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Added by
Alexander Korochkin
Publication date
2019
fields
Physics
and research's language is
English
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Cumulative optical and infrared emission from galaxies accumulated over cosmological time scales, the extragalactic background light (EBL), could be probed by complementary techniques of direct observations and source counting in the visible and infrared as well as via its imprint on the signal of distant active galactic nuclei in gamma rays. We compare the visible and infrared measurements with the gamma-ray constraints and study if the discrepancies of the measurements with different methods could be due to the presence of features in the EBL spectrum that are localised in the micron wavelength range. We combined data on time-averaged spectra of selected blazars that were obtained by Fermi and ground-based gamma-ray telescopes. We also modelled the effect of absorption on EBL while allowing for the existence of a previously unaccounted spectral feature. We show that a previously reported excess in EBL flux in the $sim 1$~micron wavelength range is consistent with gamma-ray measurements, that is, if the excess has the form of a narrow feature of the width $delta lambda < lambda$ and an overall flux of up to 15 nW/(m$^2$ sr) above the minimal EBL, which is estimated from the visible and infrared source counts. Such bump-like spectral features could originate, for example, from decaying dark-matter particles, or either axions or peculiar astrophysical processes in the course of star-formation history. We discuss the possibilities for the search of spectral features in the EBL with the Cherenkov Telescope Array (CTA).
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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.
The extragalactic background light (EBL), a diffuse photon field in the optical and infrared range, is a record of radiative processes over the Universes history. Spectral measurements of blazars at very high energies ($>$100 GeV) enable the reconstruction of the spectral energy distribution (SED) of the EBL, as the blazar spectra are modified by redshift- and energy-dependent interactions of the gamma-ray photons with the EBL. The spectra of 14 VERITAS-detected blazars are included in a new measurement of the EBL SED that is independent of EBL SED models. The resulting SED covers an EBL wavelength range of 0.56--56 $mu$m, and is in good agreement with lower limits obtained by assuming that the EBL is entirely due to radiation from cataloged galaxies.
When very high-energy photons (VHE, E>100 GeV) propagate over cosmological distances, they interact with background light by pair production. Observations of spectral features in the VHE band of extragalactic sources related to this energy-dependent absorption process with the H.E.S.S. array of Cherenkov telescopes allow measuring the spectral energy distribution (SED) of the extragalactic background light (EBL), otherwise very difficult to determine. Preliminary results on the determination of the SED of the EBL will be presented, based on the measurements of the energy spectra of blazars with H.E.S.S. at redshifts up to z = 0.2. This model independent approach shows that the shape and overall normalization of the EBL SED is accessible.
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.
The extragalactic background light records the history of infrared, optical and ultraviolet light radiation including re-radiation since the epoch of reionization. While challenging to measure directly, it can be measured indirectly via its impact on observed spectra of extragalactic gamma-ray emitters. VERITAS, a ground-based imaging atmospheric-Cherenkov telescope array sensitive to gamma rays above 100 GeV, has accrued 10 years of observations of hard-spectrum blazars. The energy and redshift range covered enables the measurement of the EBL in the range 0.56-56~$mu$m, accessing the poorly constrained cosmic infrared background region. New constraints on the EBL resulting from the joint analysis using 16 spectra from 14 VERITAS-observed blazars will be presented. The method is independent of assumptions about the shape of the EBL spectrum, and includes a full treatment of systematic and statistical uncertainties. The measured spectrum is in good agreement with lower limits from galaxy counts, limiting the potential contribution from a diffuse component.
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