X-ray Spectra from GX 5-1
This is slide 13 of 60 from the EXOSAT slide set. The following caption is the one originally provided with the slide set.
The most important characteristic of the high-frequency quasi-periodic millisecond oscillations (QPO) that are observed in GX 5-1 and related sources when they are in their horizontal branch spectral state is the strict relation of QPO frequency with X-ray intensity. In this series of power spectra obtained at different X-ray intensity levels, the QPO peak is seen to shift to higher frequencies when the X-ray intensity increases. The peak also becomes broader.