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Register a new userMeasurement of the Extragalactic Background Light with VERITAS
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Elisa Pueschel
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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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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.
Gamma-rays propagating through space are likely to be extinguished via electron-positron pair production off of the ambient extragalactic background light (EBL). The spectrum of the EBL is produced by starlight (and starlight reprocessed by dust) from all galaxies throughout the history of the Universe. The attenuation of 40 - 400 GeV gamma-rays has been observed by textit{Fermi} and used to measure the EBL spectrum over energies 1 eV -10 eV out to redshift $zsim 1$. Measurements of several TeV blazers are consistent with attenuation, attributed to the EBL at redshift $zsim 0.1$. Here we simultaneously analyze a set of TeV blazers at $zsim 0.1$ to measure the optical depth for 100 GeV - 10 TeV gamma-rays, which interact with EBL of energies 0.05 eV - 5 eV. Using a suite of models for the EBL, we show that the optical depth indicated by TeV blazar attenuation is in good agreement with the optical depths measured by textit{Fermi} at lower gamma-ray energies and higher redshifts.
The extragalactic background light (EBL) is the diffuse radiation with the second highest energy density in the Universe after the cosmic microwave background. The aim of this study is the measurement of the imprint of the EBL opacity to gamma-rays on the spectra of the brightest extragalactic sources detected with the High Energy Stereoscopic System (H.E.S.S.). The originality of the method lies in the joint fit of the EBL optical depth and of the intrinsic spectra of the sources, assuming intrinsic smoothness. Analysis of a total of ~10^5 gamma-ray events enables the detection of an EBL signature at the 8.8 std dev level and constitutes the first measurement of the EBL optical depth using very-high energy (E>100 GeV) gamma-rays. The EBL flux density is constrained over almost two decades of wavelengths (0.30-17 microns) and the peak value at 1.4 micron is derived as 15 +/- 2 (stat) +/- 3 (sys) nW / m^2 sr.
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.
Context. Measurements of the Extragalactic Background Light (EBL) are a fundamental source of information on the collective emission of cosmic sources. Aims. At infrared wavelengths, however, these measurements are precluded by the overwhelming dominance from Interplanetary Dust emission and the Galactic infrared foreground. Only at $lambda > 300 mu$m, where the foregrounds are minimal, has the Infrared EBL (IR EBL) been inferred from analysis of the COBE maps. The present paper aims to assess the possibility of evaluating the IR EBL from a few $mu$m up to the peak of the emission at >100 $mu$m using an indirect method that avoids the foreground problem. Methods. To this purpose we exploit the effect of pair-production from gamma-gamma interaction by considering the highest energy photons emitted by extragalactic sources and their interaction with the IR EBL photons. We simulate observations of a variety of low redshift emitters with the forthcoming Imaging Atmospheric Cherenkov Telescope (IACT) arrays (CTA in particular) and water Cherenkov observatories (LHAASO, HAWC, SWGO) to assess their suitability to constrain the EBL at such long wavelengths. Results. We find that, even under the most extremely favorable conditions of huge emission flares, extremely high-energy emitting blazars are not very useful for our purpose because they are much too distant (>100 Mpc the nearest ones, MKN 501 and MKN 421). Observations of more local Very High Energy (VHE) emitting AGNs, like low-redshift radio galaxies (M87, IC 310, Centaurus A), are better suited and will potentially allow us to constrain the EBL up to $lambda simeq 100 mu$m.
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