A Multiwavelength Campaign on γ Cassiopeiae. I. The Case for Surface X-Ray Flaring

In 1996 March we obtained simultaneous Rossi X-Ray Timing Explorer RXTE Proportional Counter Array (PCA) and Hubble Space Telescope (HST) Goddard High-Resolution Spectrograph (GHRS) light curves for the B0.5e star γ Cas in order to compare its X-ray and ultraviolet continuum flux behaviors. The GHRS data set consisted of a nearly continuous sequence of UV spectra covering a 21+ hr interval. Each 40 A spectrum was centered on the Si IV λλ1394-1403 lines and registered 8100 counts in each 1 s exposure. Combining spectra and integrating over >100 continuum pixels allowed us to define a UV continuum light curve binned to 1 minute with a signal-to-noise ratio of a few thousand pixel-1. We found that the light curve exhibited variations over a time comparable to the rotation period of the star, showing two broad minima 10 hr apart, which had depths of 0.8% and 1.8%. The long-term trends in the UV are anticorrelated with the X-ray fluxes, with the X-rays exhibiting increases of ~10% and ~40% during times of UV flux minima. The stability of the long-term X-ray variations on γ Cas is supported by phasing our March data with contemporaneous ASCA data, suggesting a possible period of 1.125 days (or a close alias). We also get agreement of dip patterns for an assumed 1.123 day period by phasing the GHRS continuum flux curve with IUE light curves in various wavelengths from 2 months earlier. We take this as an estimate of the star's rotational period. We conclude that the X-ray emission from γ Cas probably consists of two components. The first is a slowly varying "basal" flux representing the minimum level seen during any given phase. Superimposed on this are rapid fluctuations ("shots") that have lifetimes ranging from <10 s to ≥10 minutes. The character of these components varies from one spacecraft orbit to the next, indicating that the emissions are not produced in a truly "stationary" chaotic environment. Moreover, both the number and amplitude of the shots increase during UV minima. The shot profiles are typically symmetric and can have decay times of a few seconds or less. The shots also have a slightly harder flux distribution than the basal component, suggesting that the two emission regions are not cospatial. The time-averaged X-ray spectrum indicates a quasi temperature of ~108 K, in agreement with earlier studies. We present a picture in which magnetically generated structures on and over the star's surface are responsible for the basal and shot X-ray components. The energies and luminosities of the shots are so high that even the weakest events we measure are comparable in strength to the most luminous flares on cool active stars. Using general cooling relations for a thermal plasma, one finds that the source region for the shots probably have a size scale of ≤104 km and densities of ~1013-14 cm-3. From a simple flare model, we find that generally only a small fraction of the shot energy is radiated during the event itself. The remainder of the hot plasma expands to fill a confined volume, possibly a magnetic loop, connected to the original flare site. A collection of these loops may then account for the basal emission. With this model, we estimate that the individual loops have a characteristic density of ~1011 cm-3 and dimensions of ≥0.1R*. We note that the magnetic interpretation for the shot and basal emission poses several theoretical questions, such as how complex, dynamic fields can exist on a star that does not have a convective envelope. These results suggest that γ Cas is a member of an arguably new group of hot stars that flare continuously in X-rays. This group may represent an extension of the hotter Bp stars to high values of rotation.