A Coordinated X-ray and Optical Campaign of the Nearby Massive Binary $\delta$ Orionis Aa: II. X-ray Variability

TL;DR

This study analyzes phase- and time-resolved X-ray spectral variability in the δ Orionis Aa binary, revealing flux and emission line variations linked to wind interactions, with implications for understanding stellar wind collisions.

Contribution

It presents the first detection of phase-dependent X-ray emission line width variability in a binary, supported by 3D hydrodynamic wind collision modeling.

Findings

X-ray flux varies by about +/-15% within observations.

Emission lines such as S XV, Si XIII, and Ne IX are variable.

Line width variations correlate with binary phase, indicating wind-wind collision effects.

Abstract

We present time-resolved and phase-resolved variability studies of an extensive X-ray high-resolution spectral dataset of the $\delta$ Orionis Aa binary system. The four observations, obtained with Chandra ACIS HETGS, have a total exposure time of ~479 ks and provide nearly complete binary phase coverage. Variability of the total X-ray flux in the range 5-25 $\AA$ is confirmed, with maximum amplitude of about +/-15% within a single ~125 ks observation. Periods of 4.76d and 2.04d are found in the total X-ray flux, as well as an apparent overall increase in flux level throughout the 9-day observational campaign. Using 40 ks contiguous spectra derived from the original observations, we investigate variability of emission line parameters and ratios. Several emission lines are shown to be variable, including S XV, Si XIII, and Ne IX. For the first time, variations of the X-ray emission line widths as a function of the binary phase are found in a binary system, with the smallest widths at phase=0.0 when the secondary $\delta$ Orionis Aa2 is at inferior conjunction. Using 3D hydrodynamic modeling of the interacting winds, we relate the emission line width variability to the presence of a wind cavity created by a wind-wind collision, which is effectively void of embedded wind shocks and is carved out of the X-ray-producing primary wind, thus producing phase-locked X-ray variability.