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

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Abstract
We use the new results of the HEGRA detector on the TeV γ-ray emission from Mrk 501 to set upper limits on the energy density of the cosmic infrared background (CIRB). Contrary to previous interpretations of the γ-ray spectrum of Mrk 421 as showing an intergalactic absorption cutoff at 5 TeV, the observed spectrum of Mrk 501 extends beyond 10 TeV and appears to be unattenuated by γ-γ collisions with the low-energy CIRB photons. The upper limits on the CIRB intensity—derived both assuming an a priori shape for the CIRB spectrum and without model-dependent assumptions—are thus quite strong and almost come into conflict with the observational evaluations based on deep surveys of extragalactic sources in the near- and mid-infrared. If spectra at TeV energies for extragalactic γ-ray sources like this for Mrk 501 are confirmed with improved statistics, we may be forced to conclude that the process of γ-γ interaction in the intergalactic space is more complex than expected and that the average intergalactic magnetic field is extremely weak (B−11 G).