Constraints on the IR background based on CAT spectrum of Mkn 501.

The TeV and X-ray data obtained by the imaging Cherenkov telescope CAT and X-ray satellite BeppoSAX during the remarkable flare of Mkn 501 in April 16, 1997 are used to constrain the flux of the Cosmic Infrared Background (CIB) using different CIB models. We show that a non-negligible absorption of γ-rays due to the CIB could take place already in the low-energy (sub-TeV) domain of the spectrum of Mkn 501. The density of the near-infrared background with typical “starlight spectrum” around 1 μm should be between 5 and 35 nW m[sup −2 ] sr[sup −1] (99% CL), with the most likely value around 20 nW m[sup −2] sr[sup −1]. We also argue that the CAT γ-ray data alone allow a rather robust upper limits on the CIB, λF[sub λ] ≤ 60 nW m[sup −2] sr[sup −1] at 1 μm, based on the belief that, within the conventional models of δ-ray production, the differential source spectrum of γ-rays hardly could be flatter than dN/dE ∝ E[sup −1]. The high flux of CIB at ≤ few μm wavelengths, λF[sub λ] ≥ 10 nW m[sup −2] sr [sup −1] implies significant distortion of the initial (source) sub-TeV spectrum of γ-rays from Mkn 501, therefore has a strong impact in derivation of the jet parameters. [ABSTRACT FROM AUTHOR]