Probing the Cosmic Web with Fast Radio Bursts: I. Scattering

Scattering of FRB signals as probes of ionized gas density fluctuations in the cosmic web.

Sharon Lapiner, Nir Mandelker, Paz Beniamini, S. Peng Oh

We study the formation of multiphase gas in the post-accretion-shock regions of cosmic sheets, filaments, and the circumgalactic medium (CGM) of haloes, i.e. cosmic web objects (CWOs). Local instabilities in the hot medium result in fragmentation and cooling, eventually forming small-scale overdensities with temperatures of $\sim10^{4}{\,\rm K}$ that are in pressure equilibrium with the hot environment. Such dense, ionised inhomogeneities can affect the propagation of radio waves from fast radio bursts (FRBs), thereby offering us a way to probe their presence and properties in CWOs through scattering signatures in the observed FRB flux.We find that high-$z$ filaments and sheets make a negligible contribution to the total observed scattering. The high rates of FRBs that are expected, even at high redshifts, may still enable their detection from high-temperature filaments along rare sightlines, and we suggest other methods for such systems in a companion paper. Our model further predicts that if turbulent cloudlets exist in the CGM of intervening massive haloes with a volume-filling fraction of $f_{\rm V}\gtrsim10^{-3}$, they are expected to cause considerable cumulative scattering along an average sightline, which would result in a significant correlation between the total scattering time and source redshifts. The lack of such a correlation in current observations may imply that the cool gas in the CGM has substantial non-thermal pressure, which would reduce its density, or significant damping of small-scale density fluctuations. Forthcoming localised FRB samples can map these constraints onto bounds on volume-filling fractions, densities, cloud sizes, and the strength of turbulence.

Published paper

Cumulative scattering time ($\tau_{\rm t}$, y-axis) at $\nu=1{\,\rm GHz}$, as a function of the average extragalactic DM (${\rm DM_{IGM}}$, x-axis) or the FRB source redshift ($z_{\rm S}$, top x-axis) due to cloudlets in intercepted haloes with $M_{\rm h}\geq10^{12}{\,\rm M_{\odot}}$ (median impact parameter of $b=0.7$). The colored lines represent different volume-filling fractions ($f_{\rm V}$) of embedded cold cloudlets. The horizontal dashed line represents a crude threshold of detectable scattering time, $\tau_{\rm t}\gtrsim0.1{\,\rm ms}$. If turbulent cold cloudlets on the scale of the minimum cooling length exist in the CGM with $f_{\rm V}\gtrsim10^{-3}$, the fiducial model predicts that $\tau_{\rm t}$ increases with $z_{\rm S}$ for sources at $z_{\rm S}\lesssim1$ due to several encounters with haloes. The lack of such a correlation in current observations (albeit uncertain due to the relatively small localized samples) may imply that gas in the CGM deviates from the fiducial assumptions, and we explore several possibilities for deviations in the paper.