Polarization Signatures of Plunging Hot Spots around Kerr Black Holes

Polarimetric interferometry is a powerful tool for probing black hole accretion astrophysics and the background spacetime. Current models aiming to explain the observed multi-wavelength flares in Sgr A* often assume hotspots moving on stable Keplerian orbits. However, a hotspot formed within or close to the innermost stable circular orbit (ISCO) must plunge towards the black hole. In this talk, we show that the addition of plunging motion produces a distinctive observational signature: a precessing unwinding of the polarimetric Stokes QU looping motion, which sharply contrasts with the QU loop patterns formed from stable orbits. We also show how the morphology of this polarimetric signature depends on black hole spin, inclination, and magnetic field configuration. Current and near-future interferometric observations may detect these linear polarization signatures, offering a method to probe the physics of matter crossing the ISCO and the predictions of relativistic velocities of plunging matter.