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تسجيل مستخدم جديدThe lack of X-ray pulsations in the extreme helium star BD+37$^{circ}$442 and its possible stellar wind X-ray emission
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We report the results of a new textit{XMM-Newton} observation of the helium-rich hot subdwarf BD+37$^{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}$442 itself, rather than in an accreting neutron star companion. The X-ray spectrum is well fit by thermal plasma emission with a temperature of 0.22 keV and non-solar element abundances. Besides the overabundance of He, C and N already known from optical/UV studies, the X-ray spectra indicate also a significant excess of Ne. The soft X-ray spectrum and the ratio of X-ray to bolometric luminosity, L$_{rm X}$/L$_{rm BOL}sim2times10^{-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 $dot M_{rm W} leq10^{-8}$ ${rm M}_{odot}$ yr$^{-1}$) of low-mass hot stars.
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