Journal Article

PUBDB-2026-00640

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Constraining the nature of the most extreme Galactic particle accelerator

Acharyya, A. ; Aharonian, F. ; Ashkar, H. ; Backes, M. ; Batzofin, R. ; Berge, D.DESY* ; Bernlöhr, K. ; Böttcher, M. ; Boisson, C. ; Bolmont, J. ; Brun, F. ; Bruno, B. ; Burger-Scheidlin, C. ; Bylund, T. ; Casanova, S. ; Celic, J. ; Cerruti, M. ; Chen, A. ; Chernyakova, M. ; Chibueze, J. O. ; Chibueze, O. ; Cornejo, B. ; Cotter, G. ; de Assis Scarpin, J. ; de Bony de Lavergne, M. ; de Naurois, M. ; de Ona Wilhelmi, E. M.DESY* ; Delgado Giler, A. G. ; Devin, J. ; Djannati-Ataï, A. ; Dmytriiev, A. ; Egberts, K. ; Egg, K. ; Ernenwein, J.-P. ; Escañuela Nieves, C. ; Fauverge, P. ; Feijen, K. ; Filipovic, M. D. ; Fontaine, G. ; Funk, S. ; Gabici, S. ; Gallant, Y. A. ; Glicenstein, J. F. ; Glombitza, J. ; Goswami, P. ; Grondin, M.-H. ; Heckmann, L. ; Heß, B. ; Hinton, J. A. ; Hofmann, W. ; Holch, T. L. ; Holler, M. ; Jamrozy, M. ; Jankowsky, F. ; Jardin-Blicq, A. ; Jaroschewski, I. ; Jimeno, D. ; Jung-Richardt, I. ; Katarzyński, K. ; Kerszberg, D. ; Khélifi, B. ; Komin, N. ; Kosack, K. ; Kostunin, D.DESY* ; Lang, R. G. ; Lazarević, S. ; Lemière, A. ; Lemoine-Goumard, M. ; Lenain, J.-P. ; Liniewicz, P. ; Luashvili, A. ; Mackey, J. ; Malyshev, D. ; Marandon, V. ; Mayer, M. G. F. ; Mehta, A. ; Mitchell, A. M. W. ; Moderski, R. ; Mohrmann, L. ; Montanari, A. ; Moulin, E. ; Niemiec, J. ; Olivera-Nieto, L. ; Moghadam, M. O. ; Panny, S. ; Parsons, R. D. ; Pensec, U. ; Pichard, P. ; Preis, T. ; Pühlhofer, G. ; Punch, M. ; Quirrenbach, A. ; Reimer, A. ; Reimer, O. ; Reis, I. ; Remy, Q. ; Ren, H. X. ; Reville, B. ; Rieger, F. ; Roellinghoff, G. ; Rowell, G. ; Rudak, B. ; Sabri, K. ; Safi-Harb, S. ; Sahakian, V. ; Santangelo, A. ; Sasaki, M. ; Schüssler, F. ; Shapopi, J. N. S. ; Si Said, W. ; Sol, H. ; Stawarz, Ł. ; Steinmassl, S. ; Tanaka, T. ; Taylor, A. M. ; Taylor, G. L. ; Terrier, R. ; Tian, Y. ; Timmermans, A. ; Tsirou, M. ; Tsuji, N. ; Unbehaun, T. ; van Eldik, C. ; Vecchi, M. ; Venter, C. ; Vink, J. ; Voitsekhovskyi, V. ; Wagner, S. J. ; Wierzcholska, A. ; Zacharias, M. ; Zdziarski, A. A. ; Zech, A. ; Zhong, W. ; Takekawa, S.

2026
EDP Sciences Les Ulis

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Please use a persistent id in citations: doi:10.1051/0004-6361/202557532

Report No.: arXiv:2511.10537

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±0.04stat and 0.06°±0.01syst±0.01stat, 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 ngas ≲ 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.

Note: cc-by

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Contributing Institute(s):

1. Gammaastronomie (Z_GA)

Research Program(s):

1. 613 - Matter and Radiation from the Universe (POF4-613) (POF4-613)

Experiment(s):

1. High Energy Stereoscopic System

Appears in the scientific report 2026

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Database coverage:
; Clarivate Analytics Master Journal List ; Current Contents - Physical, Chemical and Earth Sciences ; Ebsco Academic Search ; Essential Science Indicators ; IF >= 5 ; JCR ; SCOPUS ; Science Citation Index Expanded ; Web of Science Core Collection

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