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Register a new userThe Gamma Ray Opacity of the Universe -- Indirect Measurements of the Extragalactic Background Light
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F. Krennrich
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Indirect constraints on the intensity of the Extragalactic Background Light (EBL) were provided by recent studies of extragalactic sources emitting sub-TeV to multi-TeV photons. These constraints are provided thanks to the absorption of gamma rays by soft photons from the EBL (UV/optical/IR) via pair production by gamma - gamma interactions. This paper provides an overview of recent results that have led to substantially reduced uncertainties on the EBL intensity over a wide range of wavelengths from 0.1 to 15 micron.
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We extend our previous model-independent determination of the intergalactic background light (IBL), based purely on galaxy survey data, out to a wavelength of 5 microns. Our approach enables us to constrain the range of photon densities, based on the uncertainties from observationally determined luminosity densities and colors. We further determine a 68% confidence upper and lower limit on the opacity of the universe to gamma-rays up to energies of 1.6/(1+z) TeV. A comparison of our lower limit redshift-dependent opacity curves to the opacity limits derived from the results of both ground-based air Cherenkov telescope and Fermi-LAT observations of PKS 1424+240 allows us to place a new upper limit on the redshift of this source, independent of IBL modeling.
Axion-like particles (ALPs) provide a feasible explanation for the observed low TeV opacity of the Universe. If the low TeV opacity is caused by ALP, then the $>{rm TeV}$ fluxes of unresolved extragalactic point sources will be correspondingly enhanced, resulting in an enhancement of the observed EGB spectrum at high energies. In this work, we for the first time investigate the ALP effect on the EGB spectrum. Our results show that the existence of ALPs can cause the EGB spectrum to deviate from a pure EBL absorption case. The deviation occurs at about $sim$1 TeV and current EGB measurements by Fermi-LAT cannot identify such an effect. The observation from forthcoming VHE instruments like LHAASO and CTA may be useful for studying this effect. We find that although most of the sensitive ALP parameters have been ruled out by existing ALP results, some unrestricted parameters could be probed with the EGB observation around 10 TeV.
The origin of the extragalactic $gamma$-ray background (EGB) has been debated for some time. { The EGB comprises the $gamma$-ray emission from resolved and unresolved extragalactic sources, such as blazars, star-forming galaxies and radio galaxies, as well as radiation from truly diffuse processes.} This letter focuses on the blazar source class, the most numerous detected population, and presents an updated luminosity function and spectral energy distribution model consistent with the blazar observations performed by the {it Fermi} Large Area Telescope (LAT). We show that blazars account for 50$^{+12}_{-11}$,% of the EGB photons ($>$0.1,GeV), and that {it Fermi}-LAT has already resolved $sim$70,% of this contribution. Blazars, and in particular low-luminosity hard-spectrum nearby sources like BL Lacs, are responsible for most of the EGB emission above 100,GeV. We find that the extragalactic background light, which attenuates blazars high-energy emission, is responsible for the high-energy cut-off observed in the EGB spectrum. Finally, we show that blazars, star-forming galaxies and radio galaxies can naturally account for the amplitude and spectral shape of the background in the 0.1--820,GeV range, leaving only modest room for other contributions. This allows us to set competitive constraints on the dark-matter annihilation cross section.
We have previously calculated the intergalactic background light (IBL) as a function of redshift in the far ultraviolet to near infrared range, based purely on data from deep galaxy surveys. Here we utilize similar methods to determine the mid- and far infrared IBL out to a wavelength of 850 microns. Our approach enables us to constrain the range of photon densities, based on the uncertainties from observationally determined luminosity densities and colors. By also including the effect of the 2.7 K cosmic background photons, we determine 68% confidence upper and lower limits on the opacity of the universe to gamma-rays up to PeV energies. Our direct results on the IBL are consistent with those from complimentary gamma-ray analyses using observations from the Fermi $gamma$-ray space telescope and the H.E.S.S. air Cherenkov telescope. Thus, we find no evidence of previously suggested processes for the modification of gamma-ray spectra other than that of absorption by pair production alone.
Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN) and gamma ray bursts (GRBs) give upper limits on the extragalactic background light (EBL) from the UV to the mid-IR that are only a little above the lower limits from observed galaxies. These upper limits now rule out some EBL models and purported observations, with improved data likely to provide even stronger constraints. We present EBL calculations both based on multiwavelength observations of thousands of galaxies and also based on semi-analytic models, and show that they are consistent with these lower limits from observed galaxies and with the gamma-ray upper limit constraints. Such comparisons close the loop on cosmological galaxy formation models, since they account for all the light, including that from galaxies too faint to see. We compare our results with those of other recent works, and discuss the implications of these new EBL calculations for gamma ray attenuation. Catching a few GRBs with groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov detectors could provide important new constraints on the high-redshift star formation history of the universe.
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