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

TL;DR

This study investigates the plasma properties of the host galaxy of FRB 20190520B, revealing that its large dispersion measure and scattering are primarily due to the host galaxy's interstellar medium, and proposes a new method to estimate FRB redshifts.

Contribution

The paper provides detailed analysis linking FRB scattering and dispersion to host galaxy plasma, and introduces a novel technique for estimating FRB redshifts based on the tau-DM relation.

Findings

Host galaxy's plasma accounts for the large DM and scattering.

Estimated scattering amplification and host galaxy properties.

Implied source-scattering region distance is less than 100 pc.

Abstract

The repeating FRB 20190520B is localized to a galaxy at $z=0.241$, much closer than expected given its dispersion measure $\rm DM=1205\pm4\ pc\ cm^{-3}$. 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 $\tau(1.41\ {\rm GHz})=10.9\pm1.5$ ms, which is attributed to the host galaxy, and a mean scintillation bandwidth $\nu_{\rm d}(1.41\ {\rm GHz})=0.21\pm0.01$ MHz, which is attributed to the Milky Way. Balmer line measurements for the host imply an H$\alpha$ emission measure (galaxy frame) $\rm EM_s=620$ pc cm$^{-6} \times (T/10^4 {\rm K})^{0.9}$, implying $\rm DM_{\rm H\alpha}$ of order the value inferred from the FRB DM budget, $\rm DM_h=1121^{+89}_{-138}$ pc cm$^{-3}$ for plasma temperatures greater than the typical value $10^4$ K. Combining $\tau$ and $\rm DM_h$ yields a nominal constraint on the scattering amplification from the host galaxy $\tilde{F} G=1.5^{+0.8}_{-0.3}$ (pc$^2$ km)$^{-1/3}$, where $\tilde{F}$ 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 $\tau$ arises from the host galaxy and $\Delta \nu_{\rm d}$ from the Milky Way, the implied distance between the FRB source and dominant scattering material is $\lesssim100$ pc. The host galaxy scattering and DM contributions support a novel technique for estimating FRB redshifts using the $\tau-\rm DM$ relation, and are consistent with previous findings that scattering of localized FRBs is largely dominated by plasma within host galaxies and the Milky Way.