Constraining the nature of the most extreme Galactic particle accelerator: H.E.S.S. observations of the microquasar V4641 Sgr

Acharyya A, Aharonian F, Ashkar H, Backes M, Batzofin R, Berge D, 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 JO, Chibueze O, Cornejo B, Cotter G, De Assis Scarpin J, De Bony De Lavergne M, De Naurois M, De Oña Wilhelmi E, Delgado Giler AG, Devin J, Djannati-Ataï A, Dmytriiev A, Egberts K, Egg K, Ernenwein JP, Escañuela Nieves C, Fauverge P, Feijen K, Filipovic MD, Fontaine G, Funk S, Gabici S, Gallant YA, Glicenstein JF, Glombitza J, Goswami P, Grondin MH, Heckmann L, Heß B, Hinton JA, Hofmann W, Holch TL, Holler M, Jamrozy M, Jankowsky F, Jardin-Blicq A, Jaroschewski I, Jimeno D, Jung-Richardt I, Katarzyski K, Kerszberg D, Khélifi B, Komin N, Kosack K, Kostunin D, Guedes Lang R, Lazarevic S, Lemière A, Lemoine-Goumard M, Lenain JP, Liniewicz P, Luashvili A, Mackey J, Malyshev D, Marandon V, Takekawa S, Mayer M, Mehta A, Mitchell A, Moderski R, Mohrmann L, Montanari A, Moulin E, Niemiec J, Olivera-Nieto L, Moghadam MO, Panny S, Parsons RD, Pensec U, Pichard P, Preis T, Pühlhofer G, Punch M, Quirrenbach A, Reimer A, Reimer O, Reis I, Remy Q, Ren HX, Reville B, Rieger F, Roellinghoff G, Rowell G, Rudak B, Sabri K, Safi-Harb S, Sahakian V, Santangelo A, Sasaki M, Schüssler F, Shapopi JN, Si Said W, Sol H, Stawarz , Steinmassl S, Tanaka T, Taylor AM, Taylor GL, Terrier R, Tian Y, Timmermans A, Tsirou M, Tsuji N, Unbehaun T, van Eldik C, Vecchi M, Venter C, Vink J, Voitsekhovskyi V, Wagner SJ, Wierzcholska A, Zacharias M, Zdziarski AA, Zech A, Zhong W (2026)

Publication Type: Journal article

Publication year: 2026

Journal

Book Volume: 706

Article Number: A8

DOI: 10.1051/0004-6361/202557532

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 (3I) 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.

Authors with CRIS profile

Involved external institutions

Jagiellonian University / Uniwersytet Jagielloński (UJ) Poland (PL) Max-Planck-Institut für Kernphysik (MPIK) / Max Planck Institute for Nuclear Physics Germany (DE) Konan University / 甲南大学 Japan (JP) Deutsches Elektronen-Synchrotron DESY Germany (DE) Ruprecht-Karls-Universität Heidelberg Germany (DE) Université Paris Cité France (FR) University of Tokyo Japan (JP) Nicolaus Copernicus University (NCU) / Uniwersytet Mikołaja Kopernika w Toruniu (UMK) Poland (PL) Sorbonne Université France (FR) University of the Witwatersrand (WITS) South Africa (ZA) Université Paris-Saclay France (FR) Université de Bordeaux France (FR) Eberhard Karls Universität Tübingen Germany (DE) Institute of Research into the Fundamental Laws of the Universe / Institut de recherche sur les lois fondamentales de l'univers (IRFU) France (FR) University of Southern Denmark / Syddansk Universitet Denmark (DK) DIAS Dunsink Observatory Ireland (IE) University of Namibia (UNAM) Namibia (NA) University of Adelaide Australia (AU) Nicolaus Copernicus Astronomical Centre of the Polish Academy of Sciences Poland (PL) University of Montpellier / Université Montpellier France (FR) University of Manitoba Canada (CA) Yerevan Physics Institute (YerPhI) / Երևանի Ֆիզիկայի Ինստիտուտ Armenia (AM) Laboratoire Leprince-Ringuet (LLR) France (FR) Paris Observatory / Observatoire de Paris France (FR) University of Western Sydney Australia (AU) Noordwes-Universiteit / North-West University South Africa (ZA) Polska Akademia Nauk (PAN) / Polish Academy of Sciences Poland (PL) Universität Potsdam Germany (DE) Leopold-Franzens-Universität Innsbruck / University of Innsbruck Austria (AT) Humboldt-Universität zu Berlin Germany (DE) University of Groningen / Rijksuniversiteit Groningen Netherlands (NL) University of Amsterdam Netherlands (NL) Kanagawa University / 神奈川大学 Japan (JP) Institut national de physique nucléaire et de physique des particules (IN2P3) France (FR) University of Oxford United Kingdom (GB)

How to cite

APA:

Acharyya, A., Aharonian, F., Ashkar, H., Backes, M., Batzofin, R., Berge, D.,... Zhong, W. (2026). Constraining the nature of the most extreme Galactic particle accelerator: H.E.S.S. observations of the microquasar V4641 Sgr. Astronomy & Astrophysics, 706. https://doi.org/10.1051/0004-6361/202557532

MLA:

Acharyya, A., et al. "Constraining the nature of the most extreme Galactic particle accelerator: H.E.S.S. observations of the microquasar V4641 Sgr." Astronomy & Astrophysics 706 (2026).

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