The dispersion measure and scattering of FRBs: Contributions from the intergalactic medium, foreground halos, and hosts

TL;DR

This study uses cosmological simulations to analyze how the intergalactic medium, foreground, and host halos contribute to the dispersion measure and scattering of fast radio bursts, matching observations and estimating redshifts.

Contribution

It provides a detailed simulation-based analysis of the contributions of different cosmic media to FRB dispersion and scattering, including a model for estimating redshifts from DM.

Findings

Foreground halos contribute about 30% of the IGM DM with larger variance.

The DM distribution from mock sources aligns with observations.

Host and foreground halos are key for scattering, especially at higher DM values.

Abstract

We investigate the dispersion measure(DM) and scattering of FRBs by the intergalactic-medium(IGM), foreground and host halos, using cosmological hydrodynamical simulation. We find that the median DM caused by foreground halos is around 30\% of that caused by the IGM, but has a much larger variance. The DM induced by hosts deviates from a log-normal distribution, but exhibits an extended distribution in the range of $1-3000 \ {\rm{pc\, cm^{-3}}}$ with a median value $\sim 100 \ {\rm{pc\, cm^{-3}}}$. Then we produce mock FRB sources, assuming a uniform distribution in the range $z\sim 0-0.82$, to consider the propagation effect of IGM, foreground and host halos on FRB signals simultaneously. The DM distribution of mock sources agrees well with the observation. The fitted DM-redshift relation of the mock sources can provide a rough estimation of the redshifts of observed events with errors $\delta z \lesssim 0.15$. The distribution of mock sources in the DM-scattering time($\tau$) space can also match the observation, assuming a Kolmogorov turbulence model with the inner and outer scale is 1000 km to 1 AU, and 0.2-10 pc respectively. Finally, we estimate the relative importance of these medium on DM and $\tau$ in our models. The IGM and host halos are the primary and secondary sources to the extragalactic DM, $\rm{DM_{exg}}$. Meanwhile, the contribution from foreground halos increases as $\rm{DM_{exg}}$ increases. The host and foreground halos may be the most important medium for scattering. Statistically, the latter may dominate the scattering of events with $\rm{DM_{exg}} \gtrsim 200 \ {\rm{pc\, cm^{-3}}}$.