Supranova Model for the Delayed Reddened Optical Excesses Detected in Several GRBs

Excess reddened optical emission has been measured in the power-law decay afterglow light curves of several GRBs some tens of days after the GRB events. This emission is claimed to be a signature of a supernova (SN) taking place at the same time as the GRB event, in support of the collapsar scenario. Observations of features in prompt and afterglow X-ray spectra of GRBs require high density, high metallicity material near GRB sources, supporting the supranova (SA) scenario. These conflicting lines of evidence are resolved in likely regimes of parameter space if the supernova remnant (SNR) shell is illuminated and heated by a pulsar wind (PW), and cools following the GRB event. The heating of the SNR shell by the PW is parameterized, and simplified expressions for the synchrotron, synchrotron self-Compton, and thermal radiation fields within the SNR shell and pulsar wind bubble are derived for monoenergetic injection of wind electrons. A cooling SNR shell could produce the excess emission detected from GRB 970228, GRB 980326, and GRB 011121. X-ray synchrotron radiation from the PW electrons provides an important source of ionizing radiation.