Discovery of Kilohertz Quasi-periodic Oscillations in GX 17+2

Abstract

We observed the low-mass X-ray binary and Z source GX 172 with the Rossi X-Ray Timing Explorer during 1997 February 6-8, April 1-4, and July 26-27. 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 an 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 of 2. Farther down the Z track, the QPO becomes undetectable, with rms upper limits typically of 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 beat-frequency model as the explanation for at least one of these two phenomena.