 shorter observation is not confirmed, thus dismissing the evidence for a binary nature. This implies that the observed soft X-ray emission, with a luminosity of a few 10<SUP>31</SUP> erg s<SUP>-1</SUP>, originates in BD+37°442 itself, rather than in an accreting neutron star companion. The X-ray spectrum is well fit by thermal plasma emission with a temperature of 0.22 keV and non-solar element abundances. Besides the overabundance of He, C and N already known from optical/UV studies, the X-ray spectra indicate also a significant excess of Ne. The soft X-ray spectrum and the ratio of X-ray to bolometric luminosity, L<SUB>X</SUB>/L<SUB>BOL</SUB> ∼ 2 × 10<SUP>-7</SUP>, are similar to those observed in massive early-type stars. This indicates that the mechanisms responsible for plasma shock-heating can work also in the weak stellar winds (mass-loss rates dot{M}_W≤ 10^{-8} M<SUB>☉</SUB> yr<SUP>-1</SUP>) of low-mass hot stars.