Radio Wave Propagation and the Provenance of Fast Radio Bursts

We analyze plasma dispersion and scattering of fast radio bursts (FRBs) to identify the dominant locations of free electrons along their lines of sight and thus constrain the distances of the burst sources themselves. We establish the average $tau$-DM relation for Galactic pulsars and use it as a benchmark for discussing FRB scattering. Though scattering times $tau$ for FRBs are large in the majority of the 17 events we analyze, they are systematically smaller than those of Galactic pulsars that have similar dispersion measures (DMs). The lack of any correlation between $tau$ and DM for FRBs suggests that the intergalactic medium (IGM) cannot account for both $tau$ and DM. We therefore consider mixed models involving the IGM and host galaxies. If the IGM contributes significantly to DM while host galaxies dominate $tau$, the scattering deficit with respect to the mean Galactic trend can be explained with a $tau$-DM relation in the host that matches that for the Milky Way. However, it is possible that hosts dominate both $tau$ and DM, in which case the observed scattering deficits require free electrons in the host to be less turbulent than in the Galaxy, such as if they are in hot rather than warm ionized regions. Our results imply that distances or redshifts of FRB sources can be significantly overestimated if they are based on the assumption that the extragalactic portion of DM is dominated by the IGM.