The 2004 hyperflare from SGR 1806-20: Further evidence for global torsional vibrations

We report an analysis of archival RXTE data from the 2004 December hyperflare from SGR 1806-20. In addition to the approximate to 90 Hz QPO first discovered by Israel et al., we report the detection of higher frequency oscillations at approximate to 150, 625, and 1840 Hz. We also find evidence of oscillations at; 720, and 2384 Hz, but with lower significances. The 150 Hz QPO has a width (FWHM) of about 17 Hz, an average amplitude (rms) of 6.8% and is associated with the strongest peak in the pulse profile. The 625 Hz oscillation was detected in an average power spectrum from nine Successive cycles beginning approximately 180 s after the initial hard spike. It has a width (FWHM) of approximate to 2 Hz and an average amplitude (rms) during this interval of 8.5%. We find a strong detection of the 625 Hz oscillation in a pair of successive rotation cycles beginning about 230 s after the start of the flare. In these cycles we also detect the 1840 Hz QPO. When the 625 Hz QPO is detected we also confirm the simultaneous presence of 30 and 92 Hz QPOs. The centroid frequency of the 625 Hz QPO detected with RXTE is within 1 Hz of the 626 Hz oscillation recently found in RHESSI data by Watts & Strohmayer. We argue that these new findings provide further evidence for a connection of these oscillations with global oscillation modes of neutron stars, in particular, the high-frequency signals may represent toroidal modes with at least one radial node in the crust. We discuss their implications in the context of this model and for the depth of neutron star crusts.