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 theγ-ray data. We also analyzed archivalFermiLarge Area Telescope data to constrain the source spectrum atγ-ray energies > 10 GeV.Results.We report the detection ofγ-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 5.4σconfidence level. The power lawγ-ray spectrum of HESS J1702-420A extends with an index of Γ = 1.53 ± 0.19stat ± 0.20sysand without curvature up to the energy band 64−113 TeV, in which it was detected by H.E.S.S. at a 4.0σconfidence 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−1and 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 ± 0.20sysand 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 > 5σconfidence level, whose combined emissions result in the source HESS J1702-420. We detected HESS J1702-420A at a 4.0σconfidence 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 hardγ-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.
Highlights
The acceleration sites of cosmic rays1 are a century-old unknown in modern astrophysics (Hess 1912)
This paper reports on new H.E.S.S. observations of HESS J1702-420 that have been processed with improved techniques
For each of the four observation groups, the list of reconstructed events was reduced to a binned data cube, with spatial dimensions corresponding to an analysis region of interest (RoI) of 4◦ × 4◦ centered at the HGPS position of HESS J1702-420
Summary
The acceleration sites of cosmic rays are a century-old unknown in modern astrophysics (Hess 1912). The H.E.S.S. Galactic Plane Survey (HGPS) catalog (H.E.S.S. Collaboration 2018b) lists 78 VHE γ-ray sources, most of which have been identified as of today – or at least likely associated – with multi wavelength counterparts. It was discovered during the first Galactic plane survey campaign with a significance of 4σ, based on a 5.7 hr observation livetime (Aharonian et al 2006). The surrounding ISM does not exhibit any clear morphological association with the VHE γ-ray source (Lau et al 2018), a fact that challenges a hadronic interpretation of the TeV emission. Deep X-ray observations of the VHE source with Suzaku revealed the presence of two faint point-like sources close to the line of sight of HESS J1702-420, and the absence of extended emission, with an X-ray flux at least 12 times lower than the TeV flux in the Suzaku field of view (FoV) (Fujinaga et al 2011). Independent crosschecks for the classical morphological and spectral analyses presented in Sect. 2.2 and Appendix A were carried out using the standard HAP software
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