Discovery of Rapid X-ray Oscillations in the Tail of the SGR 1806-20 Hyperflare

摘要

We have discovered rapid Quasi Periodic Oscillations (QPOs) in RXTE/PCAmeasurements of the pulsating tail of the 27th December 2004 giant flare of SGR1806-20. QPOs at about 92.5Hz are detected in a 50s interval starting 170safter the onset of the giant flare. These QPOs appear to be associated withincreased emission by a relatively hard unpulsed component and are seen onlyover phases of the 7.56s spin period pulsations away from the main peak. QPOsat about 18 and 30Hz are also detected, 200-300s after the onset of the giantflare. This is the first time that QPOs are unambiguously detected in the fluxof a Soft Gamma-ray Repeater, or any other magnetar candidate. We interpret thehighest QPOs in terms of the coupling of toroidal seismic modes with Alfvenwaves propagating along magnetospheric field lines. The lowest frequency QPOmight instead provide indirect evidence on the strength of the internalmagnetic field of the neutron star.