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 det ection 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 hig h-intensity state). This QPO was detected on the so-called normal-branch an d 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-intensit y state. The rms amplitude of this QPO decreased from 2.8 per cent between 2 and 3.1 keV to <1.9 per cent between 5.0 and 6.5 keV. Above 6.5 keV, its amplitude rapidly increased to similar to 12 per cent rms above 16 keV. The time lags of the QPO were consistent with being zero below 5 keV (compared with the 2-3.1 keV band), but they rapidly increased to similar to 70 ms ( 140 degrees) around 10 keV, above which the time lags remained approximatel y constant near 70 ms. The photon energy dependences of the rms amplitude a nd the time lags are very similar to those observed for the NBO with other satellites (Ginga, EXOSAT) at different (i.e. lower) intensity states.