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 emis sion from Mrk 501 to set upper limits on the energy density of the cos mic infrared background (CIRB). Contrary to previous interpretations o f the gamma-ray spectrum of Mrk 421 as showing an intergalactic absorp tion cutoff at 5 TeV, the observed spectrum of Mrk 501 extends beyond 10 TeV and appears to be unattenuated by gamma-gamma collisions with t he low-energy CIRB photons. The upper limits on the CIRB intensity-der ived both assuming an a priori shape for the CIRB spectrum and without model-dependent assumptions-are thus quite strong and almost come int o 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 gamma-ray sources like this for Mrk 501 ar e confirmed with improved statistics, we may be forced to conclude tha t the process of gamma-gamma interaction in the intergalactic space is more complex than expected and that the average intergalactic magneti c field is extremely weak (B < 10(-11) G).