The lack of X-ray pulsations in the extreme helium star BD+37${}^{\circ }$442 and its possible stellar wind X-ray emission

Abstract

We report the results of a new \textit{XMM-Newton} observation of the helium-rich hot subdwarf BD+37${}^{\circ }$442 carried out in February 2016. The possible periodicity at 19 s seen in a 2011 shorter observation is not confirmed, thus dismissing the evidence for a binary nature. This implies that the observed soft X-ray emission, with a luminosity of a few 10${}^{31}$ ergs${}^{-1}$, originates in BD+37${}^{\circ }$442 itself, rather than in an accreting neutron star companion. The X-ray spectrum is well fit by thermal plasma emission with a temperature of 0.22 keV and non-solar element abundances. Besides the overabundance of He, C and N already known from optical/UV studies, the X-ray spectra indicate also a significant excess of Ne. The soft X-ray spectrum and the ratio of X-ray to bolometric luminosity, L${}_{\mathrm{X}}$/L${}_{\mathrm{BOL}}\sim 2\times 10^{-7}$, 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 ${\dot{M}}_{\mathrm{W}}\le 10^{-8}$ ${\mathrm{M}}_{\odot }$ yr${}^{-1}$) of low-mass hot stars.

arXiv comment4 pages, 3 figures, 1 table. Accepted for publication in Monthly Notices of the Royal Astronomical Society

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Cited by in corpus (2)

• TESS photometry of helium-rich hot subdwarfs: r modes in BD+37 442 and BD+37 1977
• Follow-up observations of X-ray emitting hot subdwarf stars: the compact He-poor sdO star Feige 34