ar X iv : a st ro - p h / 99 04 05 7 v 1 5 A pr 1 99 9 Evidence for Intergalactic Absorption in the TeV Gamma - Ray Spectrum of Mkn 501

The recent HEGRA observations of the blazar Mkn 501 show strong curvature in the very high energy γ-ray spectrum. Applying the γ-ray opacity derived from an empirically based model of the intergalactic infrared background radiation field (IIRF), to these observations, we find that the intrinsic spectrum of this source is consistent with a power-law: dNγ/dE ∝ E −α with α = 2.00 ± 0.03 over the range 500GeV – 20TeV. Within current synchrotron self-Compton scenarios, the fact that the TeV spectral energy distribution of Mkn 501 does not vary with luminosity, combined with the correlated, spectrally variable emission in X-rays, as observed by the BeppoSAX and RXTE instruments, also independently implies that the intrinsic spectrum must be close to α = 2. Thus, the observed curvature in the spectrum is most easily understood as resulting from intergalactic absorption. Subject headings: BL Lac objects: individual: Mkn 501: active galaxies – γ-rays: theory