# Ultraviolet Activity as indicator of Small-scale Magnetic Fields in gamma Cassiopeiae

## TL;DR

This study investigates ultraviolet activity in gamma Cassiopeiae to understand its high-energy phenomena, suggesting the presence of small-scale magnetic fields through spectral analysis and light curve phasing.

## Contribution

It provides indirect evidence for small-scale magnetic fields in gamma Cas by analyzing ultraviolet and optical data, including migrating subfeatures and spectral line responses.

## Key findings

- Phasing of ultraviolet and optical light curves supports rotational interpretation.
- Migrating subfeatures likely indicate surface magnetic activity.
- Spectral line responses correlate with X-ray variations, implying magnetic fields.

## Abstract

The recorded activity of gamma 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. Our analysis focuses first on the phasing of ultraviolet and optical light curves from different epochs. The rotational interpretation of the 1.22d optical signal is justifiedin part on this phasing. Next we detail observations of "migrating subfeatures," 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 rapid rotating Be stars like gamma Cas. We also analyze the important responses of the semi-forbidden resonance S IV line at \lambda1404.8 to simultaneous X-ray variations. These results offer indirect support for the existence of small-scale magnetic fields on this Be star.