Follow-up observations of X-ray emitting hot subdwarf stars: the He-rich sdO BD +37

We report on the results of the first XMM-Newton satellite observation of the luminous and helium–rich O–type subdwarf BD +37° 1977 carried out in April 2014. X-ray emission is detected with a flux of about 4 × 10-14 erg cm-2 s-1 (0.2−1.5 keV), corresponding to a fX/fbol ratio ~ 10-7; the source spectrum is very soft, and is well fit by the sum of two plasma components at different temperatures. Both characteristics are in agreement with what is observed in the main-sequence early-type stars, where the observed X-ray emission is due to turbulence and shocks in the stellar wind. A smaller but still significant stellar wind has also been observed in BD +37° 1977; therefore, we suggest that in this case the detected X-ray flux has the same origin