Radio Scattering Horizons for Galactic and Extragalactic Transients
  Radio wave scattering can cause severe reductions in detection sensitivity
for surveys of Galactic and extragalactic fast ($\sim$ms duration) transients.
While Galactic sources like pulsars are subject to scattering in the Milky Way
interstellar medium (ISM), extragalactic fast radio bursts (FRBs) can also
experience scattering in their host galaxies and other galaxies intervening
their lines-of-sight. We assess Galactic and extragalactic scattering horizons
for fast radio transients using a combination of NE2001 to model the dispersion
measure (DM) and scattering time ($\tau$) contributed by the Milky Way, and
independently constructed electron density models for other galaxies' ISMs and
halos that account for different galaxy morphologies, masses, densities, and
strengths of turbulence. For FRB source redshifts $z_{\rm s} \lesssim 1$, an
all-sky, isotropic FRB population has values of $\tau$ ranging between $\sim 1\
\mu$s and $\sim 2$ ms at 1 GHz (observer frame) that are dominated by host
galaxies. For a hypothetical, high-redshift ($z_{\rm s}\sim5$) FRB population,
$\tau$ ranges from $\sim 0.01 - 100$s of ms at 1 GHz, and is largely dominated
by intervening galaxies. About $20\%$ of these high-redshift FRBs are predicted
to have $\tau > 5$ ms at 1 GHz (observer frame), and $\gtrsim 40\%$ of FRBs
between $z_{\rm s} \sim 0.5 - 5$ are predicted to have $\tau \gtrsim 1$ ms for
$\nu\leq 800$ MHz. The percentage of FRBs selected against from scattering may
be substantially larger because our scattering predictions are conservative
compared to localized FRBs, and if circumgalactic turbulence causes density
fluctuations larger than those observed from nearby halos.
