Microlensing and the Surface Brightness Profile of the Afterglow Image of Gamma‐Ray Burst 000301C

The optical afterglow of Gamma-Ray Burst (GRB) 000301C exhibited asignificant, short-timescale deviation from the power-law flux decline expectedin the standard synchrotron shock model. Garnavich, Loeb & Stanek found thatthis deviation was well-fit by an ad hoc model in which a thin ring of emissionis microlensed by an intervening star. We revisit the microlensinginterpretation of this variability, first by testing whether microlensing ofafterglow images with realistic surface brightness profiles (SBPs) can fit thedata, and second by directly inverting the observed light curve to obtain anon-parametric measurement of the SBP. We find that microlensing of realisticSBPs can reproduce the observed deviation, provided that the optical emissionarises from frequencies above the cooling break. Conversely, if the variabilityis indeed caused by microlensing, the SBP must be significantlylimb-brightened. Specifically, greater than 60% of the flux must originate fromthe outer 25% of the area of the afterglow image. The latter requirement issatisfied by the best fit theoretical SBP. The underlying optical/infraredafterglow lightcurve is consistent with a model in which a jet is propagatinginto a uniform medium with the cooling break frequency below the optical band.Comment: 14 pages, including 3 figures and 1 Table, very minor changes. Accepted to ApJ main journal. To appear in the November 1, 2001 issue (v561