Giant Flares as Mini Gamma Ray Bursts

On December 27, 2004, practically all gamma-ray satellites were saturated by a giant flare from SGR1806−20 (Borkowski et al. 2004; Hurley et al. 2004; Mazets et al. 2004). This giant flare was strong enough to affect our ionosphere (Campbell et al. 2005) and for its reflection from the moon to be detected (Golenetskii et al. 2004). It was followed by an extremely bright radio afterglow, with an observable extended size (Cameron & Kulkarni 2005c; Gaensler et al. 2005a). Here we show that the inner workings of such intense emission are loaded fireballs, essentially like those invoked for their brighter brothers GRBs. In contrast with pure fireballs, made of radiation and pairs, the energy is carried out from a compact source by baryons or Poynting flux to large radii where it is converted to radiation. The observed radio afterglow supports this interpretation. The giant flare from SGR1806−20 that lasted less than 0.25sec, was so powerful that we have only lower limits on its fluence. A particle detector on the RHESSI satellite provides the best lower limit (Boggs et al. 2005): 0.3 erg cm, corresponding to energy larger than 0.8× 10 erg at a distance of 15kpc (Corbel & Eikenberry 2004). The reflection from the moon, as detected by Helicon-Corona-F (Golenetskii et al. 2004) with a fluence of 7.5 × 10erg cm, provides an alternative mean of estimating the fluence immuned to saturation but limited to the energy range 25-400keV. The albedo of the moon in this energy range is around (Nakar & Piran 2005) 0.25, resulting in an isotropic energy release of about 10 erg. The spectrum is hard with significant emission above (Boggs et al. 2005) 650keV, implying a larger total energy. This energy exceeds the energies of the giant flares from SGR 0526−66 (the famous March 5h 1979 event) and from SGR 1900+14 (August 27h 1998) by a factor of a hundred (Mazets et al. 1999). Like the two other giant flares this flare was followed by a pulsed softer X-ray emission