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