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 limits only have been found for distortion parameters y and m: y <1.5 10^-5 and m 9 10^-5 at 95% of confidence. FIRAS made differential measurements between sky and blackbody calibrator. The knowledge of its emissivity is important as far as y and m measurements as concerned. We have re-estimated the XCAL emissivity by using Ray-Tracing techniques. Diffraction problems have been analyzed by a model based on waveguide approximation: it allowed us to define a propagation function t inside the XCAL V-groove. The emission of the external calibrator distorted with respect to the Black-Body emission may result in lower upper limits to y and m. New upper limits for distortion parameters y and m have been calculated considering the new XCAL emissivity. The results are: y < 3.1 10^-5 and m < 4.5 10^-4 at 95% of confidence