X-ray response to disc evolution in two γ Cas stars

Abstract

ABSTRACT The Hα emission of a set of southern γ 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, namely HD 119682 and V767 Cen, were found to display slowly decreasing disc emissions. These decreases were not perfectly monotonic and several temporary and limited rebuilding events were observed. For HD 119682, the emission component in Hα disappeared in 2020 mid-July. In X-rays, the X-ray flux was twice smaller than that recorded two decades ago but of a similar level as observed a decade ago. The X-ray flux decreased over the campaign by 30 per cent, but the hardness remained similar in data sets of all epochs. In particular, the γ Cas character remained as clear as before even when there was no trace of disc emission in the Hα line. For V767 Cen, the full disappearance of disc emission in Hα never occurred. We followed closely a disc rebuilding event, but no significant change in flux or hardness was detected. These behaviours are compared to those of other γ Cas stars and their consequences on the X-ray generation are discussed.