Curvature in the very-high energy gamma-ray spectrum of M 87^⋆

¹ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
² Max-Planck-Institut für Kernphysik, PO Box 103980 69029 Heidelberg, Germany
³ Yerevan State University, 1 Alek Manukyan St, Yerevan 0025, Armenia
⁴ Yerevan Physics Institute, 2 Alikhanian Brothers St., 0036 Yerevan, Armenia
⁵ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
⁶ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
⁷ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau, France
⁸ University of Namibia, Department of Physics, Private Bag 13301, Windhoek 10005, Namibia
⁹ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
¹⁰ Deutsches Elektronen-Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
¹¹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
¹² Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
¹³ Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden
¹⁴ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁵ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, CNRS, Université Paris Cité, 5 Pl. Jules Janssen, 92190 Meudon, France
¹⁶ Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Énergies, LPNHE, 4 Place Jussieu, 75252 Paris, France
¹⁷ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁸ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Nikolaus-Fiebiger-Str. 2, 91058 Erlangen, Germany
¹⁹ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
²⁰ Instytut Fizyki Jdrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²¹ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
²² School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg 2050, South Africa
²³ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
²⁴ School of Physical Sciences, University of Adelaide, Adelaide 5005, Australia
²⁵ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstraße 25, 6020 Innsbruck, Austria
²⁶ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
²⁷ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
²⁸ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
²⁹ Aix-Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
³⁰ Université Bordeaux, CNRS, LP2I Bordeaux, UMR 5797, 33170 Gradignan, France
³¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
³² 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
³³ Department of Physics, Konan University, 8-9-1 Okamoto, Higashinada, Kobe, Hyogo 658-8501, Japan
³⁴ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³⁵ Department of Physics, University of Wisconsin–Madison, Madison, WI 53706, USA

Received: 11 December 2023
Accepted: 15 February 2024

Abstract

The radio galaxy M 87 is a variable very-high energy (VHE) gamma-ray source, exhibiting three major flares, which were reported in 2005, 2008, and 2010. Despite extensive studies, the origin of the VHE gamma-ray emission is not yet fully understood. In this study, we investigate the VHE gamma-ray spectrum of M 87 during states of high gamma-ray activity, utilizing 20.2 h of H.E.S.S. observations. Our findings indicate a preference for a curved spectrum, characterized by a log-parabola model with extra-galactic background light (EBL) model above 0.3 TeV at the 4σ level, compared to a power-law spectrum with EBL. We investigate the degeneracy between the absorption feature and the EBL normalization and derive upper limits on EBL models that are mainly sensitive in the wavelength range of 12.4 μm–40 μm.