The CMBR spectrum: new upper limits for the distortion parameters y and mu

The CMBR Spectrum has been measured by the COBE-FIRAS experiment. Upper lim its only have been found for distortion parameters y and mu: y(up) = 1.5 x 10(-5) and mu(up) = 9 x 10(-5) at 95% of confidence. FIRAS made differentia l measurements between sky and blackbody calibrator. The knowledge of its e missivity is important as far as y and mu measurements as concerned. We hav e re-estimated the XCAL emissivity by using Ray-Tracing techniques. Diffrac tion problems have been analyzed by a model based on waveguide approximatio n: it allowed us to define a propagation function tau inside the XCAL V-gro ove. The emission of the external calibrator distorted with respect to the Black-Body emission may result in lower upper limits to y and mu. New upper limits for distortion parameters y and mu have been calculated considering the new XCAL emissivity. The results are: y(up) = 3.1 x 10(-5) and mu(up) = 4.5 x 10(-4) at 95% of confidence. (C) 2000 Elsevier Science B.V. All rig hts reserved.