Shading and Smothering of Gamma-Ray Bursts

Abstract

GRB 980425 is distinctive in that it seems to be associated with SN 1998bw, has no X-ray afterglow, and has a rounded, structureless, single-peak light curve and a relatively soft spectrum. The supernova is itself unusual in that its expansion velocity exceeds c/6. We suggest that many of these features can be accounted for with the hypothesis that we observe the gamma-ray burst along a penumbral line of sight that contains mainly photons that have scattered off ejected baryons. The hypothesis suggests a baryon-poor jet existing within a baryon-rich outflow. The sharp distinction can be attributed to whether or not the magnetic field lines thread an event horizon. Such a configuration suggests that there will be some nonthermal acceleration of pickup ex-neutrons within the baryon-poor jet. This scenario might produce observable spallation products and neutrinos.