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

¹ University of Namibia, Department of Physics, Private Bag 13301, Windhoek 10005, Namibia
² Laboratoire Leprince-Ringuet, École Polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau, France
³ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁴ Max-Planck-Institut für Kernphysik, PO Box 103980 69029 Heidelberg, Germany
⁵ High Energy Astrophysics Laboratory, RAU, 123 Hovsep Emin St, Yerevan 0051, Armenia
⁶ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
⁷ Laboratoire d’Annecy de Physique des Particules, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, LAPP, 74000 Annecy, France
⁸ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁰ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
¹¹ Instytut Fizyki Jdrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
¹² DESY, 15738 Zeuthen, Germany
¹³ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
¹⁴ Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden
¹⁵ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹⁶ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, CNRS, Université de Paris, 92190 Meudon, France
¹⁷ Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 Place Jussieu, 75252 Paris, France
¹⁸ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁹ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
²⁰ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
²¹ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg 2050, South Africa
²² Université Bordeaux, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²³ School of Physical Sciences, University of Adelaide, Adelaide 5005, Australia
²⁴ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
²⁵ Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
²⁶ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester LE1 7RH, UK
²⁷ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
²⁸ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
²⁹ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
³⁰ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
³¹ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
³² Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
³³ Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
³⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
³⁵ Department of Physics, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa
³⁶ Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³⁷ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
³⁸ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study (UTIAS), The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8583, Japan

Received: 31 March 2021
Accepted: 7 June 2021

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 ± 0.19_stat ± 0.20_sys and without curvature up to the energy band 64−113 TeV, in which it was detected by H.E.S.S. at a 4.0σ 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 ± 0.49_stat ± 0.62_sys) × 10⁻¹³ cm⁻² s⁻¹ and a radius of (0.06 ± 0.02_stat ± 0.03_sys)°, 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 ± 0.10_stat ± 0.20_sys 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.