Detection withRHESSIof High-Frequency X-Ray Oscillations in the Tailof the 2004 Hyperflare from SGR 1806-20

The recent discovery of high frequency oscillations in giant flares from SGR1806-20 and SGR 1900+14 may be the first direct detection of vibrations in aneutron star crust. If this interpretation is correct it offers a novel meansof testing the neutron star equation of state, crustal breaking strain, andmagnetic field configuration. Using timing data from RHESSI, we have confirmedthe detection of a 92.5 Hz Quasi-Periodic Oscillation (QPO) in the tail of theSGR 1806-20 giant flare. We also find another, stronger, QPO at higherenergies, at 626.5 Hz. Both QPOs are visible only at particular (but different)rotational phases, implying an association with a specific area of the neutronstar surface or magnetosphere. At lower frequencies we confirm the detection ofan 18 Hz QPO, at the same rotational phase as the 92.5 Hz QPO, and report theadditional presence of a broad 26 Hz QPO. We are however unable to make arobust confirmation of the presence of a 30 Hz QPO, despite higher countrates.We discuss our results in the light of neutron star vibration models.Comment: 5 pages, 5 figures using emulateapj. Corrected minor typo that appeared in journal versio