ar X iv : a st ro - p h / 98 03 26 7 v 1 2 3 M ar 1 99 8 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 a FWHM of 15.9±4.2 Hz. Its frequency increased with count rate and its amplitude increased with photon energy. In addition a peaked noise component was found with a cut-off frequency around 2 Hz, a power law index of around –4, and an rms amplitude of ∼1.8%, probably the well known atoll source high frequency noise. It was only found when the QPO was detected. Very low frequency noise was present with a power law index of ∼1, and an rms amplitude of ∼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 (HBO) in Z sources. That HBO might turn up in the persistently bright atoll sources was previously suggested on the basis of the magnetospheric beat frequency model for HBO. We discuss the properties of the new phenomenon within the framework of this model. Subject headings: accretion, accretion disk stars: individual (GX 13+1) stars: neutron X-rays: stars Astronomical Institute ’Anton Pannekoek’, University of Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands; [email protected], [email protected], [email protected] Space Radiation Laboratory, California Institute of Technology, 220-47 Downs, Pasadena, CA 91125; [email protected] Astrophysics, University of Oxford, Nuclear and Astrophysics Laboratory, Keble Road, Oxford OX1 3RH, United Kingdom; [email protected]