An Upper Limit on the Infrared Background Density from HEGRA data on   Mkn501
  The energy spectrum of Mkn501 in the TeV energy regime, as measured by the HEGRA (High Energy Gamma Ray Astronomy) Cerenkov telescopes during its low state in 1995/96 and during a fraction of the 1997 outburst in the TeV energy regime, is shown to place stringent upper limits on the still unknown infrared photon density in the energy region between 3 x 10**(-3) and 3 xt 10**(-1) eV. Assuming two different shapes for the unknown infrared photon spectrum in this energy range we calculate upper limits on the infrared photon density on the basis of the power-law fit obtained for the observed spectrum up to the maximum energy. 
B. Funk, N. Magnussen, H. Meyer, W. Rhode, S. Westerhoff, B.   Wiebel-Sooth (University of Wuppertal)