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

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

Preprint

PUBDB-2025-05485

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.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 Lavergne, M. d. B. ; de Naurois, M. ; de Ona Wilhelmi, E. M.DESY* ; Giler, A. G. D. ; Devin, J. ; Djannati-Ataï, A. ; Dmytriiev, A. ; Egberts, K. ; Egg, K. ; Ernenwein, J.-P. ; Nieves, C. E. ; 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. ; Katarzynski, K. ; Kerszberg, D. ; Khélifi, B. ; Komin, N. ; Kosack, K. ; Kostunin, D. ; Lang, R. G. ; Lazarevic, 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. ; Said, W. S. ; 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. ; H. E. S. S. Collaboration (Collaboration author)

2025

[10.3204/PUBDB-2025-05485]

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Please use a persistent id in citations: doi:10.3204/PUBDB-2025-05485

Report No.: arXiv:2511.10537

Abstract: 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. Using $\approx$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. We detected multi-TeV emission around V4641~Sgr with a high significance. The emission region is elongated. We found a power-law spectrum with an index $\approx$1.8, and together with results from other gamma-ray instruments, this reveals a spectral energy distribution that peaks at energies of $\approx$100~TeV for the first time. We found indications (3$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. The peak of the SED at $\approx$100~TeV identifies V4641~Sgr as a candidate cosmic-ray accelerator beyond the so-called knee. The absence of dense target gas places stringent energetic constraints on hadronic interpretations, however. The H.E.S.S. measurement requires an unusually hard ($\approx 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 \xray emission across the gamma-ray emission region requires a magnetic field strength $\lesssim3$~$μ$G, 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: Accepted for publication in A&A. Machine-readable versions of the maps and spectra are available on Zenodo: https://zenodo.org/records/17570420

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