  We report the discovery of complex high frequency variability during the
August 27, 1998 giant flare from SGR 1900+14 using the Rossi X-ray Timing
Explorer (RXTE). We detect an 84 Hz oscillation (QPO) during a 1 s interval
beginning approximately 1 min after the initial hard spike. The modulation
amplitude is energy dependent, reaching a maximum of 26% (rms) for photons
above 30 keV, and is not detected below 11 keV, with a 90% confidence upper
limit of 14% (rms). Remarkably, additional QPOs are detected in the average
power spectrum of data segments centered on the rotational phase at which the
84 Hz signal was detected. Two signals, at 53.5 and 155.1 Hz, are strongly
detected, while a third feature at 28 Hz is found with lower significance.
These QPOs are not detected at other rotational phases. The phenomenology seen
in the SGR 1900+14 flare is similar to that of QPOs recently reported by Israel
et al. from the December 27, 2004 flare from SGR 1806-20, suggesting they may
have a common origin, perhaps torsional vibrations of the neutron star crust.
Indeed, an association of the four frequencies (in increasing order) found in
SGR 1900+14 with l = 2, 4, 7, and 13 toroidal modes appears plausible. We
discuss our findings in the context of this model and show that if the stars
have similar masses then the magnetic field in SGR 1806-20 must be about twice
as large as in SGR 1900+14, broadly consistent with magnetic field estimates
from pulse timing.
