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

TL;DR

This paper explores how small-scale inhomogeneities in the cosmic web's gas can affect fast radio burst signals, offering a new method to probe the properties of cosmic structures like filaments, sheets, and the circumgalactic medium.

Contribution

It introduces a model linking scattering signatures in FRBs to the presence and properties of multiphase gas in the cosmic web, providing a novel observational probe.

Findings

High-redshift filaments contribute negligibly to scattering.

Detection of scattering from high-temperature filaments is possible along rare sightlines.

Absence of expected scattering correlations suggests non-thermal pressure or damping in the CGM.

Abstract

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 $\sim 10^{4}{\,\rm K}$ 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 \& sheets have a negligible contribution to the total observed scattering. The high rates of FRBs expected even at high redshifts may still allow 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 $f_{\rm v}\gtrsim 10^{-3}$, they are expected to cause considerable cumulative scattering along an average sightline, resulting 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, reducing its density, or significant damping of small-scale density fluctuations. Forthcoming localised FRB samples can map these constraints into bounds on volume-filling fractions, densities, cloud sizes, and the strength of turbulence.