X-Ray Variability of Stars

Abstract

Most stars emit X-rays. The hot X-ray emitting gas is not gravitationally bound to the star, but is magnetically confined in loops. It is generally assumed that rotation in convective stars relates to magnetic field generation. The X-ray intensity correlates with the rotation rate of the star.Most stars are known to be highly variable in X-ray emission. X-ray flares with rise times of tens of seconds and decay times of tens of minutes to hours are seen in the red dwarf flare stars, in the RS CVn stars, in the Hyades and Pleiades star clusters. In addition, many X-ray sources have been found which probably relate to very young stars, prior to the start of hydrogen burning (pre-main sequence stars).Some of the previously known so-called "fast X-ray transient sources" are identified with RS CVn stars. They represent the largest known increase in X-ray flux, indicating volume emission measures for the hot plasma which may be a million times that of the largest solar flare.In this review I will discuss the observational evidence of X-ray variability obtained so far and give some of the observed properties of red dwarf flare stars and RS CVn stars. A full scale theoretical interpretation of the flare mechanism does not exist for solar flares, let alone for the stellar flares. Stellar flare observations, however, may help to solve the problem.