Discovery of 5 HZ Quasi-periodic Oscillations in Cygnus X-2

An 8 hr HEAO A-1 observation of 5 Hz quasi-periodic oscillations (QPOs) in Cyg X-2 is reported. The observed QPO frequency variations are erratic and uncorrelated with intensity, and the average deviation from the mean frequency is four percent, much less than the variation observed in Cyg X-2 by Hasinger et al. (1986). The data are inconsistent with an extrapolation of the intensity-QPO frequency curve for Cyg X-2 by Hasinger et al., assuming the beat frequency model of Lamb et al. (1985). This model also predicts a positive correlation between QPO and low-frequency noise/red noise strength; however, the authors here find no relationship between these parameters. It is concluded that the QPOs observed here are a mode distinct from those seen earlier and call for a different model.