Gamma-ray Burst Afterglows: Time-Varying Extinction, Polarization, and
  Colors due to Rotational Disruption of Dust Grains
  Prompt optical emission of gamma-ray bursts (GRBs) is known to have important
effects on the surrounding environment. In this paper, we study rotational
disruption and alignment of dust grains by radiative torques (RATs) induced by
GRB afterglows and predict their signatures on the observational properties of
GRB afterglows. We first study grain disruption using RAdiative Torque
Disruption (RATD) mechanism and find that large grains (size $>0.1 \mu\rm m$)
within a distance of $d< 40$ pc from the source can be disrupted into smaller
grains. We then model the extinction curve of GRB afterglows and find that
optical-NIR extinction is rapidly decreased, and UV extinction increases due to
the conversion of large grains into smaller ones via RATD. The
total-to-selective visual extinction ratio is found to decrease from the
standard value of $R_{V}\sim 3.1$ to $\sim 1.5$ after disruption time $t_{\rm
disr} \lesssim 10^{4}$ s. Next, we study grain alignment by RATs induced by GRB
afterglows and model the wavelength-dependence polarization produced by grains
aligned with magnetic fields. We find that polarization degree first increases
due to enhanced alignment of small grains and then decreases when grain
disruption by RATD begins. The maximum polarization wavelength $ \lambda_{\rm
max}$ decreases rapidly from the standard value of $\sim 0.55 \mu\rm m$ to
$\sim 0.15 \mu\rm m$ over alignment time of $t_{\rm align} \lesssim 30$ s due
to enhanced alignment of small grains. Finally, we found that RATD induces a
significant decrease in optical/NIR extinction, producing an optical
re-brightening in the observed light curve of GRB afterglows. We show that our
theoretical predictions can explain various observational properties of GRB
afterglows, including steep extinction curves, time-variability of colors, and
optical re-brightening of GRB afterglows.
