Theoretical X-ray line profiles from colliding wind binaries

Abstract

We present theoretical X-ray line profiles from a range of model colliding wind systems. In particular, we investigate the effects of varying the stellar mass-loss rates, the wind speeds and the viewing orientation. We find that a wide range of theoretical line profile shapes is possible, varying with orbital inclination and phase. At or near conjunction, the lines have approximately Gaussian profiles, with small widths (HWHM ∼ 0.1v∞) and definite blueshifts or redshifts (depending on whether the star with the weaker wind is in front or behind). When the system is viewed at quadrature, the lines are generally much broader (HWHM ∼v∞), flat-topped and unshifted. Local absorption can have a major effect on the observed profiles — in systems with mass-loss rates of a few times 10−6 M⊙ yr−1 the lower energy lines (E ≲ 1 keV) are particularly affected. This generally results in blueward-skewed profiles, especially when the system is viewed through the dense wind of the primary. The orbital variation of the linewidths and shifts is reduced in a low-inclination binary. The extreme case is a binary with i= 0°, for which we would expect no line profile variation.