The Large Dispersion and Scattering of FRB 20190520B Are Dominated by the Host Galaxy

The repeating fast radio burst FRB 20190520B is localized to a galaxy at z = 0.241, much closer than expected given its dispersion measure DM = 1205 ± 4 pc cm-3. Here we assess implications of the large DM and scattering observed from FRB 20190520B for the host galaxy's plasma properties. A sample of 75 bursts detected with the Five-hundred-meter Aperture Spherical radio Telescope shows scattering on two scales: a mean temporal delay τ(1.41 GHz) = 10.9 ± 1.5 ms, which is attributed to the host galaxy, and a mean scintillation bandwidth δν d(1.41 GHz) = 0.21 ± 0.01 MHz, which is attributed to the Milky Way. Balmer line measurements for the host imply an Hα emission measure (galaxy frame) EMs = 620 pc cm-6 × (T/104 K)0.9, implying DMHα of order the value inferred from the FRB DM budget, DMh=1121-138+89 pc cm-3 for plasma temperatures greater than the typical value 104 K. Combining τ and DMh yields a nominal constraint on the scattering amplification from the host galaxy FG=1.5-0.3+0.8(pc2km)-1/3, where F describes turbulent density fluctuations and G represents the geometric leverage to scattering that depends on the location of the scattering material. For a two-screen scattering geometry where τ arises from the host galaxy and Δν d from the Milky Way, the implied distance between the FRB source and dominant scattering material is ≤100 pc. The host galaxy scattering and DM contributions support a novel technique for estimating FRB redshifts using the τ-DM relation, and are consistent with previous findings that scattering of localized FRBs is largely dominated by plasma within host galaxies and the Milky Way