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

Abstract

Aims. The identification of PeVatrons, hadronic particle accelerators reaching the knee of the cosmic ray spectrum (few × 1015 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. Methods. 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 likelihood analysis to model the source region and adjust non thermal radiative spectral models to the γ-ray data. We also analyzed archival Fermi Large Area Telescope data to constrain the source spectrum at γ-ray energies > 10 GeV. Results. We report the detection of γ-rays up to 100 TeV from a specific region of HESS J1702-420, which is well described by a new source component called HESS J1702-420A that was separated from the bulk of TeV emission at a 5.4σ confidence level. The power law γ-ray spectrum of HESS J1702-420A extends with an index of Γ = 1.53 ± 0.19stat ± 0.20sys and without curvature up to the energy band 64−113 TeV, in which it was detected by H.E.S.S. at a 4.0σ confidence level. This makes HESS J1702-420A a compelling candidate site for the presence of extremely high energy cosmic rays. With a flux above 2 TeV of (2.08 ± 0.49stat ± 0.62sys) × 10−13 cm−2 s−1 and a radius of (0.06 ± 0.02stat ± 0.03sys)°, HESS J1702-420A is outshone – below a few tens of TeV – by the companion HESS J1702-420B. The latter has a steep spectral index of Γ = 2.62 ± 0.10stat ± 0.20sys and an elongated shape, and it accounts for most of the low-energy HESS J1702-420 flux. Simple hadronic and leptonic emission models can be well adjusted to the spectra of both components. Remarkably, in a hadronic scenario, the cut-off energy of the particle distribution powering HESS J1702-420A is found to be higher than 0.5 PeV at a 95% confidence level. Conclusions. For the first time, H.E.S.S. resolved two components with significantly different morphologies and spectral indices, both detected at > 5σ confidence level, whose combined emissions result in the source HESS J1702-420. We detected HESS J1702-420A at a 4.0σ confidence level in the energy band 64−113 TeV, which brings evidence for the source emission up to 100 TeV. In a hadronic emission scenario, the hard γ-ray spectrum of HESS J1702-420A implies that the source likely harbors PeV protons, thus becoming one of the most solid PeVatron candidates detected so far in H.E.S.S. data. However, a leptonic origin of the observed TeV emission cannot be ruled out either.