Towards 21-cm intensity mapping at z=2.28 with uGMRT using the tapered gridded estimator – IV. Wideband analysis

We present a Wideband Tapered Gridded Estimator (TGE), which incorporates baseline migration and variation of the primary beam pattern for neutral hydrogen (H I) 21-cm intensity mapping (IM) with large frequency bandwidth radio-interferometric observations. Here we have analysed 394-494 MHz (z = 1.9 - 2.6) uGMRT data to estimate the Multi-frequency Angular Power Spectrum (MAPS) C_ℓ(Δν) from which we have removed the foregrounds using the polynomial fitting (PF) and Gaussian Process Regression (GPR) methods developed in our earlier work. Using the residual C_ℓ(Δν) to estimate the mean squared 21-cm brightness temperature fluctuation Δ^2(k), we find that this is consistent with 0 ± 2 σ in several k bins. The resulting 2σ upper limit Δ^2(k) < (4.68)^2 mK^2 at k=0.219 Mpc^-1 is nearly 15 times tighter than earlier limits obtained from a smaller bandwidth (24.4 MHz) of the same data. The 2σ upper limit [Ω_ H I b_ H I] < 1.01 × 10^-2 is within an order of magnitude of the value expected from independent estimates of the H I mass density Ω_ H I and the H I bias b_ H I. The techniques used here can be applied to other telescopes and frequencies, including ∼ 150 MHz Epoch of Reionization observations.