Follow-up observations of X-ray emitting hot subdwarf stars: the compact He-poor sdO star Feige 34. (arXiv:1905.00682v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Palombara_N/0/1/0/all/0/1">N. La Palombara</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Mereghetti_S/0/1/0/all/0/1">S. Mereghetti</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Esposito_P/0/1/0/all/0/1">P. Esposito</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Tiengo_A/0/1/0/all/0/1">A. Tiengo</a> (1,2,3) (1 – INAF/IASF Milano, Italy, 2 – IUSS Pavia, Italy, 3 – INFN Pavia, Italy)
We report on results obtained with the XMM-Newton observation of Feige 34
carried out in April 2018. This is the first spectroscopic X-ray observation of
a compact and helium-poor hot subdwarf star. The source was detected at a flux
level $f_{rm X}$ = 3.4$times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the energy
range 0.2-3 keV, which implies an X-ray-to-bolometric flux ratio $f_{rm
X}/f_{rm bol} simeq 10^{-6.5}$. The source spectrum can be described with the
sum of two thermal-plasma components with subsolar abundances at temperatures
of $simeq$ 0.3 and 1.1 keV. These properties are similar to what is observed
in early-type main-sequence stars, where the X-ray emission is attributed to
turbulence and shocks in the stellar wind. Therefore, the same phenomenon could
explain the X-ray properties of Feige 34. However, it is not possible to
reproduce the observed spectrum with a thermal-plasma model if the elemental
abundances are fixed at the values obtained from the optical and UV
spectroscopy. Moreover, we show that the X-ray luminosity and spectrum are
consistent with those expected from a young main-sequence star of late spectral
type. Therefore, we discuss the possibility that the observed X-ray emission is
due to the companion star of M0 spectral type, whose presence is suggested by
the IR excess in the spectral energy distribution of Feige 34.
We report on results obtained with the XMM-Newton observation of Feige 34
carried out in April 2018. This is the first spectroscopic X-ray observation of
a compact and helium-poor hot subdwarf star. The source was detected at a flux
level $f_{rm X}$ = 3.4$times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the energy
range 0.2-3 keV, which implies an X-ray-to-bolometric flux ratio $f_{rm
X}/f_{rm bol} simeq 10^{-6.5}$. The source spectrum can be described with the
sum of two thermal-plasma components with subsolar abundances at temperatures
of $simeq$ 0.3 and 1.1 keV. These properties are similar to what is observed
in early-type main-sequence stars, where the X-ray emission is attributed to
turbulence and shocks in the stellar wind. Therefore, the same phenomenon could
explain the X-ray properties of Feige 34. However, it is not possible to
reproduce the observed spectrum with a thermal-plasma model if the elemental
abundances are fixed at the values obtained from the optical and UV
spectroscopy. Moreover, we show that the X-ray luminosity and spectrum are
consistent with those expected from a young main-sequence star of late spectral
type. Therefore, we discuss the possibility that the observed X-ray emission is
due to the companion star of M0 spectral type, whose presence is suggested by
the IR excess in the spectral energy distribution of Feige 34.
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