2020ApJ...895...16H

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. We first show that large grains (size >0.1 µm) within a distance d < 40 pc from the source can be disrupted into smaller grains by the RAdiative Torque Disruption (RATD) mechanism. We then model the extinction curve of GRB afterglows and find that optical-near-infrared extinction decreases, and ultraviolet (UV) extinction increases due to the enhancement of small grains. The total-to-selective visual extinction ratio, R_V, is found to decrease from the standard value of ∼3.1 to ∼1.5 after disruption time t_disr <= 10⁴ 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 optical-NIR polarization degree first increases due to enhanced alignment of small grains and then decreases when RATD begins. The maximum polarization wavelength, λ_max, decreases rapidly from the standard value of ∼0.55 µm to ∼0.15 µm over alignment time of t_align <= 30 s due to enhanced alignment of small grains. Our theoretical predictions can explain various observational properties of GRB afterglows, including steep extinction curves, time-variability of colors, and optical rebrightening of GRB afterglows.