  We have discovered rapid Quasi Periodic Oscillations (QPOs) in RXTE/PCA
measurements of the pulsating tail of the 27th December 2004 giant flare of SGR
1806-20. QPOs at about 92.5Hz are detected in a 50s interval starting 170s
after the onset of the giant flare. These QPOs appear to be associated with
increased emission by a relatively hard unpulsed component and are seen only
over phases of the 7.56s spin period pulsations away from the main peak. QPOs
at about 18 and 30Hz are also detected, 200-300s after the onset of the giant
flare. This is the first time that QPOs are unambiguously detected in the flux
of a Soft Gamma-ray Repeater, or any other magnetar candidate. We interpret the
highest QPOs in terms of the coupling of toroidal seismic modes with Alfven
waves propagating along magnetospheric field lines. The lowest frequency QPO
might instead provide indirect evidence on the strength of the internal
magnetic field of the neutron star.
