Understanding AXPs and SGRs through the December 2004 SGR 1806–20's Hyperflare

Abstract

On 27th December 2004 SGR 1806?20, one of the most active Soft ?-ray Repeaters (SGRs), displayed an extremely rare event, also known as giant flare, during which up to 1047 erg were released in the ~1?1000 keV range in less than 1s. Follow-up VLA observations after the giant flare largely increased the position accuracy of SGR 1806?20, allowing to perform sensitive searches at shorter wavelengths. In particular we carried out observations by using IR adaptive optics (NAOS-CONICA) mounted on VLT which provided images of unprecedented quality (FWHM better than 0.1''). We discovered the likely IR counterpart to SGR 1806?20 based on positional coincidence with the VLA uncertainty region and flux variability of a factor of about 2 correlated with that at higher energies. Among the several high energy datasets obtained during the 27th December 2004 event, we analyzed those taken from the Rossi?XTE which provided the highest ever statistics for a giant flare. We discovered rapid Quasi Periodic Oscillations (QPOs) in RXTE/PCA data of the pulsating tail of the 27th December 2004 giant flare. QPOs at ~92.5 Hz are detected in a 50 s interval starting 170 s 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 Alfv?n waves propagating along magnetospheric field lines.