Ultraviolet Activity as Indicators of Small-scale Magnetic Fields in γ Cassiopeiae

Abstract

The recorded activity of γ Cas (B0.5 IVe) in the ultraviolet is important to an understanding of the mechanism behind this prototypical Be star’s high-energy activity, especially its hard X-ray emissions. The analysis here focuses first on the phasing of ultraviolet and optical light curves from different epochs. The rotational interpretation of the 1.22 d optical signal is justified in part on this phasing. Next, observations of “migrating subfeatures” are detailed, which traverse blue to red across line profiles in high-quality spectra. These are likely to be proxies for surface magnetic fields, which have not been detected yet by spectropolarimetric means in rapidly rotating Be stars like γ Cas. Also, the important responses of the semi-forbidden resonance S IV line at λ1404.8 to simultaneous X-ray variations are analyzed. These results offer indirect support for the existence of small-scale magnetic fields on this Be star.