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

TL;DR

This study conducted simultaneous optical and X-ray observations of pi Aqr to investigate the origin of its peculiar X-ray emission, revealing variability without clear phase correlation and challenging existing models of gamma Cas stars.

Contribution

First simultaneous optical and X-ray monitoring of pi Aqr during active phase, providing new insights into its emission variability and constraints on gamma Cas phenomena.

Findings

X-ray emission varies without clear orbital phase correlation.

Optical brightness and Halpha emission were high during observations.

X-ray spectral absorption increased, but overall properties remained consistent with past data.

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.