H.E.S.S. detection and multi-wavelength study of the z  ∼  1 blazar PKS 0346−27

¹ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory Dublin D15 XR2R, Ireland
² Max-Planck-Institut für Kernphysik P.O. Box 103980 D 69029 Heidelberg, Germany
³ University of Namibia, Department of Physics Private Bag 13301 Windhoek 10005, Namibia
⁴ Centre for Space Research, North-West University Potchefstroom 2520, South Africa
⁵ Institut für Physik und Astronomie, Universität Potsdam Karl-Liebknecht-Strasse 24/25 D 14476 Potsdam, Germany
⁶ Université Paris Cité, CNRS, Astroparticule et Cosmologie F-75013 Paris, France
⁷ Department of Physics and Electrical Engineering, Linnaeus University 351 95 Växjö, Sweden
⁸ Deutsches Elektronen-Synchrotron DESY Platanenallee 6 15738 Zeuthen, Germany
⁹ Institut für Physik, Humboldt-Universität zu Berlin Newtonstr. 15 D 12489 Berlin, Germany
¹⁰ Institut für Astronomie und Astrophysik, Universität Tübingen Sand 1 D 72076 Tübingen, Germany
¹¹ LUX, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université 5 Pl. Jules Janssen 92190 Meudon, France
¹² Sorbonne Université, CNRS/IN2P3, Laboratoire de Physique Nucléaire, et de Hautes Energies, LPNHE 4 place Jussieu 75005 Paris, France
¹³ IRFU, CEA, Université Paris-Saclay F-91191 Gif-sur-Yvette, France
¹⁴ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics Nikolaus-Fiebiger-Str. 2 91058 Erlangen, Germany
¹⁵ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue Braamfontein Johannesburg 2050, South Africa
¹⁶ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University Glasnevin Dublin D09 W6Y4, Ireland
¹⁷ University of Oxford, Department of Physics, Denys Wilkinson Building Keble Road Oxford OX1 3RH, UK
¹⁸ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS, Institut Polytechnique de Paris F-91128 Palaiseau, France
¹⁹ Aix Marseille Université, CNRS/IN2P3, CPPM Marseille, France
²⁰ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72 Place Eugène Bataillon F-34095 Montpellier Cedex 5, France
²¹ School of Science, Western Sydney University Locked Bag 1797 Penrith South DC NSW 2751, Australia
²² Landessternwarte, Universität Heidelberg Königstuhl D 69117 Heidelberg, Germany
²³ Universität Innsbruck, Institut für Astro- und Teilchenphysik Technikerstraße 25 6020 Innsbruck, Austria
²⁴ Universität Hamburg, Institut für Experimentalphysik Luruper Chaussee 149 D 22761 Hamburg, Germany
²⁵ Obserwatorium Astronomiczne, Uniwersytet Jagielloński ul. Orla 171 30-244 Kraków, Poland
²⁶ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University Grudziadzka 5 87-100 Torun, Poland
²⁷ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences ul. Bartycka 18 00-716 Warsaw, Poland
²⁸ Instytut Fizyki Jadrowej PAN, ul. Radzikowskiego 152 ul. Radzikowskiego 152 31-342 Kraków, Poland
²⁹ Yerevan Physics Institute 2 Alikhanian Brothers St. 0036 Yerevan, Armenia
³⁰ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study (UTIAS), Japan
³¹ Department of Physics, Konan University, 8-9-1 Okamoto Higashinada Kobe Hyogo 658-8501, Japan
³² Kapteyn Astronomical Institute, University of Groningen Landleven 12 9747 AD Groningen, The Netherlands
³³ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam Science Park 904 1098 XH Amsterdam, The Netherlands

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Received: 12 September 2025
Accepted: 20 December 2025

Abstract

Context. PKS 0346-27 is a low synchrotron peaked blazar at redshift 0.991. The very high energy (VHE; E > 100 GeV) spectra of blazars are always affected by γγ absorption by the extragalactic background light (EBL), and subsequently no blazars have been detected in VHE γ-rays at redshifts exceeding 1.

Aims. This is the goal of a target-of-opportunity (ToO) programme by H.E.S.S.: to observe flaring high-redshift (z ≳ 1) blazars. Importantly, extending the redshift range of VHE-detected blazars to z ≳ 1 will yield insights into the cosmological evolution of both the VHE blazar population and the EBL.

Methods. We report H.E.S.S. ToO and multi-wavelength observations of the blazar PKS 0346−27. We analysed and modelled the H.E.S.S. data together with simultaneous data from Fermi-LAT, Swift (XRT and UVOT), using single-zone leptonic and hadronic models.

Results. PKS 0346-27 was detected by H.E.S.S. at a significance of 6.3σ during one night on 3 November 2021, while for other nights before and after this day, upper limits on the VHE flux have been determined. No evidence for intra-night γ-ray variability has been found. A flare in high-energy (E > 100 MeV) γ-rays detected by Fermi-LAT preceded the H.E.S.S. detection by 2 days. A fit with a single-zone emission model to the contemporaneous spectral energy distribution during the detection night was possible with a proton-synchrotron-dominated hadronic model, requiring a proton-kinetic-energy-dominated jet power temporarily exceeding the source’s Eddington limit, although alternative (e.g. multi-zone) models cannot be ruled out. A one-zone leptonic model is, in principle, also able to fit the flare-state spectral energy distribution. However, it requires implausible parameter choices, in particular, extreme Doppler and bulk Lorentz factors of ≳80.