On the origin of highest energy gamma-rays from Mkn 501

The spectra of very high energy gamma-radiation observed 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 BL Lac objects Mkn 421 and Mkn 501, especially at energies above 10 TeV. This implies that the source spectrum of Mkn 501 corrected for the intergalactic absorption may contain a sharp-pile which generally contradicts to the predictions of current models of TeV emission of BL Lac objects, and thus leads to the so-called "IR-TeV crisis". To overcome this difficulty, in this paper we study two possibilities assuming that (i) the TeV gamma-rays from Mkn 501 have a secondary origin, i.e. are formed during 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 the ultrarelativistic cold conical outflow (jet) with bulk motion Lorentz factor \Gamma ~ 3.3 x 10^7. We show that the first hypothesis cannot reproduce the spectral shape of the TeV emission from Mkn 501. At the same time we demonstrate that the inverse Compton radiation of the ultrarelativistic cold jet hypothesis can quite satisfactorily explain the unusual spectral features of the "reconstructed" TeV radiation. We briefly discuss the astrophysical implications of this hypothesis.

photon: cosmic radiation
cosmic radiation: particle source
blazar
cosmic radiation: primary
cosmic background radiation
photon: absorption
cosmic radiation: energy spectrum
cosmic radiation: cascade
cosmic radiation: flux
numerical calculations: interpretation of experiments

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