Abstract for Proposal 084031

The impact of Be disks on the X-ray emission of gamma Cas stars

gamma Cas stars are Oe/Be stars that display an intriguing bright and hard X-ray emission. The origin of this X-ray emission could be either accretion by a compact companion or an interaction between the Be star and its decretion disk. Evidence favouring the latter scenario was obtained over recent years. This includes observations of a quasi-simultaneous decrease by a factor 10 of the X-ray emission of HD45314 as the disk of this star nearly dissipated in 2016. We aim at further constraining the gamma Cas phenomenon by taking advantage of such spectacular variations. For this purpose, we propose observing a gamma Cas star next time that its disk undergoes either an outburst or in case it clears away.



Details on Observing Strategy and Trigger Criteria
The goal is to observe a gamma Cas star if its disk suffers an outburst (H-alpha equivalent width becoming at least 1.5 times more negative than during quiescence) or nearly disappears (absolute value H-alpha equivalent width reducing to less than 25% of its quiescence value). The targets are monitored in optical spectroscopy to provide the trigger criteria. We will inform the SOC should any of the eight stars that we monitor fulfill either of the trigger criteria. Given the typical timescales of disk outbursts or dissipation, a reaction time of about ten days is sufficient.