H.E.S.S. observations of the Carina nebula and its enigmatic colliding
  wind binary Eta Carinae
  The massive binary system Eta Carinae and the surrounding HII complex, the
Carina Nebula, are potential particle acceleration sites from which
very-high-energy (VHE; E > 100 GeV) \gamma-ray emission could be expected. This
paper presents data collected during VHE \gamma-ray observations with the
H.E.S.S. telescope array from 2004 to 2010, which cover a full orbit of Eta
Carinae. In the 33.1-hour data set no hint of significant \gamma-ray emission
from Eta Carinae has been found and an upper limit on the \gamma-ray flux of
7.7 x 10-13 ph cm-2 s-1 (99% confidence level) is derived above the energy
threshold of 470 GeV. Together with the detection of high-energy (HE; 0.1 GeV >
E > 100 GeV) \gamma-ray emission by the Fermi-LAT up to 100 GeV, and assuming a
continuation of the average HE spectral index into the VHE domain, these
results imply a cut-off in the \gamma-ray spectrum between the HE and VHE
\gamma-ray range. This could be caused either by a cut-off in the accelerated
particle distribution or by severe \gamma-\gamma\ absorption losses in the wind
collision region. Furthermore, the search for extended \gamma-ray emission from
the Carina Nebula resulted in an upper limit on the \gamma-ray flux of 4.2 x
10-12 ph cm-2 s-1 (99% confidence level). The derived upper limit of ~23 on the
cosmic-ray enhancement factor is compared with results found for the old-age
mixed-morphology supernova remnant W 28.
