On re-brightening afterglows of XRFs, Soft GRBs and GRB081028

TL;DR

This paper challenges the off-axis jet interpretation of GRB081028's re-brightening, proposing instead that the cannonball model explains the afterglow as a synchrotron flare, aligning with observed properties and extending to soft GRBs and XRFs.

Contribution

It demonstrates that the cannonball model accurately explains the late-time afterglow re-brightening of GRB081028 and predicts similar behavior for soft GRBs and XRFs.

Findings

Re-brightening explained by the CB model as a synchrotron flare.

All observed properties of GRB081028 match the CB model.

Soft GRBs and XRFs have similar afterglow decay as ordinary GRBs.

Abstract

It has been claimed recently that Swift has captured for the first time a late-time afterglow re-brightening of clear nonflaring origin after the steep decay of the prompt emission in a long gamma-ray burst (GRB), which may have been produced by a narrow jet viewed far off-axis. However, this interpretation of the observed re-brightening of the X-ray afterglow (AG) of GRB081028 is unlikely in view of its large equivalent isotropic gamma-ray energy. Moreover, we show that the late-time re-brightening of the AG of GRB081028 is well explained by the cannonball (CB) model of GRBs as a synchrotron flare emitted when the jet that produced the GRB in a star formation region (SFR) in the host galaxy crossed the SFR boundary into the interstellar medium or the halo of the host galaxy. We also show that all the other observed properties of GRB081028 and its afterglow are well reproduced by the CB model. On the other hand, we demonstrate that far-off axis GRBs, which in the CB model are `soft' GRBs and XRFs and consequently have much smaller isotropic equivalent gamma-ray energies, have slowly rising afterglows with a late-time power-law decay identical to that of ordinary GRBs, in good agreement with the CB model predictions.