Level Of Extragalactic Background Light Research Articles

Hard MeV–GeV spectra of blazars

Very high energy (VHE) gamma-ray emission from a distant source (z >~0.2) can be efficiently absorbed my means of the electron-positron pair creation process. Analyses of the unabsorbed spectra imply that the intrinsic TeV emission of some blazars is hard, with spectral indices 0.5 < alpha < 1. The absorption depends on the level of extragalactic background light (EBL) that is difficult to measure directly. This implies that it is difficult to estimate the slope of the intrinsic TeV emission. To test our blazar emission scenario that is capable to reproducing the hard spectra, we therefore used the observations made by the Fermi Gamma-ray Space Telescope in the unabsorbed MeV-GeV energy range. We assume that the X-ray and gamma-ray emission of TeV blazars is produced in a compact region of a jet uniformly filled by particles of relatively high energy (g >~ 10^3, E=g m_e c^2). In other words, we assume a low energy cut-off in the particle energy distribution. The emission produced by the particles with this energy spectrum can explain hard intrinsic spectra in the energy range from MeV up to TeV. We demonstrate how to estimate the basic physical parameters of a source in this case and how to explain the observed spectra by a precise simulation of the particle energy evolution. To test our estimation methods, we use the observations of two blazars with exceptionally hard spectral indices (alpha <~ 0.5) in the MeV-GeV range and known redshifts: RGB J0710+591 and 1ES 0502+675. The estimated values of the Doppler factor and magnetic field are compared with our numerical simulations, which confirm that the particle energy distribution with a low energy cut--off can explain the observed hard spectra well. In addition, we demonstrate that the radiative cooling caused by the inverse-Compton emission in the Klein-Nishina regime may help us to explain the hard spectra.

A low level of extragalactic background light as revealed by γ-rays from blazars

The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the 'first stars', which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light. An alternative approach is to study the absorption features imprinted on the gamma-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons. Here we report the discovery of gamma-ray emission from the blazars H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources-in particular from the first stars formed. This result also indicates that intergalactic space is more transparent to gamma-rays than previously thought.