A multiwavelength campaign on gamma Cassiopeiae. II. The case for corotating, circumstellar clouds

Simultaneous X-ray/UV observations over a full day on 1996 March 14-15 have been made of the prototypical B0.5e star gamma Cas using the Rossi X-Ray T iming Explorer satellite and the Goddard High Resolution Spectrograph (GHRS ) on board the Hubble Space Telescope. The GHRS spectra, taken in the regio n of the Si Iv lambda lambda 1394-1403 doublet, also permitted the construc tion of an extremely precise light curve from a nearby "pseudocontinuum" re gion. The continuum UV and X-ray light curves reveal a pair of X-ray maxima similar to 10 hr apart that coincide in time with UV continuum flux "dips" of similar to 1%. In the first paper in this series we attributed the long -term X-ray Variations to magnetic activity sites on the surface of the sta r that undergo rotational modulation on a similar to 1.125 days period. In the current study we find that flux and color curves generated from a 33 hr sequence of Inter national Ultraviolet Explorer (IUE) echellegrams obtaine d in 1996 January display dip features similar to those in the GHRS data. C omparing the timings of the continuum flux dips and the Si Iv line strength variations in both the GHRS and IUE data sets gives a slightly revised per iod of 1.123 days for both the UV and X-ray activities. This strengthens th e argument that high-energy activity on gamma Cas is modulated by rotation of long-lived structures close to its surface. Analysis of the pseudocontin uum light curves constructed from the GHRS and IUE light curves shows at le ast two surprising characteristics for the flux dips: (1) the dips last onl y similar to 0.3 cycles, which is too brief for rotation modulation of surf ace features, and (2) their amplitudes increase from long to short waveleng ths, which attain a maximum near 1206 Angstrom. The character of the variat ions of the photospheric Si Iv line profiles is unexpected in that the equi valent width fluctuations do not correlate with the slow undulations of the continuum flux. Moreover, the profile variations do not show an expected b lue-to-red migration of microfeatures. We show that the continuum character istics and absence of migration of features in the Si Iv lines can be expla ined by the presence of very cool, optically thin clouds that corotate with the star. Assuming a tilt of the rotational axis of +45 degrees to the obs erver's line of sight, our model simulations of the two major dips in the U V light curves indicate that the clouds have radii of a few tenths of a ste llar radius and are attached to points on the surface at low to mid-latitud es on the near hemisphere. These findings support the conclusion of the fir st paper in this series that gamma Cas is a member of a small group of OB s tars that have magnetospheres associated with X-ray activity.