Detailed X-Ray Line Properties of Theta2 Ori A in Quiescence

TL;DR

This study provides detailed analysis of X-ray emission lines from Theta2 Ori A, revealing a close stellar origin, weak wind shocks, and high-temperature components inconsistent with simple wind or companion models, suggesting magnetic confinement effects.

Contribution

First detailed spectral analysis of Theta2 Ori A's X-ray lines in quiescence, constraining emission region and wind properties, and proposing magnetic confinement as a key mechanism.

Findings

X-ray lines originate within two stellar radii of the O-star surface.

Line widths confirm emission close to the stellar surface.

High-temperature components challenge standard wind models.

Abstract

We investigate X-ray emission properties of the peculiar X-ray source Theta2 Ori A in the Orion trapezium region using more than 500 ksec of HETGS spectral data in the quiescent state. The amount of exposure provides tight constraints on several important diagnostics involving O, Ne, Mg, and Si line flux ratios from He-like ion triplets, resonance line ratios of the H- and He-like lines and line widths. Accounting for the influence of the strong UV radiation field of the O9.7V star we can now place the He-like line origin well within two stellar radii of the O-star's surface. The lines are resolved with average line widths of 341+-38 km/s confirming a line origin relatively close to the stellar surface. In the framework of standard wind models this implies a rather weak, low opacity wind restricting wind shocks to temperatures not much larger than 2x10^6 K. The emission measure distribution of the X-ray spectrum, as reported previously, includes very high temperature components which are not easily explained in this framework. The X-ray properties are also not consistent with coronal emissions from an unseen low-mass companion nor with typical signatures from colliding wind interactions. The properties are more consistent with X-ray signatures observed in the massive Trapezium star Theta1 Ori C which has recently been successfully modeled with a magnetically confined wind model.