Time variabilities of X-ray binary stars

Abstract

The hard and the soft power law components and the disk blackbody component of the black hole candidate X-ray binaries have their inherent normalized power spectrum densities and phase lags. These short term variabilities are also similar to those of the X-ray energy spectral components of the non-pulsating neutron star X-ray binaries, although X-ray energy spectra of these two kinds of X-ray binaries are different in their high X-ray intensity state. There are two states in the black hole candidate X-ray binaries; the power law hard state and the power law soft state. Some of the black hole candidate-X-ray binaries show large hysteretic behavior between the states and the X-ray intensity. There seems to be no observation of large hysteretic behavior of this sort in the non-pulsating neutron star X-ray binaries. A model is assumed that in the X-ray emitting region of the accretion disk, the power law hard state corresponds to the advection dominated optically thin accretion disk and the power law soft state corresponds to the cooling dominated optically thick accretion disk, and this model is compared with observations and discussed. It is concluded that to describe long term variabilities of X-rays from the black hole candidate-X-ray binaries and the non-pulsating neutron star-X-ray binaries, other factors such as magnetic field seem to be necessary.