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 \geq 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 \times 10 ^ { -13 } \mathrm { 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 \leq E \leq 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 \times 10 ^ { -12 } \mathrm { ph cm ^ { -2 } s ^ { -1 } } ( 99 % confidence level ) .
The derived upper limit of \sim 23 on the cosmic-ray enhancement factor is compared with results found for the old-age mixed-morphology supernova remnant W 28 .