Intensity-dependent quasi-periodic oscillations in the X-ray flux of GX5-1

The X-ray flux of the bright galactic bulge source GX5-1 shows intensity-dependent quasi-periodic oscillations between ~20 and ~40 Hz, appearing as a broad peak in the power spectrum whose centroid frequency, width and integrated excess power strongly depend on the source intensity. The strength and steepness of low-frequency noise present in the power spectra below 15 Hz also depend on the source intensity. No evidence is found for coherent X-ray pulsations between 0.5 and 2,000 Hz. We discuss possible mechanisms to explain these new phenomena.