Observational implications of a plerionic environment for gamma-ray bursts

We consider the possibility that at least some gamma-ray burst (GRB) explosions take place 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 loses its rotational energy on a time-scale of months to tens of years 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 analyse in some detail the observational implications of such a plerionic environment on the afterglow 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 their X-ray afterglow 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 the emission from the PWB should persist well into the afterglow, and the lack of detection of such a component 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 afterglow (external Compton, EC) should lead to high-energy emission during the early afterglow that might explain the GeV photons detected by EGRET for a few GRBs, and should be detectable by future missions such as GLAST.