Surprises in the simultaneous X-ray and optical monitoring of π Aquarii

Abstract

To help constrain the origin of the peculiar X-ray emission of γ Cas stars, we conducted a simultaneous optical and X-ray monitoring of π Aqr in 2018. At that time, the star appeared optically bright and active, with a very strong Hα emission. Our monitoring covers three 84 d orbital cycles, allowing us to probe phase-locked variations as well as longer-term changes. In the new optical data, the radial velocity variations seem to span a smaller range than previously reported, which might indicate possible biases. The X-ray emission is variable, but without any obvious correlation with orbital phase or Hα line strength. Furthermore, the average X-ray flux and the relative range of flux variations are similar to those recorded in previous data, although the latter data were taken when the star was less bright and its disk had nearly entirely disappeared. Only the local absorption component in the X-ray spectrum appears to have strengthened in the new data. This absence of large changes in X-ray properties despite dramatic disk changes appears at odds with previous observations of other γ Cas stars. It also constrains scenarios proposed to explain the γ Cas phenomenon.