Well, I have no background info on GRBs and insufficient math, so most of the paper went over my head. I wrote the following before I read any of the other responses, just so's as to see what came out.

"I love the way information on these mysterious things trickles in: we already know that the speed with which the bursts develop hints at a compact source, and the paper to hand suggests that the gamma rays may be tightly beamed during the first stages of the GRB. If the transient described in the paper was a GRB afterglow, and if the initial gamma ray birst was missed because we weren't in the beam path, then it seems that perhaps because we did see the afterglow that GRBs might have a strong magnetic field creating a tightly focused beam early on but that the field changes rapidly as the GRB evolves."

Doctor Don hints that the source may be less compact than the radiation front makes it seem, ie the evolution of the GRB may be temporally compressed from our viewpoint. I wonder how compact the zone of activity could be and still fit the observations?

And it appears my conjecture about magnetic fields constraining the radiation was wonky. Ah well.

Don Stahl