A. Acharyya, F. Aharonian, Halim Ashkar, Michael Backes, R Batzofin, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, F. Brun, B. Bruno, C. Burger-Scheidlin, T. Bylund, S. Casanova, J. Celic, M. Cerruti, A. Chen, M. Chernyakova, James O. Chibueze, O. Chibueze, B. Cornejo, G. Cotter, J. de Assis Scarpin, Mathieu de Bony de Lavergne, M. de Naurois, E. de Oña Wilhelmi, A.G. Delgado Giler, J. Devin, A. Djannati-Ataï, Anton Dmytriiev, K. Egberts, K. Egg, J.-P. Ernenwein, C. Escañuela Nieves, Paul Fauverge, K. Feijen, M. D. Filipović, G. Fontaine, S. Funk, S. Gabici, Y. A. Gallant, J. F. Glicenstein, Jonas Glombitza, P. Goswami, M.-H. Grondin, Lea Heckmann, B. A. Hess, J. A. Hinton, W. Hofmann, T. L. Holch, M. Holler, M. Jamrozy, F. Jankowsky, A. Jardin-Blicq, I. Jaroschewski, D. Jimeno, I. Jung-Richardt, K. Katarzyński, D. Kerszberg, B. Khélifi, Nu. Komin, K. Kosack, D. Kostunin, Rodrigo Guedes Lang, Sanja Lazarević, A. Lemière, M. Lemoine-Goumard, J.-P. Lenain, P. Liniewicz, A. Luashvili, Jonathan Mackey, D. Malyshev, V. Marandon, M.G.F. Mayer, A. K. Mehta, A. M. W. Mitchell, R. Moderski, L. Mohrmann, Alessandro Montanari, E. Moulin, J. Niemiec, Laura Olivera-Nieto, M. O. Moghadam, Sebastian Panny, R. D. Parsons, U. Pensec, Pierre Pichard, T. Preis, G. Pühlhofer, M. Punch, A. Quirrenbach, A. Reimer, O. Reimer, Igor Adriano de Oliveira Reis, Q. Remy, H. X. Ren, Brian Reville, Frank Rieger, G. Roellinghoff
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.