Horizontal-branch quasi-periodic oscillation from Cygnus X-2 with a second harmonic

Cyg X-2 was observed by the Rossi X-Ray Timing Explorer during in-orbit checkout in 1996 January and was found in the horizontal, normal, and flaring branches. Horizontal-branch quasi-periodic oscillations were seen with an rms variability of approximately 6% in the fundamental. The second harmonic was also seen with an rms variability about three-fourths that of the fundamental. In the course of 2 hr, the centroid frequency of the fundamental ranged from 17 to 22 Hz, with significant variations on timescales of tens of seconds, and it was positively correlated with source intensity in the 0-20 keV band. The ratio of the width of the fundamental to its centroid frequency was 0.15-0.2. The centroid and width of the second harmonic were 2 times those of the fundamental. The relative positions of the fundamental and harmonic, as well as their correlation with intensity, are consistent with the beat-frequency-modulated accretion model of the quasi-periodic oscillation, which predicts a magnetic field of 2.2 x 10(9) G and a spin frequency of 165 Hz for the neutron star. The relative widths of the two peaks and the time history of the fundamental frequency suggest that the observed width of the peaks is due to more than temporal variation of the frequency.