Constraints on the Extragalactic Infrared Background from Gamma-Ray Observations of Markarian 501

We use the new results of the HEGRA detector on the TeV gamma-ray emissionfrom MKN 501 to set upper limits on the energy density of the cosmic infraredbackground (CIRB). Contrary to previous interpretations of the gamma-rayspectrum of MKN 421 as showing an intergalactic absorption cutoff at 5 TeV, theobserved spectrum of MKN 501 extends beyond 10 TeV and appears to beunattenuated by gamma-gamma collisions with the low-energy CIRB photons. The upper limits on the CIRB intensity -- derived both assumingan `a priori' shape for the CIRB spectrum and without model-dependentassumptions -- are thus quite strong and come almost in conflict with theobservational evaluations based on deep surveys of extragalactic sources in thenear- and mid-IR. If spectra at TeV energies for extragalactic gamma-raysources like this for MKN 501 will be confirmed with improved statistics, wemay be forced to conclude that the process of gamma-gamma interaction in theintergalactic space is more complex than expected and the average intergalacticmagnetic field extremely weak.Comment: Revised version, 9 pages in AASTEX, 3 figures, text is revised from the original version, Fig. 2 change