Surprises in the simultaneous X-ray and optical monitoring of pi Aqr

Abstract

To help constrain the origin of the peculiar X-ray emission of gamma Cas stars, we conducted a simultaneous optical and X-ray monitoring of pi Aqr in 2018. At that time, the star appeared optically bright and active, with a very strong Halpha emission. Our monitoring covers three 84d 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 Halpha 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 gamma Cas stars. It also constrains scenarios proposed to explain the gamma Cas phenomenon.

accepted for publication in A&A

References in corpus (4)

• A propelling neutron star in the enigmatic Be-star $\gamma$~Cassiopeia
• Discovery of Rapidly Moving Partial X-ray Absorbers within gamma Cassiopeiae
• pi Aqr is another gamma-Cas object
• Evolution of the disk of pi Aqr: from near-disappearance to a strong maximum

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• Fluorescent Fe K line emission of gamma Cas stars I. Do gamma Cas stars host propelling neutron stars?
• X-ray response to disk evolution in two gamma-Cas stars
• X-ray emission of massive stars and their winds