The Case for Late-time Optical Bumps in Grb Afterglows as a Supernova Signature

Observations of late-time optical bumps have been reported for several GRBs. The timescale for such bumps, and colors of such when available, find a natural explanation as due to associated super-novae. Ground-based and HST observations of the afterglow and bump of GRB 011121, in particular, place the strongest constraints yet on the physical nature of the supernova and any alternative explanations, such as supranovae or dust echoes. I summarize the search for underlying bumps in other GRBs and make the case for the supernova hypothesis in light of observed bumps and bump non-detections (e.g., GRB 010921). There is a good deal of theoretical motivation to expect to see bumps in GRB afterglow lightcurves 1. For massive star progenitors (Woosley 1993), a bump can arise either by reflection/reprocessing of the afterglow light by surrounding dust (Waxman & Draine 2000; Esin & Blandford 2000) or from a supernova that accompanies the GRB. A bump might also arise from delayed energy injection by the central source (Dai & Lu 1998). If the afterglow encounters a shell of material with higher density at ∼ 10 17 cm, then the afterglow could rebrighten. For compact binary mergers (e.g., double neutron star coalescence), a late-time bump is not a natural expectation since, a) mergers should occur in homogeneous regions of low density, b) explosive nucleosynthesis leading to a supernova is not expected around such systems, and c) the creation of a stable neutron star after merger that is capable of re-injecting energy after ∼20 days seems implausible. D. Lazzati (this workshop) has given an excellent overview of various progenitor scenarios. Thus the mere existence of bumps offer a strong discriminator between merger and massive star scenarios. Within the confines of the massive star progenitors model, the details of the bumps and accompanying afterglows should also offer insight into the specifics of the progenitors. For example, a bump from a supernova should have the spectral and temporal characteristics of other supernovae observed in the local universe; Woosley (1993) suggested that baryon contamination of the relativistic jets could be minimized if this supernova was of type Ic. A bump from thermal dust emission could have similar rise and fall 1 For the purposes of this presentation, I define a bump as: an increase in flux above an ex-trapolated/interpolated light curve; extra source of emission, owing, as reckoned, to processes of a different physical mechanism. In Bloom et al. …