H.E.S.S. observations of the Carina nebula and its enigmatic colliding wind binary Eta Carinae

TL;DR

This study used H.E.S.S. observations to search for very-high-energy gamma-ray emission from Eta Carinae and the Carina Nebula, finding no significant emission and setting upper limits that suggest a spectral cutoff between HE and VHE ranges.

Contribution

First VHE gamma-ray observational limits for Eta Carinae and the Carina Nebula, constraining particle acceleration models and gamma-ray spectral behavior.

Findings

No significant VHE gamma-ray emission detected from Eta Carinae.

Upper limit on gamma-ray flux above 470 GeV set at 7.7 x 10^-13 ph cm^-2 s^-1.

Evidence suggests a spectral cutoff between HE and VHE gamma-ray ranges.

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.