X-ray reflection in oxygen-rich accretion discs of ultracompact X-ray binaries

Abstract

We present spectroscopic X-ray data of two candidate ultra-compact X-ray binaries: 4U 0614+091 and 4U 1543−624. We confirm the presence of a broad O VIII Lyα reflection line (at ≈ 18 Å) using XMM-Newton and Chandra observations obtained in 2012 and 2013. The donor star in these sources is carbon-oxygen or oxygen-neon-magnesium white dwarf. Hence, the accretion disc is enriched with oxygen which makes the O VIII Lyα line particularly strong. We also confirm the presence of a strong absorption edge at ≈ 14 Å so far interpreted in the literature as due to absorption by neutral neon in the circumstellar and interstellar medium. However, the abundance required to obtain a good fit to this edge is ≈ 3 − 4 times solar, posing a problem for this interpretation. Furthermore, modeling the X-ray reflection off a carbon and oxygen enriched, hydrogen and helium poor disc with models assuming solar composition likely biases several of the best-fit parameters. In order to describe the Xray reflection spectra self-consistently we modify the currently available xillver reflection model. We present initial grids that can be used to model X-ray reflection spectra in UCXBs with carbon-oxygen-rich (and hydrogen and helium poor) accretion disc. We find that the new reflection model provides a better overall description of the reflection spectra of 4U 0614+091 and 4U 1543−624 than the reflection models that assume solar abundances. Besides a strong O VIII Lyα line the new reflection model also shows a strong O VIII K-edge (at 14.23 Å). We find that the absorption edge at ≈ 14 Å present in the data can be described by a O VIII K-edge formed due to reflection in the accretion disc and a Ne I K-edge originating mostly (if not entirely) in the interstellar medium, mitigating the problem of the apparent very high neon abundance. Additionally, based on the spectral properties of 4U 1543−624 we consider a scenario in which this source is accreting near the Eddington limit.

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)). …”

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

“…Detection of broad emission-like features have also been detected in sources 4U 0614+091 (Madej et al 2010), 4U 1453-624 (Madej & Jonker 2011) and 4U 1626-67 Krauss et al (2007 and have been attributed to emission from overabundant carbon, oxygen and/or neon in the accretion disk material (e.g. Madej et al 2014). Nevertheless, prominent emission-like features in the < 1 keV energy range, have been observed in the spectra of numerous NS-and BH-XRBs of all types, during different states and at different mass accretion rates (e.g.…”

“…Nevertheless, prominent emission-like features in the < 1 keV energy range, have been observed in the spectra of numerous NS-and BH-XRBs of all types, during different states and at different mass accretion rates (e.g. Boirin & Parmar 2003;Díaz Trigo et al 2006;Ng et al 2010;Madej et al 2014) and are not necessarily an indication of non-solar composition.…”

“…An interesting point is raised by the modelling of 4U 0614+091 by Madej et al (2014). In this work, they use a version of the XILLVER X-ray reflection code (García & Kallman 2010;García et al 2011;García et al 2013) modified in order to simulate reflection by C/O-rich material (the authors refer to this version of their code as XILLVER CO ).…”

Optical Modulation in the X-Ray Binary 4U 1543–624 Revisited

“…Wang & Chakrabarty (2004) estimated d ∼ 7 kpc by assuming that mass transfer in the binary is driven by gravitational radiation. Recently, Madej et al (2014) have pointed out that this binary could be in an ultra-luminous state (Gladstone, Roberts, & Done 2009), because of its spectral similarities to other ultra-luminous X-ray sources. If this were the case, the binary would be accreting near the Eddington limit and would be at a distance of 30-40 kpc: well out in the Galactic halo.…”