The spectra of very high energy \gamma -radiation from distant extragalactic objects suffer significant deformations during the passage of primary \gamma -rays through the intergalactic medium .
The recently reported fluxes of diffuse infrared background radiation indicate that we detect , most probably , heavily absorbed TeV radiation from Mkn 421 and Mkn 501 .
This implies that the absorption-corrected spectrum of Mkn 501 may contain a sharp pile-up which contradicts to the predictions of the conventional models of TeV blazars , and thus may leads to the so-called “ IR background-TeV gamma-ray crisis ” .
To overcome this difficulty , in this paper we propose two independent hypotheses assuming that ( i ) the TeV radiation from Mkn 501 has a secondary origin , i.e .
it is formed during the development of electron-photon cascades in the intergalactic medium initiated by primary \gamma -rays ; ( ii ) the pile-up in the source spectrum is a result of comptonization ( in deep Klein-Nishina regime ) of ambient optical radiation by an ultrarelativistic conical cold outflow ( jet ) with bulk motion Lorentz factor \Gamma _ { 0 } \geq 3 \times 10 ^ { 7 } .
Within the uncertainties caused by the limited energy resolution of spectral measurements , the observed TeV radiation of Mkn 501 formally can be explained by the intergalactic cascade \gamma -rays , assuming however an extremely low intergalactic magnetic field in the direction to the source at the level of \leq 10 ^ { -18 } G .
We also demonstrate that the “ bulk motion comptonization ” scenario can quite naturally reproduce the unusual spectral features in the absorption-corrected TeV spectrum of Mkn 501 , and briefly discuss the astrophysical implications of this hypothesis .