SARAS 2 constraints on global 21-cm signals from the Epoch of Reionization

TL;DR

The SARAS 2 radiometer collected data to detect the global 21-cm signal from the Epoch of Reionization, and analysis rules out several astrophysical models, especially those with inefficient heating and rapid reionization.

Contribution

This study re-analyzes SARAS 2 data using an improved frequentist approach, providing stronger constraints on high-redshift astrophysical scenarios.

Findings

Rejected 27 models at >5σ significance

Most rejected models involve inefficient gas heating and rapid reionization

Data constrains the properties of early universe heating and ionization processes

Abstract

Spectral distortions in the cosmic microwave background over the 40--200~MHz band are imprinted by neutral hydrogen in the intergalactic medium prior to the end of reionization. This signal, produced in the redshift range $z = 6-34$ at the rest frame wavelength of 21 cm, has not been detected yet; and poor understanding of high redshift astrophysics results in a large uncertainty in the expected spectrum. The SARAS~2 radiometer was purposely designed to detect the sky-averaged 21-cm signal. The instrument, deployed at the Timbaktu Collective (Southern India) in April--June 2017, collected 63~hr of science data, which were examined for the presence of the cosmological 21-cm signal. In our previous work the first-light data from SARAS~2 radiometer were analyzed with Bayesian likelihood-ratio tests using $264$ plausible astrophysical scenarios. In this paper we re-examine the data using an improved analysis based on the frequentist approach and forward modeling. We show that SARAS~2 data rejects 27 models, out of which 25 are rejected at a significance $>5\sigma$. All the rejected models share the scenario of inefficient heating of the primordial gas by the first population of X-ray sources along with rapid reionization.