MICROLENSING OF GAMMA-RAY BURST AFTERGLOWS

The afterglow of a cosmological gamma-ray burst (GRB) should appear on the sky as a narrow emission ring of radius similar to 3 x 10(16) cm (t/day)(5/8) that expands faster than light. After a day, the ring rad ius is comparable to the Einstein radius of a solar mass lens at a cos mological distance. Thus, microlensing by an intervening star can sign ificantly modify the light curve and polarization signal from a GRB af terglow. We show that the achromatic amplification signal of the after glow flux can be used to determine the impact parameter and expansion rate of the source in units of the Einstein radius of the lens, and we probe the superluminal nature of the expansion. If the synchrotron em ission from the afterglow photosphere originates from a set of coheren t magnetic field patches, microlensing would induce polarization varia bility as a result of the transient magnification of the patches behin d the lens. The microlensing interpretation of the flux and polarizati on data can be confirmed by a parallax experiment that would probe the amplification peak at different times. The fraction of microlensed af terglows can be used to calibrate the density parameter of stellar-mas s objects in the universe.