Constraining Galaxy Haloes from the Dispersion and Scattering of Fast Radio Bursts and Pulsars

TL;DR

This paper uses precise measurements of fast radio bursts to place upper limits on the scattering caused by the Milky Way halo and other galaxy haloes, constraining their electron density fluctuations and impact on observed signals.

Contribution

It introduces a likelihood analysis method to constrain the scattering contributions of galaxy haloes to FRBs, providing new upper limits on halo electron density fluctuations.

Findings

The Milky Way halo's scattering contribution is limited to less than 12 microseconds at 1 GHz.

Electron density fluctuations in galaxy haloes are at least an order of magnitude smaller than in the Galactic disk.

Haloes of other galaxies contribute negligibly to FRB scattering, consistent with observations.

Abstract

Fast radio bursts (FRBs) can be scattered by ionized gas in their local environments, host galaxies, intervening galaxies along their lines-of-sight, the intergalactic medium, and the Milky Way. The relative contributions of these different media depend on their geometric configuration and the internal properties of the gas. When these relative contributions are well understood, FRB scattering is a powerful probe of density fluctuations along the line-of-sight. The precise scattering measurements for FRB 121102 and FRB 180916 allow us to place an upper limit on the amount of scattering contributed by the Milky Way halo to these FRBs. The scattering time $\tau\propto(\tilde{F} \times {\rm DM}^2) A_\tau$, where ${\rm DM}$ is the dispersion measure, $\tilde{F}$ quantifies electron density variations with $\tilde{F}=0$ for a smooth medium, and the dimensionless constant $A_\tau$ quantifies the difference between the mean scattering delay and the $1/e$ scattering time typically measured. A likelihood analysis of the observed scattering and halo DM constraints finds that $\tilde{F}$ is at least an order of magnitude smaller in the halo than in the Galactic disk. The maximum pulse broadening from the halo is $\tau\lesssim12$ $\mu$s at 1 GHz. We compare our analysis of the Milky Way halo with other galaxy haloes by placing limits on the scattering contributions from haloes intersecting the lines-of-sight to FRB 181112 and FRB 191108. Our results are consistent with haloes making negligible or very small contributions to the scattering times of these FRBs.