Scintillation timescale measurement of the highly active FRB 20201124A

Scintillation of compact radio sources is caused by the interference between images caused by multipath propagation, and can be used to probe the intervening scattering plasma, and the velocities of the emitting source and scattering screen. In FRB20201124A, a repeating FRB which recently entered a period of extreme activity, we obtained many burst detections in observations at the Giant Metrewave Radio Telescope (GMRT) and the Effelsberg 100 m Radio Telescope. Nearby bursts show similar scintillation patterns, and we were able to measure a scintillation timescale through the correlation of spectra between burst pairs, the first such measurement for an FRB. In addition, we form the 2D autocorrelation function (ACF) and secondary spectrum of scintillation, which hints towards anisotropic scattering. The inferred scintillation velocity is $V_{\mathrm{ISS}}\approx (64\pm7) \sqrt{d_{l}/2\,\rm{kpc}}~{\rm km~s}^{-1}$, higher than Earth's velocity using the fiducial value of the screen distance. From the measured scintillation bandwidth, we find FRB20201124A to be underscattered by a factor of ~20 compared to the NE2001 model predictions, and lower than nearby pulsars. The underscattering, along with the measured scintillation velocity are consistent with a picture where the scattering screen is more nearby the Earth at d_l ~ 400 pc, rather than at 2 kpc in the spiral arm which NE2001 predicts to be the dominant source of scattering. With future measurements, the distance, geometry, and velocity of the scattering screen could be obtained through modelling of the annual variation in $V_{\rm ISS}$, or through inter-station time delays or VLBI. Scintillation/scattering measurements of FRBs could be used to constrain and improve Galactic electron density models, particularly in the Galactic halo or at high Galactic latitudes.