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Gamma-Ray Burst Afterglows: Time-varying Extinction, Polarization, and Colors due to Rotational Disruption of Dust Grains SCI
  • Thiem Hoang; Nguyen Chau Giang; Le Ngoc Tram;
  • 2020-05-20
  • ASTROPHYSICAL JOURNAL 895 1 : 16-1~16-16
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, RV, is found to decrease from the standard value of ∼3.1 to ∼1.5 after disruption time tdisr???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, λmax, decreases rapidly from the standard value of ∼0.55μm to ∼0.15μm over alignment time of talign???30s 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.