Probing ionized bubbles around luminous sources during reionization with SKA 21-cm observations

Abstract

Detecting and characterizing individual ionized bubbles during the Epoch of Reionization using the redshifted H?i 21-cm signal provides a direct probe of the early ionizing sources and the intergalactic medium. We develop and validate a computationally efficient estimator that operates on gridded visibilities to detect ionized bubbles. This serves as an accurate alternative to the more computationally demanding bare estimator that uses all baselines and frequency channels. Further, we employ a non-parametric foreground-subtraction method based on Gaussian Process Regression, which minimizes loss of the H?i 21-cm signal and yields improved signal-to-noise ratios. Our analysis indicates that ionized bubbles at redshifts (Formula presented) can be detected with signal-to-noise ratio (SNR) (Formula presented) using (Formula presented) h of Square Kilometre Array (SKA)-Low AA(Formula presented) and AA4 observations. We further derive a scaling relation that connects the SNR to the bubble radius, redshift, total observing time, and the mean neutral hydrogen fraction of the surrounding intergalactic medium. This helps to quickly predict the observational outcome for any planned observations and is, therefore, useful for devising observational strategies. Finally, we apply a Bayesian likelihood framework with Markov Chain Monte Carlo sampling to the residual visibilities to recover ionized bubble properties, including radius, position, and the mean neutral fraction. The resulting posterior distributions demonstrate accurate recovery of the bubble parameters. This confirms the feasibility of robustly characterizing individual ionized regions with the SKA-Low. � The Author(s) 2026. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.