Time-Domain Dust Astrophysics. I. Polarization Flares, Polarization-Angle Reverberation, and Fossil Imprints in Supernova-Illuminated Clouds

Cosmic transients can dramatically enhance the local radiation field on timescales of days to months. Using the time-domain TransRAT framework, which self-consistently evolves grain heating, alignment, rotational disruption, and switching of the alignment axis between the magnetic field (B-RAT) and the radiation direction (k-RAT), we predict the time-dependent dust polarization of a dense cloud illuminated by a Type IIP supernova at different distances. We identify four key signatures. First, for D 1 pc, a polarization flare develops within days to weeks, marked by sharp increases in both thermal dust polarization and extinction-polarization efficiency; this is followed by a polarization dip as radiative torque disruption (RAT-D) destroys the large aligned grains. Second, the peak wavelength of extinction polarization, lambda max, shifts blueward as the minimum aligned-grain size decreases, providing a diagnostic largely independent of magnetic-field geometry. Third, the transition from B-RAT to k-RAT produces an abrupt polarization-angle rotation of 45 in our fiducial geometry. Fourth, as the transient fades, the return to B-RAT generates a polarization-angle reverberation governed by Larmor precession. This reverberation is the most sensitive probe of grain magnetism, with superparamagnetic grains recovering more rapidly than paramagnetic grains. At D>1 pc, SN-induced polarization properties persist long after the radiation has faded, leaving a fossil imprint. This imprint offers the most practical near-term observational test: clouds near supernova remnants younger than the relaxation timescale of about10,t gas with t gas gas damping time, should exhibit elevated polarization and blueshifted lambda max today.