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This paper presents a multiwavelength study of the M8.0 flare and itsassociated fast halo CME that originated from a bipolar active region NOAA10759 on 2005 May 13. The source active region has a conspicuous sigmoidstructure at TRACE 171 A channel as well as in the SXI soft X-ray images, andwe mainly concern ourselves with the detailed process of the sigmoid eruptionas evidenced by the multiwavelength data ranging from Halpha, WL, EUV/UV,radio, and hard X-rays (HXRs). The most important finding is that the flarebrightening starts in the core of the active region earlier than that of therising motion of the flux rope. This timing clearly addresses one of the mainissues in the magnetic eruption onset of sigmoid, namely, whether the eruptionis initiated by an internal tether-cutting to allow the flux rope to riseupward or a flux rope rises due to a loss of equilibrium to later induce tethercutting below it. Our high time cadence SXI and Halpha data shows that thefirst scenario is relevant to this eruption. As other major findings, we havethe RHESSI HXR images showing a change of the HXR source from a confinedfootpoint structure to an elongated ribbon-like structure after the flaremaximum, which we relate to the sigmoid-to-arcade evolution. Radio dynamicspectrum shows a type II precursor that occurred at the time of expansion ofthe sigmoid and a drifting pulsating structure in the flare rising phase inHXR. Finally type II and III bursts are seen at the time of maximum HXRemission, simultaneous with the maximum reconnection rate derived from theflare ribbon motion in UV. We interpret these various observed properties withthe runaway tether-cutting model proposed by Moore et al. in 2001.

The Eruption from a Sigmoidal Solar Active Region on 2005 May 13