Observational Implications of a Plerionic Environment for GRBs

We consider GRBs that occur inside pulsar wind bubbles (PWBs), in the context of the supranova model, where initially a supernova explosion takes place, leaving behind a supra-massive neutron star (SMNS), which eventually loses its rotational energy and collapses to a black hole, triggering a GRB explosion. The most natural mechanism by which the SMNS can lose its rotational energy is through a strong pulsar type wind, between the supernova and the GRB events, which is expected to create a PWB. We analyze in some detail the observational implications of such a plerionic environment on the afterglow (AG) and prompt GRB emissions, and the prospect for direct detection of the plerion emission. We find that for a simple spherical model, GRBs with iron lines detected in the X-ray AG should not have a detectable radio afterglow, and should have small jet break times and non-relativistic transition times, in disagreement with observations for some of the GRBs with X-ray lines. These discrepancies with the observations may be reconciled by resorting to a non-spherical geometry, where the PWB is elongated along the polar axis. We find that lack of detection of the plerion emission provides interesting constraints on the model parameters. Finally, we predict that the inverse Compton upscattering of the PWB photons by the relativistic electrons of the AG (external Compton, EC) should lead to high energy emission during the early AG that might explain the GeV photons detected by EGRET for a few GRBs, and should be detectable by future missions such as GLAST.