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HESS observations of the Carina nebula and its enigmatic colliding wind binary Eta Carinae
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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) γ -ray emission could be expected. This paper presents data collected during VHE γ -ray observations with the HESS telescope array from 2004 to 2010, which cover a full orbit of Eta Carinae. In the 33.1-h data set no hint of significant γ -ray emission from Eta Carinae has been found and an upper limit on the γ -ray flux of 7.7 × 10−13 ph cm−2 s−1 (99 per cent confidence level) is derived above the energy threshold of 470 GeV. Together with the detection of high energy (HE; 0.1 ≤ E ≤ 100 GeV) γ -ray emission by the Fermi Large Area Telescope 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 γ -ray spectrum between the HE and VHE γ -ray range. This could be caused either by a cut-off in the accelerated particle distribution or by severe γ –γ absorption losses in the wind collision region. Furthermore, the search for extended γ -ray emission from the Carina nebula resulted in an upper limit on the γ -ray flux of 4.2 × 10−12 ph cm−2 s−1 (99 per cent 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 W28