Constraints on the Optical Depth to Reionization from Balloon-Borne CMB Measurements
Abstract
We assess the uncertainty with which a balloon-borne experiment, nominally called Tau Surveyor (τ S), can measure the optical depth to reionization σ(τ) with given realistic constraints of instrument noise and foreground emissions. Using a τ S fiducial design with six frequency bands between 150 and 380 GHz with white and uniform map noise of 7 μK arcmin, achievable with a single mid-latitude flight, and including Planck's 30 and 44 GHz data we assess the error σ(τ) obtained with three foreground models and as a function of sky fraction f sky between 40% and 54%. We carry out the analysis using both parametric and blind foreground separation techniques. We compare σ(τ) values to those obtained with low frequency and high frequency versions of the experiment called τ S-lf and τ S-hf that have only four and up to eight frequency bands with narrower and wider frequency coverage, respectively. We find that with τ S the lowest constraint is σ(τ)=0.0034, obtained for one of the foreground models with f sky=54%. σ(τ) is larger, in some cases by more than a factor of 2, for smaller sky fractions, with τ S-lf, or as a function of foreground model. The τ S-hf configuration does not lead to significantly tighter constraints. Exclusion of the 30 and 44 GHz data, which give information about synchrotron emission, leads to significant τ mis-estimates. Decreasing noise by an ambitious factor of 10 while keeping f sky=40% gives σ(τ) =0.0031. The combination of σ(τ) =0.0034, BAO data from DESI, and future CMB B-mode lensing data from CMB-S3/S4 experiments could give σ(Σ m) = 17 meV.