Very-high-energy gamma-ray emission from high-redshift blazars
  We study the possible detection of and properties of very high-energy (VHE)
gamma-ray emission (in the energy band above 100 GeV) from high redshift
sources. We report on the detection of VHE gamma-ray flux from blazars with
redshifts z>0.5. We use the data of Fermi telescope in the energy band above
100 GeV and identify significant sources via cross-correlation of arrival
directions of individual VHE gamma-rays with the positions of known Fermi
sources. There are thirteen high-redshift sources detected in the VHE band by
Fermi/LAT telescope. The present statistics of the Fermi signal from these
sources is too low for a sensible study of the effects of suppression of the
VHE flux by pair production through interactions with Extragalactic Background
Light photons. We find that the detection of these sources with ground-based
gamma-ray telescopes would be challenging. However, several sources including
BL Lacs PKS 0426-380 at z=1.11, KUV 00311-1938 at z=0.61, B3 1307+433 at
z=0.69, PG 1246+586 at z=0.84, Ton 116 at z=1.065 as well as a flat-spectrum
radio quasar 4C +55.17 at z=0.89 should be detectable by HESS-II, MAGIC-II and
CTA. A high-statistics study of a much larger number of VHE gamma-ray sources
at cosmological distances would be possible with the proposed high-altitude
Cherenkov telescope 5@5.
