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[superscript 36] erg s–1 and 3.5 × 10[superscript 36] erg s[superscript –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[superscript –1] implying a characteristic radius of the order of <10[superscript 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[superscript 18] cm[superscript –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 Lyα 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[superscript 4] at radii of a few 10[superscript 7] cm. The line wavelengths with respect to O VIII Lyα are found variably blueshifted indicating more complex inner disk dynamics.