X-ray diagnostics of chemical composition of the accretion disk and donor star in UCXBs II: XMM-Newton observations

摘要

We search for the Fe K${\alpha}$ line in spectra of Ultra Compact X-rayBinaries (UCXBs). For this purpose we have analyzed XMM-Newton observations offive confirmed UCXBs. We find that the object 2S 0918-549 - whose opticalspectrum bears tentative signatures of a C/O accretion disk - is devoid of anyemission features in the 6-7 keV range, with an upper limit of less than 10 eVfor the equivalent width (EW) of the iron line. 4U 1916-05 - whose opticalspectrum is consistent with reflection from a He-rich accretion disk - exhibitsa bright broad iron emission line. This behavior is in agreement with thetheoretical predictions presented in Koliopanos, Gilfanov and Bildsten (2013).Namely, we expect strong suppression of the Fe K${\alpha}$ emission line inspectra originating in moderately bright (LogLx less than $\approx$ 37.5) UCXBswith C/O or O/Ne/Mg-rich donors. On the other hand the EW of the iron line inspectra from UCXBs with He-rich donors is expected to retain its nominal valueof $\approx$ 100 eV. Our analysis also reveals a strong Fe K${\alpha}$ line inthe spectrum of 4U 0614+091. This detection points towards a He-rich donor andseems 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 strongiron emission line - in the two remaining UCXB sources in our sample (XTEJ1807-294, 4U 0513-40) - as an indication of a C/O or O/Ne/Mg white dwarfdonor. From the upper limits of the Fe K${\alpha}$ line EW in 4U 0513-40, 2S0918-549 and XTE J1807-294 we obtain a lower limit on the oxygen-to-ironratio,O/Fe$\ge$ 10$\times$[O/Fe]$_{\odot}$.