Limits on the extragalactic background light in the Fermi era

摘要

Very high energy (VHE, energy approx.> 100 GeV) γ-rays from cosmological sources are attenuated due to the interaction with photons of the extragalactic background light (EBL) in the ultraviolet to infrared wavelength bands. The EBL, thus, leaves an imprint on the observed energy spectra of these objects. In the last four years, the number of extragalactic VHE sources discovered with imaging atmospheric Cherenkov telescopes (IACTs), such as MAGIC, H.E.S.S., and VERITAS, has doubled. Furthermore, the measurements with the Fermi satellite brought new insights into the intrinsic spectra of the sources at GeV energies. Here, we present upper limits on the EBL intensity that are derived by considering the most extensive VHE source sample ever used in this context. This is accomplished by constructing a large number of generic EBL shapes and combining spectral information from Fermi and IACTs together with minimal assumptions about the source physics at high and very high gamma-ray energies. In addition to previous studies, the evolution of the EBL with redshift is accounted for and the possibility of the formation of an electromagnetic cascade and the implications on the upper limits are explored. The EBL density at z = 0 is constrained over more than two orders of magnitude in wavelength, i.e., between 0.4 and 100 μm. The resulting upper limits constitute the strongest ever reported over such a broad wavelength range.