Numerical Modeling of the Radio Nebula from the 2004 December 27 Giant Flare of SGR 1806−20

Abstract

We use the relativistic hydrodynamics code Cosmos++ to model the evolution of the radio nebula triggered by the 2004 December 27 giant flare event of SGR 1806-20. We primarily focus on the rebrightening and centroid motion occurring subsequent to day 20 following the flare event. We model this period as a mildly relativistic (gamma similar to 1.07-1.67) jetted outflow expanding into the ISM. We demonstrate that a jet with total energy similar to 10(46) ergs confined to a half opening angle similar to 20 degrees fits the key observables of this event, e. g., the flux light curve, emission map centroid position, and aspect ratio. In particular, we find excellent agreement with observations if the rebrightening is due to the jet, moving at 0.5c and inclined similar to 0 degrees-40 degrees toward the observer, colliding with a density discontinuity in the ISM at a radius of several times 10(16) cm. We also find that a jet with a higher velocity, greater than or similar to 0.7c, and larger inclination, greater than or similar to 70 degrees, moving into a uniform ISM can fit the observations in general but tends to miss the details of rebrightening. The latter, uniform ISM model predicts an ISM density more than 100 times lower than that of the former model and thus suggests an independent test that might discriminate between the two. One of the strongest constraints of both models is that the data seem to require a nonuniform jet in order to be well fitted.