Two-screen scattering in CRAFT FRBs

Abstract

ABSTRACT Temporal broadening is a commonly observed property of fast radio bursts (FRBs), associated with turbulent media which cause radiowave scattering. Similarly to dispersion, scattering is an important probe of the media along the line of sight to an FRB source, such as the circumburst or circumgalactic media (CGM). Measurements of characteristic scattering times alone are insufficient to constrain the position of the dominant scattering media along the line of sight. However, where more than one scattering screen exists, Galactic scintillation can be leveraged to form strong constraints. We quantify the scattering and scintillation in 10 FRBs with (1) known host galaxies and redshifts and (2) captured voltage data enabling high-time resolution analysis. We find strong evidence for two screens in three cases. For FRBs 20190608B and 20210320C, we find evidence for scattering screens less than approximately 16.7 and 3000 kpc, respectively, from their sources, consistent with the scattering occurring in the circumburst environment, the host interstellar medium (ISM) or the CGM. For FRB 20201124A, we find a low modulation index that evolves over the burst’s scattering tail, indicating the presence of a scattering screen ≈9 kpc from the host, and excluding the circumburst environment from potential scattering sites. By assuming that pulse broadening is contributed by the host galaxy ISM or circumburst environment, the lack of observed scintillation in four FRBs in our sample suggests that existing models may be poor estimators of scattering times associated with the Milky Way’s ISM, similar to the anomalously low scattering observed for FRB 20201124A.