Anisotropy in the Microwave Sky: Results from the First Flight of BAM

Abstract

Results are reported from the first flight of a new balloon-borne instrument, BAM (Balloon-borne Anisotropy Measurement), designed to search for cosmic microwave background (CMB) anisotropy. The instrument uses a cryogenic differential Fourier transform spectrometer to obtain data in five spectral channels whose central frequencies lie in the range 3.7 cm^{-1} to 8.5 cm^{-1}. The spectrometer is coupled to an off-axis prime focus telescope; the combination yields difference spectra of two regions on the sky defined by 0\fdg 7 FWHM beams separated by 3\fdg 6. Single differences obtained at ten sky positions show statistically significant fluctuations. Assuming Gaussian correlated anisotropy, for the band average 3.1 cm^{-1} to 9.2 cm^{-1}, one finds $ΔT/T = 3.1^{+3.1}_{1.1}\times 10^{-5}$ (90% confidence interval) for a correlation angle of 1\fdg 2. This corresponds to $Q_{flat} = 35.9^{17.7}_{6.3} μK$ (1σ).