Flux upper limits for 47 AGN observed with H.E.S.S. in 2004-2011
  About 40% of the observation time of the High Energy Stereoscopic System
(H.E.S.S.) is dedicated to studying active galactic nuclei (AGN), with the aim
of increasing the sample of known extragalactic very-high-energy (VHE, E>100
GeV) sources and constraining the physical processes at play in potential
emitters. H.E.S.S. observations of AGN, spanning a period from April 2004 to
December 2011, are investigated to constrain their gamma-ray fluxes. Only the
47 sources without significant excess detected at the position of the targets
are presented. Upper limits on VHE fluxes of the targets were computed and a
search for variability was performed on the nightly time scale. For 41 objects,
the flux upper limits we derived are the most constraining reported to date.
These constraints at VHE are compared with the flux level expected from
extrapolations of Fermi-LAT measurements in the two-year catalog of AGN. The
H.E.S.S. upper limits are at least a factor of two lower than the extrapolated
Fermi-LAT fluxes for 11 objects. Taking into account the attenuation by the
extragalactic background light reduces the tension for all but two of them,
suggesting intrinsic curvature in the high-energy spectra of these two AGN.
Compilation efforts led by current VHE instruments are of critical importance
for target-selection strategies before the advent of the Cherenkov Telescope
Array, CTA.
