X-ray response to disk evolution in two gamma-Cas stars

TL;DR

This study monitors gamma-Cas stars' disk evolution and finds that their peculiar X-ray emissions persist despite significant changes or disappearance of disk emission, challenging existing models of X-ray generation.

Contribution

It provides new observational evidence linking disk evolution in gamma-Cas stars to their X-ray emission behavior, highlighting the persistence of X-ray characteristics despite disk changes.

Findings

X-ray flux decreased by 30% over the campaign.

Gamma-Cas X-ray properties remained stable despite disk emission changes.

Disappearance of Halpha emission did not alter X-ray hardness.

Abstract

The Halpha emission of a set of southern gamma-Cas stars was monitored since 2019, with the aim of detecting transition events and examining how their peculiar X-ray emission would react in such cases. Two stars, HD119682 and V767Cen, were found to display slowly decreasing disk emissions. These decreases were not perfectly monotonic and several temporary and limited re-building events were observed. For HD119682, the emission component in Halpha disappeared in mid-July 2020. In X-rays, the X-ray flux was twice smaller than recorded two decades ago but of a similar level as observed a decade ago. The X-ray flux decreased over the campaign by 30%, but the hardness remained similar in datasets of all epochs. In particular, the gamma-Cas character remained as clear as before even when there was no trace of disk emission in the Halpha line. For V767Cen, the full disappearance of disk emission in Halpha never occurred. We followed closely a disk rebuilding event, but no significant change in flux or hardness was detected. These behaviours are compared to those of other gamma-Cas stars and their consequences on the X-ray generation are discussed.