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

Abstract

We have examined soft (<2keV) "dips" in six archival XMM-Newton observations of gamma Cas (B0.5IV e) for "soft dips" (< 2keV) in X-ray light curves. We find that such events are sometimes accompanied by minor, near-simultaneous dips in the hard X-ray band. We investigate how these occurrences can be understood in the "magnetic star-disk interaction" X-ray generation hypothesis. The soft X-ray dips are interpreted as transits by dense X-ray-absorbing blobs moving across sightlines to the Be star. 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. A spectrum from 2014 July suggest that "warm" (kT = 0.6-4keV) plasma sources 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 call attention to a sudden drop in all X-ray energies of the 2014 light curve of gamma Cas and a similar sudden drop in a light curve of the analog HD110432. We speculate that these could be related to strong soft X-ray dips several hours earlier.