If the duration of reionization ∆z > 1, Simons Observatory aims to measure the average duration of reionization with a significance between 2σ and 3σ, and thus constrain models for the ionizing process. Such a measurement would be among the first to probe the properties of the first galaxies, quasars, and the intergalactic medium in the reionization epoch.

The figure shows a summary of constraints on the redshift and duration/width of reionization. The SO forecasts are reported with 68% confidence-level contours from baseline/goal configurations in combi- nation with Planck large-scale data (blue/orange). The solid navy lines show the redshift and width of reionization at constant values of the inter-galactic medium (IGM) opacity and source efficiency. The SO predictions are compared to current exclusion limits for the time of reionization from Planck (green band), recent measurements of the global 21 cm signal assuming standard thermal properties (i.e., spin temperature much larger than the CMB temperature) of the IGM (yellow band), and Gunn Peterson trough from fully absorbed Lyman alpha in quasar spectra (gray band), and to current upper limits on the duration of reionization from Planck and SPT data (brown band).