Soft and hard X-ray dips in the light curves of γ Cassiopeiae

Abstract

The available six archival XMM-Newton observations of the anomalous X-ray emitter γ Cas (B0.5 IVe) have been surveyed for the presence of soft X-ray `dips' in X-ray light curves. In addition to discovering such events in the soft band (≤2 keV), we show that sometimes they are accompanied by minor, nearly simultaneous dips in the hard X-ray band. Herein, we investigate how these occurrences can be understood in the `magnetic star-disc interaction' hypothesis proposed in the literature to explain the hard, variable X-ray emission of this Be star. In this scenario, the soft X-ray dips are interpreted as transits by comparatively dense, soft X-ray-absorbing blobs that move across the lines of sight to the surface of the Be star. We find that these blobs have similar properties as the `cloudlets' responsible for migrating subfeatures in UV and optical spectral lines and therefore may be part of a common distribution of co-rotating occulters. The frequencies, amplitudes, and longevities of these dips vary widely. Additionally, the most recent spectra from 2014 July suggest that the `warm' (kT ≈ 0.6-4 keV) plasma sources responsible for some of the soft flux are much more widely spread over the Be star's surface than the hot plasma sites that dominate the flux at all X-ray energies. We finally call attention to a sudden drop in all X-ray energies of the 2014 light curve of γ Cas and a similar sudden drop in a light curve of the `analog' HD 110432. We speculate that these could be related to appearances of particularly strong soft X-ray dips several hours earlier.