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

Abstract

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 DM_(Hα) 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 F^~G =1.5_(-0.3}){(0.8) pc² km^(-1/3), 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.

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