Microlensing of collimated gamma-ray burst afterglows

We investigate the stellar microlensing of collimated gamma-ray burst afterglows. A spherical afterglow appears on the sky as a superluminally expanding thin ring (a ringlike image), which is maximally amplified as it crosses a lens. We find that the image of the collimated afterglow becomes quite uniform (a disklike image) after the jet-break time (after the Lorentz factor of the jet drops below the inverse of the jet opening angle). Consequently, the amplification peak in the light curve after the break time is lower and broader. Therefore, detailed monitoring of the amplification history will be able to test whether afterglows are jets or not, i.e., disklike or not, if lensing occurs after the break time. We also show that some proper motion and polarization is expected, peaking around the maximum amplification. The simultaneous detection of proper motion and polarization will indicate that the brightening of the light curve is due to microlensing.