Intriguing X-ray and optical variations of the gamma Cas analog HD45314

TL;DR

This study investigates the optical and X-ray variability of the gamma Cas analog HD45314, revealing correlations between disk material and X-ray emission that support magnetic star-disk interaction as the emission source.

Contribution

It provides detailed multi-year optical and X-ray observations of HD45314, demonstrating the link between disk material and X-ray properties, supporting the magnetic interaction hypothesis.

Findings

Optical emission lines show significant morphological changes.

X-ray spectrum softens during disk dissipation phases.

X-ray emission correlates with disk material presence.

Abstract

A growing number of Be and Oe stars, named the gamma Cas stars, are known for their unusually hard and intense X-ray emission. This emission could either trace accretion by a compact companion or magnetic interaction between the star and its decretion disk. To test these scenarios, we carried out a detailed optical monitoring of HD45314, the hottest member of the class of gamma Cas stars, along with dedicated X-ray observations on specific dates. High-resolution optical spectra were taken to monitor the emission lines formed in the disk, while X-ray spectroscopy was obtained at epochs when the optical spectrum of the Oe star was displaying peculiar properties. Over the last four years, HD45314 has entered a phase of spectacular variations. The optical emission lines have undergone important morphology and intensity changes including transitions between single- and multiple-peaked emission lines as well as shell events, and phases of (partial) disk dissipation. Photometric variations are found to be anti-correlated with the equivalent width of the H-alpha emission. Whilst the star preserved its hard and bright X-ray emission during the shell phase, the X-ray spectrum during the phase of (partial) disk dissipation was significantly softer and weaker. The observed behaviour of HD45314 suggests a direct association between the level of X-ray emission and the amount of material simultaneously present in the Oe disk as expected in the magnetic star-disk interaction scenario.