Abstract
The afterglow of a cosmological gamma-ray burst (GRB) should appear on the sky as a narrow emission ring of radius ~3 × 1016 cm (t/day)5/8 that expands faster than light. After a day, the ring radius is comparable to the Einstein radius of a solar mass lens at a cosmological distance. Thus, microlensing by an intervening star can significantly modify the light curve and polarization signal from a GRB afterglow. We show that the achromatic amplification signal of the afterglow 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 emission from the afterglow photosphere originates from a set of coherent magnetic field patches, microlensing would induce polarization variability as a result of the transient magnification of the patches behind the lens. The microlensing interpretation of the flux and polarization data can be confirmed by a parallax experiment that would probe the amplification peak at different times. The fraction of microlensed afterglows can be used to calibrate the density parameter of stellar-mass objects in the universe.
Highlights
The recent discovery of delayed X-ray (Costa et al 1997), optical, and radio (Frail et al 1997) emission over hours to several months following c-ray bursts (GRB) established a new class of variable sources in astronomy
The short duration of a microlensing event could provide a test for the high Lorentz factor of the afterglow photosphere, which is predicted by all Ðreball models
In ° 2 we describe our model for gamma-ray burst (GRB) afterglows and characterize both the intensity and polarization signals that would result from a microlensing event
Summary
The recent discovery of delayed X-ray (Costa et al 1997), optical (van Paradijs et al 1997 ; Bond 1997), and radio (Frail et al 1997) emission over hours to several months following c-ray bursts (GRB) established a new class of variable sources in astronomy. As a result of relativistic beaming, the emission region seen by an external observer occupies an angle of D1/c relative to the center of the explosion, where c is the Lorentz factor. This region appears to expand faster than the speed of light and occupies an angle of D0.1È102 microarcseconds on the sky (or a physical size of D1015È1018 cm). The short duration of a microlensing event could provide a test for the high Lorentz factor of the afterglow photosphere, which is predicted by all Ðreball models (for comparison, the variations due to peculiar velocities in microlensing events of steady sources take decades rather than days). ° 4 summarizes the main conclusions of this work
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