CORRELATION BETWEEN FAST QUASI-PERIODIC OSCILLATIONS AND X-RAY SPECTRAL SHAPE IN ATOLL SOURCES

We present correlations between the frequency of the fast quasi-period ic oscillations (QPOs) and the shape of the X-ray spectrum in the atol l sources 4U 1608-52 and 4U 0614+091. We find that the photon index of a power-law component of the energy spectrum is well correlated with the QPO frequency in both sources. The correlation between the photon index and the QPO frequency most likely represents a common dependence on some physical parameter of the system.