The Galactic Halo Contribution to the Dispersion Measure of Extragalactic Fast Radio Bursts

TL;DR

This paper introduces a new model of the Milky Way halo's contribution to the dispersion measure of extragalactic fast radio bursts, incorporating X-ray observations and a dual-component gas structure.

Contribution

It presents a comprehensive MW halo DM model including spherical and disk-like hot gas components, aligning with recent X-ray data and baryon mass estimates.

Findings

Predicted mean halo DM of 43 pc cm^-3

Range of 30-245 pc cm^-3 across the sky

Accounted for 0.2 dex fluctuation in DM

Abstract

A new model of the Milky Way (MW) halo component of the dispersion measure (DM) for extragalactic sources, such as fast radio bursts (FRBs), is presented in light of recent diffuse X-ray observations. In addition to the spherical component of isothermal gas ($kT\sim0.3$ keV) in hydrostatic equilibrium with the Galactic gravitational potential, our model includes a disk-like non-spherical hot gas component to reproduce the directional dependence of the observed X-ray emission measure (EM). The total gas mass ($1.2\times10^{11}\,M_{\odot}$) is dominated by the spherical component, and is consistent with the total baryon mass of the MW expected from the dark matter mass and the cosmic baryon-to-dark-matter ratio. Our model predicts a mean halo DM of $43\:\,{\rm pc\:cm^{-3}}$, with a full range of $30$-$245\:\,{\rm pc\:cm^{-3}}$ over the whole sky. The large scatter seen in the X-ray EM data implies a $\sim0.2$ dex (rms) fluctuation of the MW halo DM. We provide an analytic formula to estimate the MW halo DM of our model along any line of sight, which can be easily used to compute the total MW component of DM toward extragalactic sources, in combination with existing DM models of the warm ionized medium associated with the Galactic disk.