X iv : a st ro - p h / 97 08 16 2 v 2 1 8 D ec 1 99 7 CONSTRAINTS ON THE EXTRAGALACTIC INFRARED BACKGROUND FROM GAMMA - RAY OBSERVATIONS OF MKN 501

We use the new results of the HEGRA detector on the TeV γ–ray emission from MKN 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 MKN 421 as showing an intergalactic absorption cutoff at 5 TeV, the observed spectrum of MKN 501 extends beyond 10 TeV and appears to be unattenuated by γγ 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 come almost in conflict with the observational evaluations based on deep surveys of extragalactic sources in the nearand mid-IR. If spectra at TeV energies for extragalactic gamma-ray sources like this for MKN 501 will be confirmed with improved statistics, we may be forced to conclude that the process of γγ interaction in the intergalactic space is more complex than expected and the average intergalactic magnetic field extremely weak (B < 10 G). Subject headings: infrared: general — gamma rays: observations — scattering