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

TL;DR

This study presents XMM-Newton observations of the helium-rich star BD+37°442, showing no pulsations and suggesting its X-ray emission originates from stellar wind shock-heating rather than a binary companion.

Contribution

It provides the first detailed X-ray spectral analysis of BD+37°442, revealing plasma properties and wind-related emission mechanisms in a low-mass hot star.

Findings

No confirmed 19 s periodicity, ruling out a binary neutron star companion.

X-ray spectrum fits thermal plasma with 0.22 keV temperature and element overabundances.

X-ray luminosity and spectrum resemble those of massive early-type stars.

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}$442 itself, rather than in an accreting neutron star companion. The X-ray spectrum is well fit by thermal plasma emission with a temperature of 0.22 keV and non-solar element abundances. Besides the overabundance of He, C and N already known from optical/UV studies, the X-ray spectra indicate also a significant excess of Ne. The soft X-ray spectrum and the ratio of X-ray to bolometric luminosity, L$_{\rm X}$/L$_{\rm BOL}\sim2\times10^{-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.