Could the Unusual Optical Afterglow of GRB 000301c Arise from a Non-relativistic Shock with Energy Injection?

Abstract

Recent observations on the GRB 000301c afterglow show that three breaks appear in the R-band light curve, and in particular the decay slope at late times is as steep as -3.0. This unusual afterglow is clearly inconsistent with the standard afterglow shock model. Here we propose a non-standard model for the unusual R-band afterglow of GRB 000301c. In this model, an ultra-relativistic shock in a dense medium ("dirty environment")rapidly evolved to the non-relativistic phase in initial 1 day. During such a phase, the shock happened to be caught up with by two energetic shells ejected from the central engine at two different times, and the shock was refreshed, leading to two flattenings of the light curve. After each interaction between the shock and shell, the afterglow decayed as $\propto t^{-3.0}$ if the electron distribution index of the shocked medium, $p\approx 3.4$, derived from the optical spectrum. Therefore, this model can provide an excellent explanation for the flattening and steepening features of the GRB 000301c optical afterglow light curve. We further point out that the energy injection shells ejected from the central engine at later times may be material shells (e.g., in the massive star progenitor models related to black holes) or radiation shells (e.g., in the millisecond pulsar progenitor models).