X-ray diagnostics of chemical composition of the accretion disc and donor star in UCXBs – II. XMM–Newton observations

We search for the Fe Kα 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 signatures of a C/O accretion disk – is devoid of any emission features in the 6-7keV range, with an upper limit of less than 10eV for the equivalent width (EW) of the iron line. 4U 1916-05 – whose optical spectrum is consistent with reflection from a He-rich accretion disk – exhibits a bright broad iron emission line. This behavior is in agreement with the theoretical predictions presented in Koliopanos, Gilfanov & Bildsten (2013). Namely, we expect strong suppression of the Fe Kα emission line in spectra originating in moderately bright (LogLx less than � 37.5) UCXBs with C/O or O/Ne/Mg-rich donors. On the other hand the EW of the iron line in spectra from UCXBs with He-rich donors is expected to retain its nominal value of � 100eV. Our analysis also reveals a strong Fe Kα line in the spectrum of 4U 0614+091. This detection points towards a He-rich donor and seems to be at odds with the source’s classification as C/O-rich. Nevertheless, a He-rich donor would explain the bursting activity reported for this system. Lastly, based on our theoretical predictions, we attribute the lack of a strong iron emission line – in the two remaining UCXB sources in our sample (XTE J1807-294, 4U 0513-40) – as an indication of a C/O or O/Ne/Mg white dwarf donor. From the upper limits of the Fe Kα line EW in 4U 0513-40, 2S 0918-549 and XTE J1807-294 we obtain a lower limit on the oxygen-to-iron ratio, O/Fe> 10 × [O/Fe]⊙.