Discovery of a variable-frequency 50-60 Hz quasi-periodic oscillation on the normal branch of GX 17+2

We report the discovery, with the Rossi X-Ray Timing Explorer, of a 50-60 Hz quasi-periodic oscillation (QPO) in GX 17+2. The QPO is seen when GX 17+2 is on the normal branch in the X-ray color-color diagram. Its frequency initially increases from 59 to 62 Hz as the source moves down the normal branch, but below the middle of the normal branch it decreases to similar to 50 Hz. Together with this frequency decrease, the QPO peak becomes much broader, from similar to 4 Hz in the upper part of the normal branch to similar to 15 Hz in the lower normal branch. The rms amplitude remains approximately constant between 1% and 2% along the entire normal branch. From a comparison of the properties of this QPO with those of QPOs previously observed along the normal branch in other Z sources, we conclude that it is most likely the horizontal-branch QPO (HBO). However, this QPO displays a number of unusual characteristics. The decrease in the QPO frequency along the lower normal branch is not in agreement with the predictions of the beat-frequency model for the HBO unless the mass flux through the inner disk decreases as the source moves down the lower normal branch. We tentatively suggest that the required decrease in the mass flux through the inner disk is caused by an unusually rapid increase in the mass flux in the radial inflow as GX 17+2 moves down the normal branch. Assuming that this explanation is correct, we can derive an upper bound on the dipole component of the star's magnetic field at the magnetic equator of 5 x 10(9) G for a 1.4 M. neutron star with a radius of 10(6) cm.