# First results on the Epoch of Reionization from First Light with SARAS 2

## TL;DR

This paper reports initial results from SARAS 2, a radiometer designed to detect the global 21-cm signal from the Epoch of Reionization, providing constraints on models of early universe heating and reionization.

## Contribution

It presents the first observational constraints on reionization models using SARAS 2 data, demonstrating the instrument's capability to probe cosmic dawn signals.

## Key findings

- Disfavors models with weak X-ray heating (f_X ≤ 0.1).
- Disfavors models with rapid reionization (peak dT_b/dz ≥ 120 mK per redshift unit).
- Provides initial limits on the timing and heating of the early universe.

## Abstract

Long wavelength spectral distortions in the Cosmic Microwave Background arising from the 21-cm transition in neutral Hydrogen are a key probe of Cosmic Dawn and the Epoch of Reionization. These features may reveal the nature of the first stars and ultra-faint galaxies that transformed the spin temperature and ionization state of the primordial gas. SARAS~2 is a spectral radiometer purposely designed for precision measurement of these monopole or all-sky global 21-cm spectral distortions. We use 63~hr night time observing of the radio background in the frequency band 110-200~MHz with the radiometer deployed at the Timbaktu Collective in Southern India to derive likelihoods for plausible redshifted 21-cm signals predicted by theoretical models. First light with SARAS 2 disfavors the class of models that feature weak X-ray heating (with $f_X \leq 0.1$) and rapid reionization (with peak $\frac{dT_b}{dz} \geq 120~\textrm{mK per unit redshift interval}$ ).