Precursor plerionic activity and high-energy gamma-ray emission in the supranova model of gamma-ray bursts

The supranova model of gamma-ray bursts (GRBs), in which the GRB event is preceded by a supernova (SN) explosion by a few months to years, has recently gained support from Fe line detections in the X-ray afterglows of some GRBs. A crucial ingredient of this model yet to be studied is the fast-rotating pulsar that should be active during the time interval between the SN and the GRB, driving a powerful wind and a luminous plerionic nebula. We discuss some observational consequences of this precursor plerion, which should provide important tests for the supranova model: (1) the fragmentation of the outlying SN ejecta material by the plerion and its implications for Fe line emission and (2) the effect of inverse Compton cooling and emission in the GRB external shock due to the plerion radiation field. The plerion-induced inverse Compton emission can dominate in the GeV-TeV energy range during the afterglow, being detectable by GLAST from redshifts z≲1.5 and by AGILE from redshifts z≲0.5 and being distinguishable from self-Compton emission by its spectrum and light curve. The prospects for direct detection and identification of the precursor plerion emission are also briefly considered.