We use the relativistic hydrodynamics code Cosmos ++ to model the evolution of the radio nebula triggered by the Dec. 27 , 2004 giant flare event of soft gamma repeater 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 \sim 1.07 - 1.67 ) jetted outflow expanding into the interstellar medium ( ISM ) .
We demonstrate that a jet with total energy \sim 10 ^ { 46 } ergs confined to a half opening angle \sim 20 ^ { \circ } fits the key observables of this event , e.g .
the flux lightcurve , 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 0.5 c and inclined \sim 0 ^ { \circ } -40 ^ { \circ } toward the observer , colliding with a density discontinuity in the ISM at a radius of several 10 ^ { 16 } cm .
We also find that a jet with a higher velocity , \gtrsim 0.7 c , and larger inclination , \gtrsim 70 ^ { \circ } , 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 which might discriminate between the two .
One of the strongest constraints of both models is that the data seems to require a non-uniform jet in order to be well fit .