Abstract
The optical afterglow of gamma-ray burst (GRB) 000301C exhibited a significant, 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 fitted by an ad hoc model in which a thin ring of emission is microlensed by an intervening star. We revisit the microlensing interpretation 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 nonparametric measurement 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 microlensing, 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 light curve is consistent with a model in which a jet is propagating into a uniform medium with the cooling break frequency below the optical band.
Highlights
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 ejects material with a relativistic bulkLorentz factor, driving a relativistic shock into the external medium
Namics, the normalized surface brightness proÐles (SBPs) within a given PLS is independent of time
For each k, there are three relevant forms for the SBP, each corresponding to a di†erent PLS : brbaa\\\l i[n[13d3pe(/x(p2la[a(\lf[1ol)r/\4lkc)[\.lamT\)0h, e([kcbo\(\3rrp2e([s)1pa[o1rn)e/pd4)ai]/n2\gfovr12(alblm(ua\\e\s[thfolpecr))/t,2bem,\aapnno13dd, a \ [(3p [ 2)/4 for b \ [p/2
Summary
Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540 ; gaudi=sns.ias.edu Racah Institute of Physics, Hebrew University, Jerusalem, 91904, Israel ; jgranot=nikki.Ðz.huji.ac.il Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 ; aloeb=cfa.harvard.edu Received 2001 May 15 ; accepted 2001 July 2
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