[Paper Review] First Results of Eta Car Observations with H.E.S.S.II

E. Leser, S. Ohm+8 more|arXiv (Cornell University)|Aug 3, 2017

Astrophysical Phenomena and Observations3 citations

TL;DR

This paper reports the first significant detection of high-energy gamma-ray emission from Eta Carinae by the H.E.S.S. II telescope array, achieving a 13.6σ significance after lowering the energy threshold to ~200 GeV via the addition of a 28-meter central telescope. The detection confirms the presence of a high-energy component extending up to 300 GeV, consistent with particle acceleration in colliding stellar winds and providing critical insights into non-thermal processes in massive binary systems.

ABSTRACT

Eta Carinae (or Eta Car) is a colliding-wind binary that shows non-thermal emission from hard X-rays to high-energy $γ$-rays. The $γ$-ray spectrum exhibits two spectral components, where the high-energy component extends up to 300 GeV. Previous observations of Eta Carinae with the High Energy Stereoscopic System (H.E.S.S.) resulted in upper limits. With the addition of the large 28-m central telescope to the H.E.S.S. array in 2012 the lower bound on the energy range has significantly been reduced. This lowers the energy threshold of the analysis compared to the published results due to the improved instrument sensitivity at energies below 400 GeV. Eta Carinae has been regularly observed in the following years with H.E.S.S. II. Here we report on the first results of Eta Carinae observations, which also cover the periastron passage in 2014 - the time of maximum $γ$-ray emission seen in GeV $γ$-rays with Fermi-LAT.

Motivation & Objective

• To detect high-energy gamma-ray emission from Eta Carinae using the upgraded H.E.S.S. II array with improved sensitivity at lower energies.
• To investigate the spectral properties of Eta Carinae in the 100–300 GeV range, particularly around the 2014 periastron passage.
• To assess the impact of strong and inhomogeneous night sky background (NSB) on Cherenkov telescope data in the Carina Nebula region.
• To validate the detection of gamma-ray emission using independent analysis chains and Monte Carlo simulations under realistic NSB conditions.
• To establish the first robust high-energy gamma-ray measurement of a colliding-wind binary system beyond Fermi-LAT's energy range.

Proposed method

• The H.E.S.S. II array, upgraded in 2012 with a 28-meter-diameter central telescope, reduced the energy threshold from 470 GeV to ~200 GeV, enabling detection of lower-energy gamma rays.
• 25 hours of observations were conducted during 2014–2015, covering the pre- and post-periastron phases of Eta Carinae’s 5.5-year orbit.
• A cleaning algorithm discarded 10% of the noisiest camera pixels, and a cut at 3 times the measured NSB rate reduced residual NSB contamination to 1.5%.
• Systematic checks were performed on shower image homogeneity and NSB distribution, with the highest NSB levels found south of Eta Carinae.
• Monte Carlo simulations were used to model NSB effects under actual observational conditions, including spatially varying NSB rates.
• Two independent analysis chains were applied to both monoscopic (single 28m telescope) and stereoscopic (all five telescopes) data to ensure result robustness.

Experimental results

Research questions

• RQ1 Does the H.E.S.S. II array detect significant high-energy gamma-ray emission from Eta Carinae in the 100–300 GeV range?
• RQ2 How does the addition of the 28-meter telescope improve the energy threshold and sensitivity for detecting Eta Carinae?
• RQ3 To what extent does the strong and inhomogeneous night sky background affect the detection of Cherenkov signals from Eta Carinae?
• RQ4 Is the observed gamma-ray emission consistent with particle acceleration in colliding stellar winds, as predicted by shock models?
• RQ5 How does the gamma-ray flux from Eta Carinae vary around the 2014 periastron passage, the time of maximum GeV emission observed by Fermi-LAT?

Key findings

• H.E.S.S. II achieved a 13.6σ pre-trial significance for gamma-ray emission from Eta Carinae, marking the first significant detection in the very high-energy band.
• The energy threshold was reduced to approximately 200 GeV, a 60% improvement over the previous H.E.S.S. threshold of 470 GeV.
• A total of 1070 gamma-ray candidate events were recorded in 25 hours of observation, distributed across monoscopic and stereoscopic data sets.
• The night sky background (NSB) was found to be 10 times higher than average in the Eta Carinae field, with the strongest NSB levels located south of the source.
• Systematic tests confirmed that NSB contamination was minimized to 1.5% of pure NSB photons after data cleaning and thresholding, with Monte Carlo simulations validating the robustness of the result.
• The detection was confirmed independently by two analysis chains, ensuring the reliability of the observed signal and its spatial consistency with Eta Carinae.