Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe

The Astrophysical Journal (Impact Factor: 5.99). 11/2010; 723(2):1082-1096. DOI: 10.1088/0004-637X/723/2/1082


The extragalactic background light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above ~10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the γ-ray flux of extragalactic sources such as blazars and gamma-ray bursts (GRBs). The Large Area Telescope on board Fermi detects a sample of γ-ray blazars with redshift up to z ~ 3, and GRBs with redshift up to z ~ 4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of γ-ray flux attenuation by the EBL. We place upper limits on the γ-ray opacity of the universe at various energies and redshifts and compare this with predictions from well-known EBL models. We find that an EBL intensity in the optical-ultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. can be ruled out with high confidence.

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Available from: Elisabetta Cavazzuti,
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    • "Put simply, VHE γ-rays that would normally not arrive from distant AGNs or GRBs could become detectable because Lorentz violation would suppress the pairproduction of electrons and positrons that would otherwise have stopped them from getting through. VHE γ-rays traveling cosmological distances encounter low-energy photons belonging to the cosmic microwave background (CMB) and the extragalactic background light (EBL) due to starlight, both primary and re-emitted after absorption by dust [13] [14] [15]. At sufficiently high energies, according to conventional quantum electrodynamics (QED) some fraction of the emitted γ-rays are absorbed as they scatter off the EBL through pair production of an electron and a positron: γ V HE +γ EBL → e + +e − [16]. "
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    ABSTRACT: The arrival of TeV-energy photons from distant galaxies is expected to be affected by their QED interaction with intergalactic radiation fields through electron-positron pair production. In theories where high-energy photons violate Lorentz symmetry, the kinematics of the process $\gamma + \gamma\rightarrow e^+ + e^-$ is altered and the cross-section suppressed. Consequently, one would expect more of the highest-energy photons to arrive if QED is modified by Lorentz violation than if it is not. We estimate the sensitivity of Cherenkov Telescope Array (CTA) to changes in the $\gamma$-ray horizon of the Universe due to Lorentz violation, and find that it should be competitive with other leading constraints.
    Journal of Cosmology and Astroparticle Physics 01/2014; 2014(06). DOI:10.1088/1475-7516/2014/06/005 · 5.81 Impact Factor
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    • "This energy range is of great importance for a wide range of scientific topics: the Universe goes from being transparent to gamma-rays to having a pronounced horizon at a redshift < 1 [1] [2] [3], limiting the number of bright very distant objects (such as e.g. Gamma Ray Bursts, or GRBs) that can be studied, but providing information on the cosmological evolution of the infrared-ultraviolet background light [4] [5] [6] [7] [8] [9]. the brightest Fermi-LAT sources [10] have spectra that steepen in this energy range. Examples for this behavior are flat-spectrum radio quasars [11], and mid-aged supernova remnants interacting with molecular clouds [12] [13] the diffuse Galactic background has a steeper spectrum than most detected Fermi-LAT sources and therefore goes from being the dominant c-ray emitter below this energy range (at $ 100 MeV) to being sub-dominant to individual sources in the TeV band [14] [15] [16] [17] [18]. "
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    ABSTRACT: The past decade has seen a dramatic improvement in the quality of data available at both high (HE: 100 MeV to 100 GeV) and very high (VHE: 100 GeV to 100 TeV) gamma-ray energies. With three years of data from the Fermi Large Area Telescope (LAT) and deep pointed observations with arrays of Cherenkov telescope, continuous spectral coverage from 100 MeV to $\sim10$ TeV exists for the first time for the brightest gamma-ray sources. The Fermi-LAT is likely to continue for several years, resulting in significant improvements in high energy sensitivity. On the same timescale, the Cherenkov Telescope Array (CTA) will be constructed providing unprecedented VHE capabilities. The optimisation of CTA must take into account competition and complementarity with Fermi, in particularly in the overlapping energy range 10$-$100 GeV. Here we compare the performance of Fermi-LAT and the current baseline CTA design for steady and transient, point-like and extended sources.
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