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P.M. Garnavich (Notre Dame), A. Loeb, K.Z. Stanek (CfA)
The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic, short time-scale variability that is difficult to reconcile with the standard relativistic shock model. We interpret the observed light curves as a microlensing event superimposed on power-law flux decays typical of afterglows. In general, a relativistic GRB shock appears on the sky as a thin ring expanding at a superluminal speed. Initially the ring is small relative to its angular separation from the lens and so its flux is magnified by a constant factor. As the ring grows and sweeps across the lens its magnification reaches a maximum. Subsequently, the flux gradually recovers its unlensed value. This behavior involves only three free parameters in its simplest formulation and was predicted theoretically by Loeb & Perna (1998). Fitting the available R-band photometric data of GRB 000301C to a simple model of the microlensing event and a broken power-law for the afterglow, we find reasonable values for all the parameters and a reduced \chi2/DOF parameter of 1.12 compared with 2.79 for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8\;days after the burst. The entire optical-IR data imply a width of the GRB ring of order 10% of its radius, similar to theoretical expectations. The angular resolution provided by microlensing is better than a micro-arcsecond. We infer a mass of approximately 0.5\;M\odot for a lens located half way to the source at z\rm s=2.04. A galaxy 2\prime\prime from GRB~000301C might be the host of the stellar lens, but current data provides only an upper-limit on its surface brightness at the GRB position. This work was supported by NASA grants NAG5-9364 (PG), NAG5-7039 (AL) and HF-01124.01 (KS).
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The author(s) of this abstract have provided an email address for comments about the abstract: pgarnavi@nd.edu