The classical B0.5e star γ Cassiopeia is known to be a unique X-ray source by virtue of its moderate LX (1033 ergs s-1), hard X-ray spectrum, and light curve punctuated by ubiquitous flares and slow undulations. The peculiarities of this star have led to a controversy concerning the origin of these emissions: whether they are from wind infall onto a putative degenerate companion, as in the case of normal Be/X-ray binaries, or from the Be star itself. Recently, much progress has been made to resolve this question: (1) the discovery that γ Cas is a moderately eccentric binary system (P = 203.59 days) with unknown secondary type, (2) the addition of RXTE observations at six epochs in 2000, adding to three others in 1996-1998, and (3) the collation of robotic telescope (Automated Photometric Telescope) B- and V-band photometric observations over four seasons that show a 3%, cyclical flux variation with cycle lengths of 55-93 days. We find that X-ray fluxes at all nine epochs show random variations with orbital phase, thereby contradicting the binary accretion model, which predicts a substantial modulation. However, these fluxes correlate well with the cyclical optical variations. In particular, the six flux measurements in 2000, which vary by a factor of 3, closely track the interpolated optical variations between the 2000 and 2001 observing seasons. The energy associated with the optical variations greatly exceeds the energy in the X-rays, so that the optical variability cannot simply be due to reprocessing of X-ray flux. However, the strong correlation between the two suggests that they are driven by a common mechanism. We propose that this mechanism is a cyclical magnetic dynamo excited by a Balbus-Hawley instability located within the inner part of the circumstellar disk. According to our model, variations in the field strength directly produce the changes in the magnetically related X-ray activity. Turbulence associated with the dynamo results in changes to the density (and therefore the emission measure) distribution within the disk and creates the observed optical variations.
Richard D. Robinson et al 2002 ApJ 575 435