Constraining the nature of the most extreme Galactic particle accelerator

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° ±0.01 syst ±0.04 stat and 0.06° ±0.01 syst ±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 (3 σ ) 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.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2025-11-14
DOI
https://doi.org/10.1051/0004-6361/202557532
Citations
3
Primary Topic
Astrophysics and Cosmic Phenomena
Type
article
Field-Weighted Citation Impact
5.61

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article

Constraining the nature of the most extreme Galactic particle accelerator

Sanja Lazarević, S. M. Caballero‐Nieves, E. de Oña Wilhelmi, T. Unbehaun et al.
3 citations
Astronomy and Astrophysics
Astrophysics and Cosmic Phenomena
5.61
article

Constraining the nature of the most extreme Galactic particle accelerator

Sanja Lazarević, S. M. Caballero‐Nieves, E. de Oña Wilhelmi, T. Unbehaun, Gavin Rowell, Michael Backes, F. Schüßler, J. A. Hinton, Miroslav D Filipović, D. Kostunin, K. Feijen, Tobias Preis, T. L. Holch, Mathieu de Bony de Lavergne, B. Bruno, R. Terrier, G. Roellinghoff, R. D. Parsons, J. Vink, A. Wierzcholska, Brian Reville, S. Casanova, Jonas Glombitza, Igor Adriano de Oliveira Reis, Alessandro Montanari, M. de Naurois, Ł. Stawarz, A. K. Mehta, G. Cotter, B. Khélifi, K. Egberts, B. A. Hess, D. Malyshev, H. Sol, O. Chibueze, A. Acharyya, K. Bernlöhr, Laura Olivera-Nieto, Jonathan Mackey, C. Venter, Hang Ren, Naomi Tsuji, T. Tanaka, T. Bylund, Frank Rieger, A. M. W. Mitchell, M. Jamrozy, P. Goswami, I. Jung-Richardt, M. Cerruti, S. Gabici, Rodrigo Guedes Lang, Lea Heckmann, M. Vecchi, V. Marandon, J. Niemiec, A. Jardin-Blicq, C. Burger-Scheidlin, Halim Ashkar, R. Moderski, L. Mohrmann, M. Sasaki, Samar Safí-Harb, F. Brun, James O. Chibueze, A. Lemière, J. N. S. Shapopi, B. Rudak, J. Bolmont, J.-P. Lenain, A.G. Delgado Giler, Anton Dmytriiev, A. A. Zdziarski, M. Punch, V. Voitsekhovskyi, S. Steinmassl, A. Santangelo, K. Kosack, S. Funk, I. Jaroschewski, A. Quirrenbach, Nu. Komin, Sebastian Panny, Karim Sabri, U. Pensec, K. Egg, O. Reimer, A. Chen, A. Zech, Anne Timmermans, M.-H. Grondin, Gene Taylor, F. Aharonian, M. Lemoine-Goumard, A.M. Taylor, F. Jankowsky, G. Pühlhofer, A. Reimer, J.-P. Ernenwein, D. Berge, E. Moulin, Y. Tian, J. Devin, C. van Eldik, W. Zhong, C. Boisson, V. Sahakian, S. Takekawa, Y. A. Gallant, Q. Remy, S. J. Wagner, M. Holler, J. F. Glicenstein, A. Luashvili, M.G.F. Mayer, M. Zacharias, Walid Mohd Said, M. Chernyakova, R Batzofin, A. Djannati-Ataï, M. O. Moghadam, M. Tsirou, W. Hofmann, M. Böttcher, D. Kerszberg, K. Katarzyński, J. de Assis Scarpin, B. Cornejo, P. Fauverge, Pierre Pichard, P. Liniewicz, D. Jimeno
article en
3 citations

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° ±0.01 syst ±0.04 stat and 0.06° ±0.01 syst ±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 (3 σ ) 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.

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), Université de Bordeaux (FR), Konan University (JP), University of Groningen (NL), University of Potsdam (DE), University of Southern Denmark (DK), 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), 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), Kanagawa University (JP), Laboratoire Univers et Particules de Montpellier (FR), Laboratoire de Physique Nucléaire et de Hautes Énergies (FR), Institute of Astronomy (RU), International Center for Relativistic Astrophysics (IT), CEA Paris-Saclay (FR), Astronomical Institute of the Slovak Academy of Sciences (SK), Nicolaus Copernicus Astronomical Center (PL), Polish Mother’s Memorial Hospital Research Institute (PL), Thüringer Landessternwarte Tautenburg (DE), Max Planck Institute for Nuclear Physics (DE), Institut de Recherche sur les Lois Fondamentales de l'Univers (FR), University of Manitoba (CA), The University of Adelaide (AU), Western Sydney University (AU), The University of Tokyo (JP), University of Tübingen (DE), University of Amsterdam (NL), Polish Academy of Sciences (PL)
Department of Science and Innovation, South Africa, National Research Foundation, Universiteit van Amsterdam, Bundesministerium für Bildung und Forschung, Austrian Science Fund, Max-Planck-Gesellschaft, Centre National de la Recherche Scientifique, National Astronomical Observatory of Japan, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, National Commission on Research, Science and Technology, University of Namibia, Bundesministerium für Bildung, Wissenschaft und Forschung, Science and Technology Facilities Council, Japan Society for the Promotion of Science, National Institutes of Natural Sciences, Institut National de Physique Nucléaire et de Physique des Particules
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Astrophysics and Cosmic Phenomena
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