Asking Fast Radio Bursts for More than Reionization History

TL;DR

This paper explores how dispersion measures of fast radio bursts can be used to probe the epoch of reionization, revealing information about ionization history, bubble sizes, and reionization duration through variance analysis.

Contribution

It introduces new estimators and variance-based methods to extract reionization history details from FRB dispersion measures, distinguishing different reionization scenarios.

Findings

DM measurements are biased by line-of-sight fluctuations during EoR.

The ratio of DM variance to mean depends on ionization bubble sizes.

DM structure function encodes reionization duration and bubble size information.

Abstract

We propose different estimators to probe the intergalactic medium (IGM) during epoch of reionization (EoR) using the dispersion measure (${\rm DM}$) of the fast radio bursts. We consider three different reionization histories, which we can distinguish with a total of $\lesssim 1000\,{\rm DM}$ measurements during EoR if their redshifts are known. We note that the redshift derivatives of ${\rm DM}$ are also directly sensitive to the reionization history. The major point of this work is to explore the variance in the ${\rm DM}$ measurements and the information encoded in them. We find that the all-sky average $\overline{{\rm DM}}(z)$ gets biased from the line-of-sight (LoS) fluctuations in the ${\rm DM}$ measurements introduced by the ionization of IGM during EoR. We find that the ratio $\sigma_{\rm DM}/\overline{{\rm DM}}$ depends directly on the ionization bubble sizes as well as the reionization history. On the other hand, we also find that angular variance (coined as $\textit{structure function}$) of ${\rm DM}$ encodes the information about the duration of reionization and the typical bubble sizes as well. We establish the usefulness of variances in ${\rm DM}$ using toy models of reionization and later verify it with the realistic reionization simulations.