Optical and X-Ray Variability of Gamma Cas-Type Stars

$\gamma\,$Cas-type stars is an enigmatic group of Be stars with unusually hard X-rays and an X-ray luminosity of $10^{31}-10^{33}\,$erg\,s$^{-1}$, which is higher than a typical value for classical Be stars. To test how the X-ray and optical radiation from $\gamma\,$Cas-type stars is generated, we review the optical observations of these stars and an analysis of their X-ray light curves. The periods of the line profiles variations appeared to be close to the periods of changes in their X-ray luminosities. We suppose that a significant fraction of X-ray emission is formed close to the region where optical emission does. The X-ray brightness of the binary $\gamma\,$Cas-type stars HD\,45314, HD\,45995 and NGC\,6649-9 vary with periods of ~50-90 seconds, which may be the rotation periods of white dwarfs in Be+WD binary systems. It means that at least hard part of the X-ray emission from these $\gamma\,$Cas-type stars is formed as result of an accretion onto rapidly rotating white dwarfs. The soft part of X-rays from $\gamma\,$Cas-type stars can be interpreted as a result of a reconnection of the local magnetic field lines of Be star and those in the circumstellar decretion disk.