Discovery of a 57-69 Hz quasi-periodic oscillation in GX 13+1

We report the discovery of a quasi-periodic oscillation (QPO) at 61.0 +/- 1.7 Hz with the Rossi X-Ray Timing Explorer in the low-mass X-ray binary and persistently bright atoll source GX 13+1 (4U 1811-17). The QPO had an rms amplitude of 1.7% +/- 0.2% (2-13.0 keV) and an FWHM of 15.9 +/- 4.2 Hz. Its frequency increased with the count rate, and its amplitude increased with the photon energy. In addition, a peaked noise component was found with a cutoff frequency of similar to 2 Hz, a power-law index of around -4, and an rms amplitude of similar to 1.8%, probably the well-known atoll source high-frequency noise. It was found only when the QPO was detected. Very low frequency noise was present, with a power-law index of similar to 1 and an rms amplitude of similar to 4%. A second observation showed similar variability components. In the X-ray color-color diagram, the source did not trace out the usual banana branch but showed a two-branched structure. This is the first detection of a QPO in one of the four persistently bright atoll sources in the Galactic bulge. We argue that the QPO properties indicate that it is the same phenomenon as the horizontal-branch oscillations (HBOs) in Z sources. That HBOs might turn up in the persistently bright atoll sources was previously suggested on the basis of the magnetospheric beat-frequency model for HBOs. We discuss the properties of the new phenomenon within the framework of this model.