Observations of quasi-periodic oscillations in CYG X-2

The EXOSAT observations of Cyg X-2, which reveal two types of periodic oscillations (QPO) associated with different spectral behavior are discussed. The QPO in the range 18 to 55 Hz with a hard spectrum are found in all observations where the source is on the horizontal branch in a hardness-versus-intensity diagram. On the vertical branch, however, QPO's with a soft spectrum are found at a stable frequency of 5.6 Hz. A multitude of QPO frequency-intensity relations, found in different horizontal branch observations, coalesce to an almost unique relation when the QPO frequency is correlated with spectral hardness. A cross-correlation analysis of the rapid variability reveals a time lag of hard photons by 1.5 to 3.8 ms; the lag being smaller when the QPO frequency is larger. Results are consistent with a model where QPO's represent the Kepler frequency at the edge of the neutron star magnetosphere. The data do not require the assumption of a beat frequency. The derived surface magnetic field strength is of the order of 10 billion G.