Evidence of 100 TeV $\gamma$-ray emission from HESS J1702-420: A new PeVatron candidate

Abstract

The identification of PeVatrons, hadronic particle accelerators reaching the knee of the cosmic ray spectrum (few $10^{15}$ eV), is crucial to understand the origin of cosmic rays in the Galaxy. We provide an update on the unidentified source HESS J1702-420, a promising PeVatron candidate. We present new observations of HESS J1702-420 made with the High Energy Stereoscopic System (H.E.S.S.), and processed using improved analysis techniques. The analysis configuration was optimized to enhance the collection area at the highest energies. We applied a three-dimensional (3D) likelihood analysis to model the source region and adjust non thermal radiative spectral models to the $\gamma$-ray data. We also analyzed archival data from the Fermi Large Area Telescope (LAT) to constrain the source spectrum at $\gamma$-ray energies >10 GeV. We report the detection of a new source component called HESS J1702-420A, that was separated from the bulk of TeV emission at a $5.4\sigma$ confidence level. The power law $\gamma$-ray spectrum of HESS J1702-420A extends with an index of $\Gamma =1.53\pm 0.19_{\text{stat}}\pm 0.20_{\text{sys}}$ and without curvature up to the energy band 64-113 TeV, in which it was detected by H.E.S.S. at a $4.0\sigma$ confidence level. This brings evidence for the source emission up to $100\text{TeV}$, which makes HESS J1702-420A a compelling candidate site for the presence of extremely high energy cosmic rays. Remarkably, in a hadronic scenario, the cut-off energy of the proton distribution powering HESS J1702-420A is found to be higher than 0.5 PeV at a 95% confidence level. HESS J1702-420A becomes therefore one of the most solid PeVatron candidates detected so far in H.E.S.S. data, altough a leptonic origin of its emission could not be ruled out either.

Accepted for publication in the 2. Astrophysical processes section of Astronomy & Astrophysics

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• Sensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnants
• Detection prospects of very and ultra high-energy gamma rays from extended sources with ASTRI, CTA, and LHAASO
• Gamma-ray observations of MAXI J1820+070 during the 2018 outburst
• Testing source confusion and identification capability in Cherenkov Telescope Array data
• HAWC Study of Very-High-Energy $\gamma$-ray Spectrum of HAWC J1844-034
• Constraining leptonic emission scenarios for the PeVatron candidate HESS J1702-420 with deep XMM-Newton observations
• Search for the Galactic accelerators of Cosmic-Rays up to the Knee with the Pevatron Test Statistic
• Analysis Methods for Gamma-ray Astronomy
• The expected potential of hadronic PeVatron searches with spectral $\gamma$-ray data from the Southern Wide-field Gamma-ray Observatory
• Search for long-term variability of HESS J1745-290
• Constraining the position of the knee in the galactic cosmic ray spectrum with ultra-high-energy diffuse $\gamma$-rays