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 analysed 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 disc - is devoid of any emission features in the 6-7 keV range, with an upper limit of less than 10 eV for the equivalent width (EW) of the Fe line. 4U 1916-05 - whose optical spectrum is consistent with reflection from a He-rich accretion disc - exhibits a bright broad iron emission line. This behaviour is in agreement with the theoretical predictions presented in Koliopanos et al. Namely, we expect strong suppression of the Fe Kα emission line in spectra originating in moderately bright (log L_X less than ≈37.5) UCXBs with C/O- or O/Ne/Mg-rich donors. On the other hand the EW of the Fe line in spectra from UCXBs with He-rich donors is expected to retain its nominal value of ≈100 eV. 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 and 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]_⊙.