Follow-up observations of X-ray emitting hot subdwarf star: the He-rich sdO BD +37{\deg} 1977

TL;DR

This study reports the first X-ray detection of the helium-rich hot subdwarf star BD +37° 1977 using XMM-Newton, revealing soft X-ray emission likely originating from stellar wind turbulence and shocks.

Contribution

First XMM-Newton observation of BD +37° 1977 demonstrating X-ray emission from a helium-rich hot subdwarf star and linking it to stellar wind activity.

Findings

X-ray flux detected at ~4×10^(-14) erg/cm^2/s

X-ray spectrum is very soft, fit by two plasma components

X-ray emission likely due to stellar wind turbulence and shocks

Abstract

We report on the results of the first XMM-Newton satellite observation of the luminous and helium-rich O-type subdwarf BD +37{\deg} 1977 carried out in April 2014. X-ray emission is detected with a flux of about 4*10^(-14) erg/cm2/s (0.2-1.5 keV), corresponding to a f_X/f_bol ratio about 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 been observed also in BD +37{\deg} 1977; therefore, we suggest that also in this case the detected X-ray flux has the same origin.