Discovery of KiloHertz Quasi-Periodic Oscillations in GX 17+2

We observed the low-mass X-ray binary and Z source GX 17+2 with the Rossi Xray Timing Explorer during 6–8 Feb 1997, 1–4 Apr 1997, and 26–27 Jul 1997. The X-ray color-color diagram shows a clear Z track. Two simultaneous kHz quasi-periodic oscillations (QPOs) are present in each observation, whose frequencies are well correlated with the position of the source on the Z-track. At the left end of the horizontal branch (HB) only the higher frequency peak is observed, at 645±9 Hz, with an rms amplitude of 5.7±0.5%, and a FWHM of 183±35 Hz. When the source moves down the Z track to the upper normal branch the frequency of the kHz QPO increases to 1087±12 Hz, and the rms amplitude and FWHM decrease by a factor 2. Further down the Z the QPO becomes undetectable, with rms upper limits of typically 2.0%. Halfway down the HB a second QPO appears in the power spectra with a frequency of 480±23 Hz. The frequency of this QPO also increases when the source moves along the Z track, up to 781±11 Hz halfway down the normal branch, while the rms amplitude and FWHM stay approximately constant at 2.5% and 70 Hz. The QPO frequency difference is constant at 293.5±7.5 Hz. Simultaneously with the kHz QPOs we detect HB QPOs (HBOs). The simultaneous presence of HBOs and kHz QPOs excludes the magnetospheric beatfrequency model as the explanation for at least one of these two phenomena. Subject headings: accretion, accretion disks — stars: individual (GX 17+2) — stars: neutron — X-rays: stars Astronomical Institute “Anton Pannekoek”, University of Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, NL-1098 SJ Amsterdam, The Netherlands; [email protected], [email protected], [email protected], [email protected], [email protected] Facultad de Ciencias Astronómicas y Geof́ısicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, 1900 La Plata, Argentina 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], [email protected] Space Radiation Laboratory, California Institute of Technology, 220-47 Downs, Pasadena, CA 91125; [email protected]