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 nois e component was found with a cutoff frequency of similar to 2 Hz, a po wer-law index of around -4, and an rms amplitude of similar to 1.8%, p robably 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 simila r to 4%. A second observation showed similar variability components. I n the X-ray color-color diagram, the source did not trace out the usua l banana branch but showed a two-branched structure. This is the first detection of a QPO in one of the four persistently bright atoll sourc es in the Galactic bulge. We argue that the QPO properties indicate th at it is the same phenomenon as the horizontal-branch oscillations (HB Os) in Z sources. That HBOs might turn up in the persistently bright a toll sources was previously suggested on the basis of the magnetospher ic beat-frequency model for HBOs. We discuss the properties of the new phenomenon within the framework of this model.