Extragalactic optical-infrared background radiation, its time evolution and the cosmic photon-photon opacity

Abstract

Context. The background radiation in the optical and the infrared cause energy loss in the propagation of high energy particles through space. In particular, TeV observations with Cherenkov telescopes of extragalactic sources are influenced by the opacity effects due to the interaction of the very high-energy source photons with the background light. Aims. With the aim of assessing with the best possible detail these opacity terms, we have modelled the extragalactic optical and infrared backgounds using available information on cosmic sources in the universe from far-UV to sub-millimeter wavelengths over a wide range of cosmic epochs. Methods. We have exploited the relevant cosmological survey data -including number counts, redshift distributions, luminosity functions -from ground-based observatories in the optical, near-IR, and sub-millimeter, as well as multi-wavelength information coming from the HST, ISO and Spitzer space telescopes. Additional constraints have been used from direct measurements or upper limits on the extragalactic backgrounds by dedicated missions (COBE). All data were fitted and interpolated with a multi-wavelength backward evolutionary model, allowing us to estimate the background photon density and its redshift evolution. From the redshift-dependent background spectrum, the photon-photon opacities for sources of high-energy emission at any redshifts were then computed. The same results can also be used to compute the optical depths for any kind of processes in the intergalactic space involving interactions with background photons (like scattering of cosmic-ray particles). Results. We have applied our photon-photon opacity estimates to the analysis of spectral data at TeV energies on a few BLAZARs of particular interest. The opacity-corrected TeV spectra are entirely consistent with standard photon-generation processes and show photon indices steeper than Γ intrinsic = 1.6. Contrary to some previous claims, but in agreement with other reports, we find no evidence for any truly diffuse background components in addition to those from resolved sources. We have tested in particular the effects of a photon background originating at very high redshifts, as would be the emissions by a primeval population of Population III stars around z ∼ 10. We could not identify any opacity features in our studied BLAZAR spectra consistent with such an emission and place a stringent limit on such a diffuse photon intensity of ∼6 nW/m 2 /sr between 1 and 4 μm. Conclusions. TeV observations of BLAZARs are consistent with background radiation contributed by resolved galaxies in the optical and IR, and exclude prominent additional components from very high-z unresolved sources.

EBL constraints using a sample of TeV gamma-ray emitters measured with the MAGIC telescopes

“…The method used is similar to [4,5] and [16]. The EBL model used here is from [12] and similar results are obtained when using models of [10,11] and [13]. Our result on the EBL scaling factor is α = 0.99(+0.10)(−0.12), statistical errors only and α = 0.99(+0.15)(−0.56) when adding systematic uncertainties.…”

“…The recent models of [10,11,12,13] can reproduce well the EBL spectrum seen at redshift z=0 and have in general good agreement among each other as well as with the indirect constraints and detections using VHE γ-ray sources. The latter suggests that there are not many unresolved sources of EBL out there.…”

The exceptional VHE gamma-ray outburst of PKS 1510-089 in May 2016

“…A curved spectrum is preferred with 4.4σ, which is well explained by the absorption from the extragalactic background light (EBL) using the model of [18]. The intrinsic spectrum is well described by a power-law F(E) = N(E 0 )×(E/E 0 ) −Γ with normalization N = (19.0±0.8 stat )×10 −10 cm −2 s −1 TeV −1 , decorrelation energy E 0 = 0.268 TeV, and photon index Γ = 2.9 ± 0.2 stat .…”

Simulations of Ultra High Energy Cosmic Rays Propagation

“…The IRB and the IGMF are currently poorly known. The existing model calculations of the IRB spectral density [13][14][15] may differ by factor of 2 at redshift z = 0 and moreover the uncertainty grows with redshift. We use the Kneiske et.…”