SIMBAD references

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 2.8per cent between 2 and 3.1keV to <1.9per cent between 5.0 and 6.5keV. Above 6.5keV, its amplitude rapidly increased to ∼12per cent rms above 16keV. The time lags of the QPO were consistent with being zero below 5keV (compared with the 2-3.1keV band), but they rapidly increased to ∼70ms (140°) around 10keV, above which the time lags remained approximately constant near 70ms. The photon energy dependences 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.