Detection of cosmic microwave background structure in a second field with the Cosmic Anisotropy Telescope

We describe observations at frequencies near 15 GHz of the second 2 × 2deg2 field imaged with the Cambridge Cosmic Anisotropy Telescope (CAT). After the removal of discrete radio sources, structure is detected in the images on characteristic scales of about half a degree, corresponding to spherical harmonic multipoles in the range ℓ ≈ 330-680. A Bayesian analysis confirms that the signal arises predominantly from the cosmic microwave background (CMB) radiation for multipoles in the lower half of this range; the average broad-band power in a bin with centroid ℓ = 422 (θ ≈ 51 arcmin) is estimated to be ΔT/T <2.1-0.5+0.4 × 10-5. For multipoles centred on ℓ = 615 (θ ≈ 35 arcmin), we find contamination from Galactic emission is significant, and constrain the CMB contribution to the measured power in this bin to be ΔT/T <2.0 × 10-5 (1σ upper limit). These new results are consistent with the first detection made by CAT in a completely different area of sky. Together with data from other experiments, this new CAT detection adds weight to earlier evidence from CAT for a downturn in the CMB power spectrum on scales smaller than 1 deg. Improved limits on the values of H0 and Ω are determined using the new CAT data.