Constraints on Extragalactic Background Light from Cherenkov telescopes:
  status and perspectives for the next 5 years
  Very high energy (VHE, E > 30 GeV) gamma-rays are absorbed via interaction
with low-energy photons from the extragalactic background light (EBL) if the
involved photon energies are above the threshold for electron-positron pair
creation. The VHE gamma-ray absorption, which is energy dependent and increases
strongly with redshift, distorts the VHE energy spectra observed from distant
objects. The observed energy spectra of the AGNs carry therefore an imprint of
the EBL. Recent detections of hard spectra of distant blazars (z = 0.11 - 0.54)
by H.E.S.S. and MAGIC put strong constraints on the EBL density in the optical
to near infrared waveband. It is, however, not yet possible to distinguish
between an intrinsic softening of blazar spectra and a softening caused by the
interaction with low energy EBL photons. In this paper, we give an overview of
the EBL constraints, their limitations and perspectives for the joint efforts
of the Fermi Gamma-Ray Space telescope and imaging atmospheric Cherenkov
telescopes.
