Detection of very-high-energy γ-ray emission from the colliding wind binary η Car with H.E.S.S.

Abstract

Aims. Colliding wind binary systems have long been suspected to be high-energy (HE; 100 MeV < E < 100 GeV) γ-ray emitters. η Car is the most prominent member of this object class and is confirmed to emit phase-locked HE γ rays from hundreds of MeV to ~100 GeV energies. This work aims to search for and characterise the very-high-energy (VHE; E >100 GeV) γ-ray emission from η Car around the last periastron passage in 2014 with the ground-based High Energy Stereoscopic System (H.E.S.S.). Methods. The region around η Car was observed with H.E.S.S. between orbital phase p = 0.78−1.10, with a closer sampling at p ≈ 0.95 and p ≈ 1.10 (assuming a period of 2023 days). Optimised hardware settings as well as adjustments to the data reduction, reconstruction, and signal selection were needed to suppress and take into account the strong, extended, and inhomogeneous night sky background (NSB) in the η Car field of view. Tailored run-wise Monte-Carlo simulations (RWS) were required to accurately treat the additional noise from NSB photons in the instrument response functions. Results. H.E.S.S. detected VHE γ-ray emission from the direction of η Car shortly before and after the minimum in the X-ray light-curve close to periastron. Using the point spread function provided by RWS, the reconstructed signal is point-like and the spectrum is best described by a power law. The overall flux and spectral index in VHE γ rays agree within statistical and systematic errors before and after periastron. The γ-ray spectrum extends up to at least ~400 GeV. This implies a maximum magnetic field in a leptonic scenario in the emission region of 0.5 Gauss. No indication for phase-locked flux variations is detected in the H.E.S.S. data.

Colliding-Wind Binaries as a Source of TeV Cosmic Rays

“…Emission from ηCar, in the range of 100 GeV up to ∼400 GeV, has also been detected by H.E.S.S. (H. E. S. S. Collaboration et al, 2020). The combined data in the range from few up to hundreds of GeVs show a spectral index of −0.7 to −0.5.…”

INTEGRAL View of TeV Sources: A Legacy for the CTA Project

“…At γ-ray energies, detections have been reported with AGILE [92] and Fermi [93], while the H.E.S.S. telescope detected emission up to 400 GeV [94,95]. Although the analysis indicated a point-like morphology, the picture was not sufficiently clear and later observations did not indicate variability associated with lower energy measurements [95] and no flare similar to the one reported with AGILE has been revealed.…”

The 2020 periastron of Eta Carinae at high-energies

“…Synchrotron emission from the source is undetected, likely due to the high opacity of the stellar winds, while it has a confirmed non-thermal X-ray [12] and GeV-TeV γ-ray [6,13] components. Previous studies above 100 MeV with Fermi LAT have established that η Carinae displays two distinct components above (High Energy; HE) and below (Low Energy; LE) 10 GeV.…”