Abstract
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.
Highlights
The Carina Nebula is one of the largest and most active HII regions in our Galaxy and a place of ongoing star formation
The AGILE collaboration reported on a two-day γ-ray flare from the direction of Eta Carinae which occurred in October 2008
The H.E.S.S. flux Upper limits (ULs) for the individual subsets above the threshold energies of ∼ 0.5 TeV are all well below the extrapolated hard HE γ-ray component measured by Fermi -LAT 3
Summary
The Carina Nebula is one of the largest and most active HII regions in our Galaxy and a place of ongoing star formation. The AGILE collaboration reported on a two-day γ-ray flare from the direction of Eta Carinae which occurred in October 2008 This increased γ-ray flux could not be confirmed by Farnier et al (2011), Walter & Farnier (2011) found that the high-energy component flux shows a drop in the yearly light curve. Both these findings point to a possible origin of the HE γ-ray emission in the colliding wind region of Eta Carinae. The more advanced data analysis method that is used in this work is discussed later, and achieves a significantly better point-source sensitivity (Ohm et al 2009)
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