Gamma-Ray Bursts in Pulsar Wind Bubbles: Putting the Pieces Together

Abstract

We present the main observational features expected for GRBs that occur inside pulsar wind bubbles (PWBs). This is the most natural outcome of supranova model where initially a supernova explosion takes place, leaving a supra-massive NS, which loses its rotational energy over a time t_sd and collapses to a BH, triggering a GRB explosion. We find that the time delay t_sd between the supernova and GRB events is the most important parameter that determines the behavior of the system. We consider the afterglow, prompt GRB and direct emission from the PWB. The observational signatures for different t_sd are described and joined together into one coherent framework. Constraints on the model are derived for a spherical PWB, from the lack of detection of the PWB emission and afterglow observations. For very low values of t_sd<1 hr the supranova model reduces to the collapsar model; 0.4<t_sd/1yr<1 is required to produce the iron lines seen in some X-ray afterglows. However, we find that for a spherical PWB, this implies no detectable radio afterglow, a small jet break time and non-relativistic transition time, in disagreement with observations for some of these GRBs. These discrepancies with the observations may be reconciled by resorting to a non-spherical geometry. We find that light element lines, recently detected in a few X-ray afterglows, are expected to dominate over iron lines for small t_sd, while for large t_sd the situation is reversed. Finally, we predict that external Compton scattering of the PWB photons by the afterglow relativistic electrons, should lead to high energy emission during the early afterglow that may explain the GeV photons detected by EGRET in some GRBs, and should be detectable by future missions (GLAST).