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 \chi ^ { 2 } / DOF parameter of 1.48 compared with 2.99 for the broken power-law fit alone .
The peak magnification of \sim 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 _ { 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 .