DISCOVERY OF KHZ QUASI-PERIODIC OSCILLATIONS IN THE Z-SOURCE CYGNUS-X-2

During observations with the Rossi X-Ray Timing Explorer from 1997 Jun e 31 to July 3 we discovered two simultaneous kHz quasi-periodic oscil lations (QPOs) near 500 and 860 Hz in the low-mass X-ray binary and Z source Cygnus X-2. In the X-ray color-color diagram and hardness inten sity diagram (HID), a clear Z track was traced out, which shifted in t he HID within 1 day to higher count rates at the end of the observatio n. Z track shifts are well known to occur in Cyg X-2; our observation for the first time catches the source in the act. A single kHz QPO pea k was detected at the left end of the horizontal branch (HB) of the Z track, with a frequency of 731 +/- 20 Hz and an amplitude of 4.7(-0.6) (+0.8)% rms in the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher count rates, an additional peak at 532 +/- 43 Hz was detected with an rms amplitude of 3.0(-0.7)(+1.0). When the sou rce moved down the HB, thus when the inferred mass accretion rate incr eased, the frequency of the higher frequency QPO increased to 839 +/- 13 Hz, and its amplitude decreased to 3.5(-0.3)(+0.4)% rms. The higher frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8(-0.4)(+0.6)% and a frequency of 1007 +/- 15 Hz ; its peak width did not show a clear correlation with inferred mass a ccretion rate. The lower frequency QPO was most of the time undetectab le, with typical upper limits of 2% rms; no conclusion on how this QPO behaved with mass accretion rate can be drawn. If the peak separation between the QPOs is the neutron star spin frequency (as required in s ome beat-frequency models), then the neutron star spin period is 2.9 /- 0.2 ms (346 +/- 29 Hz). This discovery makes Cyg X-2 the fourth Z s ource that displays kHz QPOs. The properties of the kHz QPOs in Cyg X- 2 are similar to those of other Z sources. Simultaneous with the kHz Q POs, the well-known horizontal-branch QPOs (HBOs) were visible in the power spectra. At the left end of the HB, the second harmonic of the H BO was also detected. We also detected six small X-ray bursts. No peri odic oscillations or QPOs were detected in any of them, with typical u pper limits of 6%-8% rms.