Microlensing and the Surface Brightness Profile of the Afterglow Image of GRB 000301C.pdf

a r X i v : a s t r o - p h / 0 1 0 5 2 4 0 v 2 2 J u l 2 0 0 1 Microlensing and the Surface Brightness Profile of the Afterglow Image of GRB 000301C B. Scott Gaudi1 Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540 gaudi@ Jonathan Granot Racah Institute of Physics, Hebrew University, Jerusalem, 91904, Israel jgranot@nikki.fiz.huji.ac.il Abraham Loeb Harvard-Smithsonian CfA, 60 Garden Street, Cambridge, MA 02138 aloeb@ ABSTRACT The optical afterglow of Gamma-Ray Burst (GRB) 000301C exhibited a sig- nificant, short-timescale deviation from the power-law flux decline expected in the standard synchrotron shock model. Garnavich, Loeb Stanek found that this deviation was well-fit by an ad hoc model in which a thin ring of emission is microlensed by an intervening star. We revisit the microlensing interpreta- tion of this variability, first by testing whether microlensing of afterglow images with realistic surface brightness profiles (SBPs) can fit the data, and second by directly inverting the observed light curve to obtain a non-parametric measure- ment of the SBP. We find that microlensing of realistic SBPs can reproduce the observed deviation, provided that the optical emission arises from frequencies above the cooling break. Conversely, if the variability is indeed caused by mi- crolensing, the SBP must be significantly limb-brightened. Specifically, ≥ 60% of the flux must originate from the outer 25% of the area of the afterglow image. The latter requirement is satisfied by the best fit theoretical SBP. The underlying optical/infrared afterglow lightcurve is consistent with a model in which a jet is propagating into a uniform medium with the cooling break frequency below the optical band. – 2 – gamma rays:bursts – gravitational lensing 1. Introduction The afterglows of gamma-ray bursts (GRBs) are observed in the X-ray, optical, near- infrared, and radio, and appear to be well-described by the synchrotron blast-wave model in which the source eje