Abstract
Abstract We analyze the archival XMM-Newton X-ray observations of 15 γ Cas analogue stars and two candidates for such objects. The EPIC spectra of the considered stars in the range of 0.2-8 keV were extracted and fitted by different models. Our estimates show that assuming the X-ray emission from γ Cas analogues to be totally thermal, their model plasma temperatures can reach anomalously high values. However including an additional power components to the model spectra leads to significant decreasing of the plasma temperatures. The spectral index of the power component is about 1.5, and the fraction of this in the total model flux is rather large (50-90%). Moreover, it decreases with expanding temperature of the X-ray emitting plasma as compared to typical OB stars. We conclude that γ Cas analogues can produce nonthermal X-ray emission within the framework of the Chen & White (1991) model, while if the nonthermal X-rays from typical OB stars exists, they should be generated by different processes.
Highlights
The γ Cas analogues are enigmatic Oe and Be stars with very strong X-ray emission and X-ray luminosities up to LX ∼ 1032 erg s−1
We revealed a decreases of the fraction of power components FPL of the model spectra of γ Cas analogues with growing plasma temperature unlike for typical OB stars (Ryspaeva & Kholtygin, 2020)
At the same time in the model spectra with an additional power component the spectral index G∼1-2, the power law (PL) component dominates at hard energies
Summary
The γ Cas analogues are enigmatic Oe and Be stars with very strong X-ray emission and X-ray luminosities up to LX ∼ 1032 erg s−1. Supposing that the X-ray emitting plasma is thermal, its temperature can reach 10-20 keV and more These values are higher than those for typical OB stars but smaller than typical temperatures for High Mass X-ray Binaries (HMXB). Later Ryspaeva & Kholtygin (2020) proposed that γ Cas analogues can be the sources of nonthermal X-rays, which are formed in a framework of Chen & White (1991) theory. In such case X-ray emission is “produced through the inverse Compton mechanism when the stellar UV photons are scattered by relativistic electrons accelerated by shocks in the stellar wind near the star”. This is a quote from Chen & White (1991) paper
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