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تسجيل مستخدم جديدX-ray diagnostics of chemical composition of the accretion disk and donor star in ultra-compact X-ray binaries
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Non-solar composition of the donor star in ultra-compact X-ray binaries may have a pronounced effect on the fluorescent lines appearing in their spectra due to reprocessing of primary radiation by the accretion disk and the white dwarf surface. We show that the most dramatic and easily observable consequence of the anomalous C/O abundance, is the significant, by more than an order of magnitude, attenuation of the Ka line of iron. It is caused by screening of the presence of iron by oxygen - in the C/O dominated material the main interaction process for a E ~ 7keV photon is absorption by oxygen rather than by iron, contrary to the solar composition case. Ionization of oxygen at high mass accretion rates adds a luminosity dependence to this behavior - the iron line is significantly suppressed only at low luminosity, log(LX) less than 37-37.5, and should recover its nominal strength at higher luminosity. The increase of the EW of the Ka lines of carbon and oxygen, on the other hand, saturates at rather moderate values. Screening by He is less important, due to its low ionization threshold and because in the accretion disk it is mostly ionized. Consequently, in the case of the He-rich donor, the iron line strength remains close to its nominal value, determined by the iron abundance in the accretion disk. This opens the possibility of constraining the nature of donor stars in UCXBs by means of X-ray spectroscopy with moderate energy resolution.
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We search for the Fe K${alpha}$ line in spectra of Ultra Compact X-ray Binaries (UCXBs). For this purpose we have analyzed XMM-Newton observations of five confirmed UCXBs. We find that the object 2S 0918-549 - whose optical spectrum bears tentative s
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