Detection of very-high-energy gamma-ray emission from Eta Carinae during its 2020 periastron passage

TL;DR

This study reports the first detection of very-high-energy gamma-ray emission from Eta Carinae during its 2020 periastron, revealing insights into particle acceleration and emission regions in this colliding-wind binary system.

Contribution

It provides the first comprehensive VHE gamma-ray observation covering the full periastron passage of Eta Carinae with upgraded H.E.S.S. telescopes, and models the emission region and particle acceleration.

Findings

VHE gamma-ray emission detected during periastron with a steep spectrum.

Hints of increased VHE flux towards periastron, but no variability during passage.

Gamma-ray spectrum is a smooth continuation of the HE spectrum from Fermi-LAT data.

Abstract

The colliding-wind binary system $\eta$ Carinae has been identified as a source of high-energy (HE, below $\sim$100\,GeV) and very-high-energy (VHE, above $\sim$100\,GeV) gamma rays in the last decade, making it unique among these systems. With its eccentric 5.5-year-long orbit, the periastron passage, during which the stars are separated by only $1-2$\,au, is an intriguing time interval to probe particle acceleration processes within the system. In this work, we report on an extensive VHE observation campaign that for the first time covers the full periastron passage carried out with the High Energy Stereoscopic System (H.E.S.S.) in its 5-telescope configuration with upgraded cameras. VHE gamma-ray emission from $\eta$ Carinae was detected during the periastron passage with a steep spectrum with spectral index $\Gamma= 3.3 \pm 0.2_{\mathrm{stat}} \, \pm 0.1_{\mathrm{syst}}$. Together with previous and follow-up observations, we derive a long-term light curve sampling one full orbit, showing hints of an increase of the VHE flux towards periastron, but no hint of variability during the passage itself. An analysis of contemporaneous Fermi-LAT data shows that the VHE spectrum represents a smooth continuation of the HE spectrum. From modelling the combined spectrum we conclude that the gamma-ray emission region is located at distances of ${\sim}10 - 20$\,au from the centre of mass of the system and that protons are accelerated up to energies of at least several TeV inside the system in this phase.