# Hydrogen 2p--2s transition: signals from the epochs of recombination and reionization

## TL;DR

This paper proposes using the 2p--2s fine structure lines of neutral hydrogen as a new method to study the epochs of recombination and reionization, analyzing their potential observability with current and future radio telescopes.

## Contribution

It introduces a novel observational approach based on hydrogen fine structure lines to probe the reionization era and evaluates their detectability.

## Key findings

- Brightness temperature of ~20 μK at z ≈ 6.3 for reionization signals.
- Recombination epoch signal peaks at ~100 μK in 5-10 MHz range.
- Detection of the recombination signal is hindered by Galactic emission.

## Abstract

We propose a method to study the epoch of reionization based on the possible observation of 2p--2s fine structure lines from the neutral hydrogen outside the cosmological H {\sc ii} regions enveloping QSOs and other ionizing sources in the reionization era. We show that for parameters typical of luminous sources observed at $z \simeq 6.3$ the strength of this signal, which is proportional to the H {\sc i} fraction, has a brightness temperature $\simeq 20 \mu K$ for a fully neutral medium. The fine structure line from this redshift is observable at $\nu \simeq 1 \rm GHz$ and we discuss prospects for the detection with several operational and future radio telescopes. We also compute the characteristics of this signal from the epoch of recombination: the peak brightness is expected to be $\simeq 100 \mu K$; this signal appears in the frequency range 5-10 MHz. The signal from the recombination era is nearly impossible to detect owing to the extreme brightness of the Galactic emission at these frequencies.