Modelling Annual Scintillation Velocity Variations of FRB 20201124A

Abstract

Compact radio sources exhibit scintillation, an interference pattern arising from propagation through inhomogeneous plasma, where scintillation patterns encode the relative distances and velocities of the source, scattering material, and Earth. In Main et al. 2022, we showed that the scintillation velocity of the repeating fast radio burst FRB20201124A can be measured by correlating pairs of burst spectra, and suggested that the scattering was nearby the Earth at $\sim0.4\,$kpc from the low values of the scintillation velocity and scattering timescale. In this work, we have measured the scintillation velocity at 10 epochs spanning a year, observing an annual variation which strongly implies the screen is within the Milky Way. Modelling the annual variation with a 1D anisotropic or 2D isotropic screen results in a screen distance $d_{l} = 0.24\pm0.04\,$pc or $d_{l} = 0.37\pm0.07\,$pc from Earth respectively, possibly associated with the Local Bubble or the edge of the Orion-Eridanus Superbubble. Continued monitoring, and using measurements from other telescopes particularly at times of low effective velocity will help probe changes in screen properties, and distinguish between screen models. Where scintillation of an FRB originates in its host galaxy or local environment, these techniques could be used to detect orbital motion, and probe the FRB's local ionized environment.Comment: 5 pages, 5 Figures, submitted to MNRAS Letter