X-ray diagnostics of chemical composition of the accretion disc and donor star in ultra-compact X-ray binaries

Abstract

We have analyzed the X-ray spectra of all known Ultra Compact X-ray Binaries (UCXBs), with the purpose of constraining the chemical composition of their accretion disk and donor star. Our investigation was focused on the presence (or absence) of the Fe Kα emission line, which was used as the probe of chemical composition of the disk, based on previously established theoretical predictions for the reflection of X-ray radiation off the surface of C/O-rich or He-rich accretion disks in UCXBs. We have contrasted the results of our spectral analysis to the history of type I X-ray bursts from these systems, which can also indicate donor star composition. We found that UCXBs with prominent and persistent iron Kα emission also featured repeat bursting activity. On the other hand, the UCXBs for which no iron line was detected, appear to have few or no type I X-ray bursts detected over more than a decade of monitoring. Based on Monte Carlo simulations, demonstrating a strong correlation between the Fe Kα line strength and the abundance of C and O in the accretion disk material and given the expected correlation between the H/He abundance and the recurrence rate of type I X-ray bursts, we propose that there is a considerable likelihood that UCXBs with persistent iron emission have He-rich donors, while those that do not, likely have C/O or O/Ne/Mg-rich donors. Our results strongly advocate for the development of more sophisticated simulations of X-ray reflection from hydrogen-poor accretion disks.

Observations of the Ultra-compact X-Ray Binary 4U 1543-624 in Outburst with NICER, INTEGRAL, Swift, and ATCA

“…The O line showed a clear evolution in the shape of its profile as the source intensity increased. The Fe line profile does not appear to change as dramatically, which is likely due to a combination of (1) there being three times less NICER collecting area in the Fe band in comparison to the O region and (2) the line emission being dampened from the overabundance in C/O from the companion (Koliopanos et al 2013). Both show a similar line profile when plotted in velocity space, indicating that they arise from a similar location within the disk.…”

Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9–342058

“…A He-dominated disk would be consistent the lack of H-features and presence of HeII emission in the optical spectra (Baglio et al 2016). We note that if RXS J1804 harbors an evolved companion, the detection of Fe-K reflection in both the soft and the hard X-ray spectral state might argue in favor of a He donor rather than a C/O or O/Ne/Mg white dwarf donor, since a high C/O abundance could screen Fe and hence suppress the Fe-K line in C/O rich systems (Koliopanos et al (2013), Koliopanos et al (2014), but see Madej et al (2014)). …”

The Evolution of Compact Binary Star Systems

“…For UCXBs, like for AM CVn stars, an analysis of details of the chemical composition of donors, which show up in their optical or X-ray spectra, seems to be a promising way for distinguishing between the possible progenitors [515, 521, 374]. For UCXBs, a different chemical composition of donors may manifest itself in properties of thermonuclear explosions (type I X-ray bursts) that occur on the surface of the accreting neutron stars with low magnetic fields [47].…”