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• W2595557353 abstract "The flux of high energy (HE, energy 100 MeV . E . 100 GeV) and very high en- ergy (VHE, E & 100 GeV) γ -rays originating from cosmological sources is atten- uated due to pair production in interactions with photons at ultraviolet to infrared wavelengths of the extragalactic background light (EBL). T he main components contributing to the EBL photon density are the starlight int egrated over cosmic time and the starlight reprocessed by dust in galaxies. Conseque ntly, the EBL is an integral measure of the cosmic star formation history. Depe nding on the source distance, the Universe should be opaque to γ -rays above a certain energy. Nevertheless, the number of detected γ -ray sources has increased continuously in recent years. VHE emitting objects beyond redshifts of z > 0 . 5 have been detected with imaging air Cherenkov telescopes (IACTs), while HE γ -rays from active galac- tic nuclei (AGN) above redshifts z & 3 have been observed with the Large Area Telescope (LAT) on board the Fermi satellite. In this work, a large sample of VHE γ -ray spectra will be combined with data of the Fermi -LAT to derive upper limits on the EBL photon density at z = 0. Generic EBL realizations are used to correct AGN spectra for absorpt ion, which are subse- quently tested against model assumptions. The evolution of the EBL with redshift is accounted for, and a possible formation of electromagnetic cascades is considered. As a result, the EBL density is constrained over almost three orders of magnitude in wavelength, between 0 . 4 μ m and 100 μ m. At optical wavelengths, an EBL intensity above 24 nW m − 2 sr − 1 is ruled out, and between 8 μ m and 31 μ m it is limited to be below 5 nW m − 2 sr − 1 . In the infrared, the constraints are within a factor ∼ 2 of lower limits derived from galaxy number counts. Additionally, the behavior of VHE spectra in the transition from the optical depth regimes τ γγ 2 is investigated. The absorption-corrected spectra consi s- tently show an upturn at high optical depths, significant at t he 4 σ level. A source in- trinsic e ff ect is unlikely to produce such a feature, since the transiti on to the τ γγ > 2 regime occurs at di ff erent energies for each source. Systematic uncertainties t hat could mimic the e ff ect are studied but found unlikely as a possible explanation . A similar study is conducted for photons detected with the Fermi -LAT. To this end, the number of expected photons in the optical thick regime is compared to the num- ber of photons observed with the LAT. Above τ γγ > 2, three photons are associated with AGN with high confidence. Under the assumption of certai n EBL models,extrapolating the unattenuated spectrum from low to high en ergies results in a prob- ability of 1 . 2 × 10 − 4 to observe these photons. However, the probability for dete cting the high optical depth photons when all LAT detected AGN with known redshift are considered sensitively depends on the the choice of the intr insic spectral model. The indication for a reduced opacity might be explained by th e oscillation of pho- tons into hypothetical axion-like particles (ALPs) in ambi ent magnetic fields. Such particles propagate unimpeded over cosmological distance s, thereby reducing the γ -ray opacity. Photon-ALP conversions are studied in di ff erent magnetic field con- figurations, including intracluster and intergalactic mag netic fields, as well as the field of the Milky Way. Optimistic values of the field strength and coherence length result in lower limits on the photon-ALP coupling, g a γ & 10 − 12 GeV − 1 . For more realistic magnetic field parameters, couplings above g a γ & 2 × 10 − 11 GeV − 1 are nec- essary to explain the indication for the reduced opacity. Th e lower limits are in reach of future dedicated ALP experiments." @default.
• W2595557353 created "2017-03-23" @default.
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• W2595557353 date "2013-09-17" @default.
• W2595557353 modified "2023-09-27" @default.
• W2595557353 title "The Opacity of the Universe for High and Very High Energy $gamma$-Rays" @default.
• W2595557353 hasPublicationYear "2013" @default.
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