Keck II spectroscopy of mHz quasi-periodic oscillations in Hercules X-1

We present Keck II spectroscopy of optical mHz quasi-periodic oscillations (QPOs) in the light curve of the X-ray pulsar binary Hercules X-1. In the p ower spectrum it appears as 'peaked noise', with a coherency similar to2, a central frequency of 35 mHz and a peak-to-peak amplitude of 5 per cent. Ho wever, the dynamic power spectrum shows it to be an intermittent QPO, with a lifetime of similar to 100 s, as expected if the lifetime of the orbiting material is equal to the thermal time-scale of the inner disc. We have dec omposed the spectral time series into constant and variable components and used blackbody fits to the resulting spectra to characterize the spectrum o f the QPO variability and constrain possible production sites. We find that the spectrum of the QPO is best fitted by a small hot region, possibly the inner regions of the accretion disc, where the ballistic accretion stream impacts on to the disc. The lack of any excess power around the QPO frequen cy in the X-ray power spectrum, created using simultaneous light curves fro m RXTE, implies that the QPO is not simply reprocessed X-ray variability.