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Astron. Astrophys. 353, 847-852 (2000)

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4. Correction for intergalactic absorption

Interaction of TeV gamma-rays with intergalactic infrared photons by pair production processes probably modifies substantially the intrinsic TeV spectrum emitted by the source (Nikishov 1962; Gould & Schreder 1965; Stecker et al. 1992). The most important domain for us is 1-10 µm as these photons can interact with TeV gamma-rays around the HEGRA threshold. The curve in Fig. 1 presents upper limits and measurements in relative agreement with recent modelling of CIB (Malkan & Stecker 1998; Primack et al. 1999). Gamma photons below 2 TeV effectively interact only with CIB photons below few µm, so only Primack models can be used. As the purpose of this section is to confirm ideas, we choose the LCDM cosmology model (cosmological constant + cold dark matter) which gives higher density and is therefore more conservative than the HCDM one (hot+cold dark matter). For a non-evolving CIB density according to this model, the amount of absorption for 0.1 to 20 TeV photons is shown in Fig. 2 for three of the studied BL Lac objects. Strictly speaking, the CIB energy density depends on redshift due to evolutionary effects, which are not well known. However, the redshift dependence could be neglected for redshifts lower than 0.15 since we do not expect significant evolution on very short time scales.

[FIGURE] Fig. 1. Experimental constraints on the CIB energy density together with LCDM model by Primack et al. 1999 (full line) and the maximum CIB given by Malkan & Stecker 1998 (dashed line). The triangles show ISO measurements at mid-infra-red (Stanev et al. 1998); the filled square shows a tentative detection at 3.5[FORMULA] (Dwek et al. 1998a); all other measurements, lower and upper limits are from a recent compilation by (Dwek et al.,1998b).

[FIGURE] Fig. 2. Percentage of VHE photons reaching the observer after absorption by the CIB using LCDM model by Primack et al. 1999.

Between 500 GeV and 1 TeV, 25% of the flux is absorbed even for a source as close as Mkn 501 with z=0.034; for the source 2E0829.1+0439 with a redshift of 0.18, this percentage rises to 75%. The figure shows that the CIB absorption becomes very large for objects with redshifts above [FORMULA].

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000