ar X iv : a st ro - p h / 97 11 21 9 v 1 1 9 N ov 1 99 7 Discovery of KiloHertz Quasi - Periodic Oscillations in the Z source Cygnus X - 2

During observations with the Rossi X-ray Timing Explorer from June 31st to July 3rd 1997 we discovered two simultaneous kHz QPOs near 500 Hz 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-intensity diagram (HID) a clear Z track was traced out, which shifted in the HID within one day to higher count rates at the end of the observation. Z track shifts are well known to occur in Cygnus X-2; our observation for the first time catches the source in the act. A single kHz QPO peak 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% 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%. When the source moved down the HB, thus when the inferred mass accretion rate increased, the frequency of the higher-frequency QPO increased to 839±13 Hz, and its amplitude decreased to 3.5 −0.3% rms. The higher-frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8 −0.4% and a frequency of 1007±15 Hz; its peak width did not show a clear correlation with inferred mass accretion rate. The lower-frequency QPO was most of the time undetectable, 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 some beat-frequency models) then the neutron star spin period is 2.9±0.2 ms (346±29 Hz). This discovery makes Cygnus X-2 the fourth Z source which displays kHz QPOs. The properties of the kHz QPOs in Cygnus X-2 are similar to those of other Z sources. Simultaneous with the kHz QPOs, 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 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, NL1098 SJ Amsterdam, The Netherlands; [email protected], [email protected], [email protected], [email protected] Astrophysics, University of Oxford, Nuclear and Astrophysics Laboratory, Keble Road, Oxford OX1 3RH, United Kingdom; [email protected] Departments of Physics, University of Alabama at Huntsville, Huntsville, AL 35899 Department of Physics and Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139; [email protected] Departments of Physics and Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801; [email protected] Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138; [email protected] Space Radiation Laboratory, California Institute of Technology, 220-47 Downs, Pasadena, CA 91125; [email protected]