Intergalactic Photon Spectra from the Far‐IR to the UV Lyman Limit for 0 <z< 6 and the Optical Depth of the Universe to High‐Energy Gamma Rays

Abstract

We calculate the intergalactic photon density as a function of both energy and redshift for 0 < z < 6 for photon energies from.003 eV to the Lyman limit cutoff at 13.6 eV in a ÃCDM universe with à ¼ 0:7 and m ¼ 0:3. The basic features of our backward-evolution model for galaxies were developed in earlier papers by Malkan & Stecker.With a few improvements, we find that this evolutionary model gives predictions of new deep number counts from Spitzer, as well as a calculation of the spectral energy distribution of the diffuse infrared background, which are in good agreement with the data. We then use our calculated intergalactic photon densities to extend previous work on the absorption of high-energy -rays in intergalactic space owing to interactions with low-energy photons and the 2.7 K cosmic microwave background radiation. We calculate the optical depth of the universe, , for -rays having energies from 4 GeV to 100 TeV emitted by sources at redshifts from 0 to 5. We also give an analytic fit with numerical coefficients for approximating (E ; z). As an example of the application of our results, we calculate the absorbed spectrum of the blazar PKS 2155À304 at z ¼ 0:117 and compare it with the spectrum observed by the HESS air Cerenkov -ray telescope array.

Transition from galactic to extragalactic cosmic rays

“…analytically, being model dependent [67]. In general the steepening of the nuclei spectrum is not as sharp as the GZK and it occurs at lower energies [66].…”

“…For heavy enough nuclei A > 10, taking into account only the CMB background, the steepening of the spectrum starts at the energy where adiabatic energy losses (due to the expansion of the universe) and losses due to pair production become equal (see energy losses in [66]), namely at E $ 20 À 50 EeV. The pair production process experienced by nuclei is affected only by the CMB radiation, while the photo-disintegration process takes place also on the Extragalactic Background Light (EBL) [67]. The latter is the process in which a nucleus with atomic mass number A loses one or more nucleons interacting with the CMB or EBL:…”

Extragalactic propagation of ultrahigh energy cosmic-rays

“…rigidity, simply could not reach us 16 . The induced suppression of the flux at low energy could be of crucial importance (between 10 17 eV and 10 18 eV, or even above in the case of strong fields) and impact the transition between Galactic and extragalactic CRs.…”

Understanding the spectrum of a distant blazar PKS 1424+240 and its implications

“…The latter parameter is uncertain, although spectral fits to other sources were obtained for values consistent with theoretical models of cosmic rays [2,3,6]. There are many models of EBL derived using different approaches [24][25][26][27][28]. For a number of sources, both "high" and "low" EBL models fit the data well, with no statistically significant difference in the goodness of fit [3].…”