Constraining the nature of the most extreme Galactic particle accelerator

H.E.S.S. observations of the microquasar V4641 Sgr

¹ University of Southern Denmark Odense, Denmark
² Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory Dublin D15 XR2R, Ireland
³ Max-Planck-Institut für Kernphysik P.O. Box 103980 D 69029 Heidelberg, Germany
⁴ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS, Institut Polytechnique de Paris F-91128 Palaiseau, France
⁵ University of Namibia, Department of Physics Private Bag 13301 Windhoek 10005, Namibia
⁶ Centre for Space Research, North-West University Potchefstroom 2520, South Africa
⁷ Institut für Physik und Astronomie, Universität Potsdam Karl-Liebknecht-Strasse 24/25 D 14476 Potsdam, Germany
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 LUX, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université 5 Pl. Jules Janssen 92190 Meudon, France
¹¹ Sorbonne Université, CNRS/IN2P3, Laboratoire de Physique Nucléaire, et de Hautes Energies, LPNHE 4 place Jussieu 75005 Paris, France
¹² IRFU, CEA, Université Paris-Saclay F-91191 Gif-sur-Yvette, France
¹³ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics Nikolaus-Fiebiger-Str. 2 91058 Erlangen, Germany
¹⁴ Instytut Fizyki Jacadrowej PAN, ul. Radzikowskiego 152 ul. Radzikowskiego 152 31-342 Kraków, Poland
¹⁵ Université Paris Cité, CNRS, Astroparticule et Cosmologie F-75013 Paris, France
¹⁶ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue Braamfontein Johannesburg 2050, South Africa
¹⁷ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University Glasnevin Dublin D09 W6Y4, Ireland
¹⁸ University of Oxford, Department of Physics, Denys Wilkinson Building Keble Road Oxford OX1 3RH, UK, United Kingdom
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 Université Bordeaux, CNRS, LP2I Bordeaux, UMR 5797 F-33170 Gradignan, France
²² School of Science, Western Sydney University Locked Bag 1797 Penrith South DC NSW 2751, Australia
²³ Landessternwarte, Universität Heidelberg Königstuhl D 69117 Heidelberg, Germany
²⁴ Institut für Astronomie und Astrophysik, Universität Tübingen Sand 1 D 72076 Tübingen, Germany
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 GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam Science Park 904 1098 XH Amsterdam, The Netherlands
³⁰ Department of Physics and Astronomy, University of Manitoba Winnipeg MB R3T 2N2, Canada
³¹ School of Physical Sciences, University of Adelaide Adelaide 5005, Australia
³² Yerevan Physics Institute 2 Alikhanian Brothers St. 0036 Yerevan, Armenia
³³ Department of Physics, Konan University, 8-9-1 Okamoto Higashinada Kobe Hyogo 658-8501, Japan
³⁴ Institute for Cosmic Ray Research, University of Tokyo, 5-1-5 Kashiwa-no-ha Kashiwa Chiba 277-8582, Japan
³⁵ Kapteyn Astronomical Institute, University of Groningen Landleven 12 9747 AD Groningen, The Netherlands
³⁶ Department of Applied Physics, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi Kanagawa-ku Yokohama Kanagawa 221-8686, Japan

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Received: 2 October 2025
Accepted: 6 November 2025

Abstract

Context. Microquasars have emerged as promising candidates to explain the cosmic-ray flux at petaelectronvolt energies. LHAASO observations revealed V4641 Sgr as the most extreme example so far. Its gamma-ray spectrum extends up to 800 TeV, which requires particles with multi-PeV energy. The TeV emission is highly extended, which challenges expectations given the reported low-inclination angle of the V4641 Sgr jets.

Aims. We spatially and spectrally resolved the gamma-ray emission from V4641 Sgr and investigated the particle acceleration in the system.

Methods. Using ≈100 h of H.E.S.S. data, we performed a spectro-morphological study of the gamma-ray emission around V4641 Sgr. We employed HI and dedicated CO observations of the region to infer the target material for cosmic-ray interactions.

Results. We detected multi-TeV emission around V4641 Sgr with a high significance. The emission region is elongated, and its major and minor axes are 0.34°_±0.01syst^±0.04_stat and 0.06°_±0.01syst^±0.01_stat, respectively. We found a power-law spectrum with an index ≈1.8, and together with results from other gamma-ray instruments, this reveals a spectral energy distribution (SED) that peaks at energies of ≈100 TeV for the first time. We found indications (3σ) of a two-component morphology, with indistinguishable spectral properties. The position of V4641 Sgr is inconsistent with the best-fit position of the single-component model and with the dip between the two components. We found no significant evidence of an energy-dependent morphology. No dense gas was found at any distance towards V4641 Sgr, which places an upper limit of n_gas ≲ 0.2 cm⁻³ within the gamma-ray emission region.

Conclusions. The peak of the SED at ≈100 TeV identifies V4641 Sgr as a candidate cosmic-ray accelerator beyond the so-called knee. The absence of dense target gas places stringent energetic constraints on hadronic interpretations, however. The H.E.S.S. measurement requires an unusually hard (≈1.5) spectral index for the protons. A leptonic scenario faces fewer obstacles if the particle transport is fast enough to avoid losses and to reproduce the observed energy-independent morphology. The absence of bright X-ray emission across the gamma-ray emission region requires a magnetic field strength ≲3 μG, however. Our findings favour a leptonic origin of the gamma-ray emission. This conclusion does not exclude hadron acceleration in the V4641 Sgr system.