A Review of Disk-Corona Oscillations

Low frequency (≈ 0.1-35 Hz) quasi-periodic oscillations of theX-ray flux characterize many of the black hole candidates, in particularthose which have radio evidence for jets. These QPO have amplitudes ofup to 20% in the states of black hole novae which are called veryhigh and intermediate and are correlated with the break frequency ofthe band-limited low frequency white noise. With transition to thestate called the soft high state, both the QPO and the noisedisappear. While the noise is strong in the low hard states like thatof Cyg X-1, the QPO, when present, are weak and broad. In their strongmanifestations, these QPO have the curious property of appearing tohave the spectrum of the power-law component which dominates in thelow state, while correlations between their frequency and the diskcomponent in the spectrum imply control by the disk. The correlations,the harmonic structure of the QPO, and the phase lags have complexbehavior in the same source (GRS 1915+105, XTE J1550-564). Thephenomena point to interaction between the disk and the corona, for which there are several interesting ideas.