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 undergo 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 Galactic disk, and independently constructed electron density models for the Galactic halo and other galaxies' ISMs and halos that account for different galaxy morphologies, masses, densities, and strengths of turbulence. For source redshifts $0.5\leq z_{\rm s}\leq1$, an all-sky, isotropic FRB population has simulated values of $\tau(1\rm~GHz)$ ranging from $\sim1~\mu$s to $\sim2$ ms ($90\%$ confidence, observer frame) that are dominated by host galaxies, although $\tau$ can be $\gg2$ ms at low Galactic latitudes. A population at $z_{\rm s}=5$ has $0.01\lesssim\tau\lesssim300$ ms at 1 GHz ($90\%$ confidence), dominated by intervening galaxies. About $20\%$ of these high-redshift FRBs are predicted to have $\tau>5$ ms at 1 GHz (observer frame), and $\gtrsim40\%$ of FRBs between $z_{\rm s}\sim0.5-5$ have $\tau\gtrsim1$ ms for $\nu\leq 800$ MHz. Our scattering predictions may be conservative if scattering from circumsource environments is significant, which is possible under specific conditions. The percentage of FRBs selected against from scattering could also be substantially larger than we predict if circumgalactic turbulence causes more small-scale ($\ll1$ au) density fluctuations than observed from nearby halos.