Measurement of the EBL spectral energy distribution using the VHE γ-ray spectra of H.E.S.S. blazars

Abstract

Very high-energy γ rays (VHE, E 100 GeV) propagating over cosmological distances can interact with the low-energy photons of the extragalactic background light (EBL) and produce electron-positron pairs. The transparency of the Universe to VHE γ rays is then directly related to the spectral energy distribution (SED) of the EBL. The observation of features in the VHE energy spectra of extragalactic sources allows the EBL to be measured, which otherwise is very difficult. An EBL model-independent measurement of the EBL SED with the H.E.S.S. array of Cherenkov telescopes is presented. It was obtained by extracting the EBL absorption signal from the reanalysis of high-quality spectra of blazars. From H.E.S.S. data alone the EBL signature is detected at a significance of 9.5σ, and the intensity of the EBL obtained in different spectral bands is presented together with the associated γ-ray horizon.

Measurement of the extragalactic background light using MAGIC and Fermi-LAT gamma-ray observations of blazars up to z = 1

“…Fermi-LAT Abdollahi+'18 ( Figure 13. Wavelength-resolved EBL measurement using MAGIC and Fermi-LAT observations (same as in Figure 12) compared to other EBL measurements obtained with gamma-ray observations, taken from Pueschel (2017), Abdalla et al (2017), Biteau & Williams (2015), Abdollahi et al (2018) The short-wavelength EBL excess in our analysis is strongly reduced if the five PG 1553+113 spectra are excluded from the sample. In such case, the best-fit scale factor becomes α 0.18−0.62 µm = 1.6±0.9 stat , which is compatible with 1.…”

The 2014 TeV γ-Ray Flare of Mrk 501 Seen with H.E.S.S.: Temporal and Spectral Constraints on Lorentz Invariance Violation

“…constraints from γ-rays (Meyer et al 2012;Biteau & Williams 2015;Abdalla et al 2017), and results from empirical determinations (Stecker et al 2016) agree between lower and upper limits. In the following, the model of Franceschini et al (2008) is used as a reference.…”

Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation

“…While the specific intensity of the EBL remains uncertain due to the difficulties of foreground subtraction in direct observations, current-generation γ-ray observatories (in particular imaging atmospheric Cherenkov telescopes, IACTs: H.E.S.S. [12], MAGIC [13], VERITAS [14]) show agreement with expectations from galaxy counts at the ∼ 30 % level for EBL wavelengths up to a few tens of µm [15][16][17][18][19]. On the other hand, the redshift evolution of the EBL, partly probed by observations with the Fermi Large Area Telescope (LAT, [20]) up to hundreds of GeV [21,22], remains poorly constrained by ground-based observatories due to the limited number of γ-ray sources detected beyond z ∼ 0.5.…”

“…[17,[53][54][55]) and approaches where the spectral energy distribution is modeled independently from any prescriptions (only the redshift evolution is tuned to follow that of EBL models, see, e.g., Refs. [15,16,18]). Modeldependent approaches have thus far mostly relied on a simple scaling by a factor α 0 of the photon density from an existing EBL model, which results in τ (E γ , z 0 ; π EBL ) = α × τ (E γ , z 0 ).…”

“…[108]. 16 Two limiting cases are further considered: on the one hand, the blazar is assumed to have been active at the current-day average flux for 10 7 years and, on the other hand, that it has been active only in the last 10 years (the period over which this blazar has been observed by γ-ray instruments). The underlying assumption, as in other related studies, is that the current TeV emission of the blazar traces its past emission averaged over timescales comparable to that of the cascade development (on the order of a Myr for 100 GeV secondary photons and an IGMF strength of 10 −14 G [101]).…”