Normal-Branch Quasi-Periodic Oscillations during the High Intensity State of Cygnus X-2

Using data obtained with the Rossi X-ray Timing Explorer, we report the detection of a 5 Hz quasi-periodic oscillation (QPO) in the bright low-mass X-ray binary and Z source Cygnus X-2 during high overall intensities (the high intensity state). This QPO was detected on the so-called normal branch and can be identified with the normal branch QPO or NBO. Our detection of the NBO is the first one during times when Cygnus X-2 was in the high intensity state. The rms amplitude of this QPO decreased from 2.8% between 2–3.1 keV to <1.9% between 5.0–6.5 keV. Above 6.5 keV, its amplitude rapidly increased to ∼12% rms above 16 keV. The time lags of the QPO were consistent with being zero below 5 keV (compared to the 2–3.1 keV band), but they rapidly increased to ∼70 ms (140◦) around 10 keV, above which the time lags remained approximately constant near 70 ms. The photon energy dependencies of the rms amplitude and the time lags are very similar to those observed for the NBO with other satellites (Ginga, EXOSAT) at different (i.e. lower) intensity states.