2022ApJ...931...87O

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) EM_s = 620 pc cm^–6 x (T/10⁴ K)^0.9, implying DM_Hα of order the value inferred from the FRB DM budget, {\mathrm{DM}}_{{\rm{h}}}={1121}_{-138}^{+89} pc cm^–3 for plasma temperatures greater than the typical value 10⁴ K. Combining τ and DM_h yields a nominal constraint on the scattering amplification from the host galaxy \widetilde{F}G\,=\,{1.5}_{-0.3}^{+0.8}{({\mathrm{pc}}^{2}\,\mathrm{km})}^{-1/3}, where \widetilde{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.