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

Context. PKS 0346-27 is a low synchrotron peaked blazar at redshift 0.991. The very high energy (VHE; E &gt; 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 6.3 σ 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 &gt; 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.

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Journal
Astronomy and Astrophysics
Published
2026-01-13
DOI
https://doi.org/10.1051/0004-6361/202557228
Primary Topic
Astrophysics and Cosmic Phenomena
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article
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article

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

T. L. Holch, R.G. Lang, A. M. W. Mitchell, M.D. Filipovic et al.
Astronomy and Astrophysics
Astrophysics and Cosmic Phenomena
article

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

T. L. Holch, R.G. Lang, A. M. W. Mitchell, M.D. Filipovic, Y. Becherini, J. de Assis Scarpin, P. Pichard, A. Wierzcholska, S. Funk, W. Si Said, S. Steinmassl, A. Luashvili, R.D. Parsons, S.J. Wagner, J. Bolmont, Ł. Stawarz, A. Dmytriiev, K. Kosack, I.D. Davids, A. Reimer, A. Zech, B. Cornejo, C. van Eldik, M. Backes, T. Tanaka, M. Holler, R. Batzofin, K. Egg, G. Pühlhofer, P. Liniewicz, M. Mayer, E. Moulin, S. Panny, D. Berge, A. Mehta, I. Reis, K. Katarzyński, M. de Naurois, C. Boisson, L. Mohrmann, M. Jamrozy, P. Goswami, C. Venter, L. Heckmann, J.A. Hinton, E. de Oña Wilhelmi, B. Rudak, D. Malyshev, K. Bernlöhr, M. Vecchi, F. Aharonian, J.O. Chibueze, W. Hofmann, M. Sasaki, R. Terrier, A. Quirrenbach, A. Santangelo, F. Schüssler, J. Niemiec, J. Vink, S. Gabici, M. Punch, G.L. Taylor, B. Bi, K. Egberts, J. Celic, M. Böttcher, K. Sabri, G. Cotter, U. Pensec, J.N.S. Shapopi, J. Glombitza, K. Kasprzak, N. Komin, O. Chibueze, T. Unbehaun, M. Zacharias, G. Fontaine, F. Jankowsky, A. Lemière, A. Chen, J.-P. Ernenwein, R. Moderski, B. Bruno, J. Damascene Mbarubucyeye, A. Djannati-Ataï, J.F. Glicenstein, D. Cecchin Momesso, O. Reimer, M. de Bony de Lavergne, K. Feijen, I. Jung-Richardt, M. Chernyakova, J. Mackey, T. Preis, T. Wach, M.O. Moghadam, M. Genaro, D. Jimeno, D. Horns, I. Jaroschewski, J. Devin, A. Montanari, B. Khélifi, C. Esca n uela Nieves, D. Kostunin, F. Brun, V. Marandon, M. Cerruti, V. Sahakian, W. Zhong, J.-P. Lenain, S. Lazarević, B. Hess, T. Takahashi, A.M. Taylor, A.G. Delgado Giler, D. Kerszberg, C. Burger-Scheidlin
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Abstract

Context. PKS 0346-27 is a low synchrotron peaked blazar at redshift 0.991. The very high energy (VHE; E &gt; 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 6.3 σ 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 &gt; 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.

Astronomy and AstrophysicsVol. 706
Jagiellonian University (PL), Centre National de la Recherche Scientifique (FR), Dublin Institute For Advanced Studies (IE), A. Alikhanyan National Laboratory (AM), Universität Hamburg (DE), Konan University (JP), University of Groningen (NL), University of Potsdam (DE), Friedrich-Alexander-Universität Erlangen-Nürnberg (DE), Universität Innsbruck (AT), University of the Witwatersrand (ZA), Université de Montpellier (FR), Université Paris Cité (FR), Aix-Marseille Université (FR), University of Namibia (NA), Linnaeus University (SE), Heidelberg University (DE), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Université Paris Sciences et Lettres (FR), North-West University (ZA), Université Paris-Saclay (FR), Deutsches Elektronen-Synchrotron DESY (DE), Laboratoire AstroParticule et Cosmologie (FR), Humboldt-Universität zu Berlin (DE), Sorbonne Université (FR), University of Oxford (GB), Laboratoire Univers et Particules de Montpellier (FR), Laboratoire de Physique Nucléaire et de Hautes Énergies (FR), Kavli Institute for the Physics and Mathematics of the Universe (JP), Institute of Astronomy (RU), International Center for Relativistic Astrophysics (IT), CEA Paris-Saclay (FR), Astronomical Institute of the Slovak Academy of Sciences (SK), Hamburg Institut (Germany) (DE), Nicolaus Copernicus Astronomical Center (PL), Thüringer Landessternwarte Tautenburg (DE), Max Planck Institute for Nuclear Physics (DE), Institut de Recherche sur les Lois Fondamentales de l'Univers (FR), Western Sydney University (AU), University of Tübingen (DE), University of Amsterdam (NL), Polish Academy of Sciences (PL)
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