THE EXTRAGALACTIC BACKGROUND LIGHT, THE HUBBLE CONSTANT, AND ANOMALIES: CONCLUSIONS FROM 20 YEARS OF TeV GAMMA-RAY OBSERVATIONS

Abstract

Ground-based observatories have been collecting 0.2 − 20 TeV gamma rays from blazars for about twenty years. These gamma rays can experience absorption along the line of sight due to interactions with the extragalactic background light (EBL). In this paper, we show that the gamma-ray optical depth can be reduced to the convolution product of an EBL kernel with the EBL intensity, assuming a particular form for the EBL evolution. We extract the absorption signal from the most extensive set of TeV spectra from blazars collected so far and unveil a broad-band EBL spectrum from midultraviolet to far infrared. This spectrum is in good agreement with the accumulated emission of galaxies, constraining unresolved populations of sources. We propose a data-driven estimate of the Hubble constant based on the comparison of local and gamma-ray measurements of the EBL. After setting stringent upper-limits on the redshift of four TeV blazars, we investigate the 106 gammaray spectra in our sample and find no significant evidence for anomalies. The intrinsic TeV spectra are not harder than their GeV counterpart, and no spectral upturn is visible at the highest optical depths. Finally, we investigate a modification of the pair-creation threshold due to Lorentz invariance violation. A mild excess prevents us from ruling out an effect at the Planck energy, and we constrain for the first time the energy scale of the modification to values larger than sixty percent of the Planck energy.

Fermi-LAT high-z active galactic nuclei and the extragalactic background light

“…In addition to B0 218+357, the MAGIC collaboration used 11.8 h of observations from a 2014 flare of 1ES 1011+496 (z = 0.212) (Ahnen et al 2016) to obtain a scaling factor for the EBL models α DOM11 = 1.07(−0.20, +0.24) stat + sys and α FRA08 = 1.14(−0.14, +0.09) stat . This was followed by a study using eight high-frequency-peaked BL Lac and four FSRQs within the redshift range of 0.031 < z < 0.944 (which also includes the observations of the two aforementioned sources), where a scaling factor of α DOM11 = 0.99(−0.56, +0.15) stat + sys was derived (Mazin et al 2016) Correction Ahnen et al (2016) and Mazin et al (2016) (purple square); the Fermi-LAT points from Ackermann et al (2012) and the combined ground-based points from Biteau & Williams (2015) (orange circles). The black upward triangle is the value derived in this work using a combination of power-law and log-parabola models, whereas the green downward triangle shows the value derived when only considering log-parabola models.…”

AKARI near-infrared background fluctuations arise from normal galaxy populations

“…Whereas direct measurements of the NIRB report a large flux above that of the integrated light of resolved galaxies (e.g., Dwek & Arendt 1998;Matsumoto et al 2005), the levels inferred by γ-ray absorption studies are much lower (Ackermann et al 2012; H.E.S.S. Collaboration 2013; Biteau & Williams 2015;Ahnen 2016).…”

Constraints on violation of Lorentz invariance from atmospheric showers initiated by multi-TeV photons

“…Besides, they require modeling of the EBL spectrum. While the understanding of EBL has significantly improved over the last decade (see [51,54] and references therein), some uncertainties still remain [55,56]. In Refs.…”