The 2004 Hyperflare from SGR 1806-20: Further Evidence for Global Torsional Vibrations

We report an analysis of the archival Rossi X-ray Timing Explorer (RXTE) data from the December 2004 hyperflare from SGR 1806-20. In addition to the ≈ 90 Hz QPO first discovered by Israel et al., we report the detection of higher frequency oscillations at ≈ 150, 625, and 1,840 Hz. In addition, we also find indications of oscillations at ≈ 720, and 2,384 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 detected in average power spectra centered on the rotational phase of the strongest peak in the pulse profile. This is approximately half a cycle from the phase at which the 90 Hz QPO is detected. 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 ≈ 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 begining about 230 s after the start of the flare. In these cycles we also detect the 1,840 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 from this hyperflare 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, in particular for the depth of neutron star crusts. Subject headings: stars: magnetic—pulsar: individual (SGR 1806-20)—stars: neutron—stars: rotation—stars: oscillations—X-rays: stars Exploration of the Universe Div., NASA/GSFC, Greenbelt, MD 20771; [email protected] Max Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany; [email protected]