Dynamical K Edge and Line Variations in the X-Ray Spectrum of the Ultra-Compact Binary 4u 0614+091

Abstract

We observed the ultra-compact binary candidate 4U 0614+091 for a total of 200 ks with the high-energy transmission gratings on board the Chandra X-ray Observatory. The source is found at various intensity levels with spectral variations present. X-ray luminosities vary between 2.0 × 10 36 erg s −1 and 3.5 × 10 36 erg s −1 . Continuum variations are present at all times and spectra can be fit well with a power-law component, a high kT blackbody component, and a broad line component near oxygen. The spectra require adjustments to the Ne K edge and in some occasions also to the Mg K edge. Once further resolved, the Ne K edge region appears variable in terms of optical depths and morphology. The edge vicinity reveals an average velocity smear of ∼3500 km s −1 implying a characteristic radius of the order of <10 9 cm consistent with an ultra-compact binary nature. The variability proves that the excess is intrinsic to the source. The data seem to indicate excess column densities of up to several 10 18 cm −2 . However, there are some serious problems with an Ne i interpretation in ultra-compact disks and we need to consider alternative interpretations, which include a variable O viii ionization edge, as likely candidates. We test such possibilities with existing observations. The prominent soft emission line complex near the O viii Ly α position appears extremely broad and relativistic effects from near the innermost disk have to be included. Gravitationally broadened line fits also provide nearly edge-on angles of inclination consistent with the observed high dynamics near the Ne K edge. The emissions appear consistent with an ionized disk with ionization parameters of the order of 10 4 at radii of a few 10 7 cm. The line wavelengths with respect to O viii Ly α are found variably blueshifted indicating more complex inner disk dynamics.

Galactic Ultracompact X-Ray Binaries: Disk Stability and Evolution

“…If the suggested 50-51 minute orbital period of this system is correct, then the persistent nature of this system and the C/O composition of the accretion disk (Nelemans et al 2006;Schulz et al 2010;Madej et al 2010) suggest an He star donor, as for 4U 1626-67. A 30 minute orbital period would not change this conclusion, as the inferred mass-transfer rate would still be rather larger than can be supplied by standard WD evolution.…”

Unveiling the environment surrounding low-mass X-ray binary SAX J1808.4–3658

“…All models provide acceptable fits, but some of them are less feasible. Absorption by fast neutral neon was also used by Schulz et al (2010) to describe the dynamical neon edge of the UCB 4U 0614+091, but they noticed that it is difficult to produce a large amount of neutral neon in a hot accretion disk. Higher ions should be then detected at similar velocities.…”

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

“…Madej et al 2010) and K-edges stemming from absorbing material in the vicinity of the disk (e.g. Schulz et al 2010) could provide direct indication of a C/O or O/Ne-rich disk and donor star. However, due to increased interstellar absorption below 1 keV and contamination of the reflected component by the primary emission, detection of these features with sufficient accuracy, often proves to be difficult.…”

“…However, analysis of later observations, yielded standard solar-like abundances, revealing a temporal variability of the effect and thus indicating that the apparent non-solar abundances could be the result of fluctuations in the ionization state of the surrounding material (e.g. Juett & Chakrabarty 2005;Schulz et al 2010), rather than the result of non-solar composition. 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.…”