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 \gamma -rays due to the CIB could take place already in the low-energy ( sub-TeV ) domain of the spectrum of Mkn 501 .
This implies that the data of the low-energy threshold CAT telescope contain very important information about the CIB at short wavelengths , 0.4 \mu m \leq \lambda \leq 3 . \mu m .
The analysis of almost simultaneous spectroscopic measurements of Mkn 501 in a high state by CAT and BeppoSAX in the framework of the standard homogeneous Synchrotron-Self-Compton ( SSC ) framework model leads to the conclusion that the density of the near-infrared background with typical “ starlight spectrum ” around 1 \mu m should be between 5 and 35 nW m ^ { -2 } sr ^ { -1 } ( 99 \% CL ) , with most likely value around 20 nW m ^ { -2 } sr ^ { -1 } .
Also we argue that the CAT \gamma -ray data alone allow rather robust upper limits on the CIB , \lambda F _ { \lambda } \leq 60 nW m ^ { -2 } sr ^ { -1 } at 1 \mu m , taking into account that for any reasonable scenario of \gamma -ray production the differential intrinsic spectrum of \gamma -rays hardly could be flatter than { d } N / { d } E \propto E ^ { -1 } .
This estimate agrees , within statistical and systematic uncertainties , with recent reports about tentative detections of the CIB at 2.2 and 3.5 \mu m by the Diffuse Infrared Background Experiment ( DIRBE ) , as well as with the measurements of the background radiation at optical wavelengths from absolute photometry .
The high flux of CIB at \leq few \mu m wavelengths implies a significant distortion of the shape of the initial ( source ) spectrum of \gamma -rays from Mkn 501 at sub-TeV energies .
The “ reconstructed ” intrinsic \gamma -ray spectrum shows a distinct peak in the Spectral Energy Distribution ( SED ) around 2 TeV with a flux by a factor of 3 higher than the measured flux .
The energy spectrum of gamma radiation from both sides of the peak has power-law behavior with photon index \alpha \simeq 1.5 below 2 TeV , and \alpha \simeq 2.5 above 2 TeV .
This agrees with predictions of SSC model .
We also discuss the impact of the intergalactic absorption effect in derivation of the SSC parameters for the jet in Mkn 501 .