J/A+A/706/A8 Microquasar V4641 Sgr image (Acharyya+, 2026)
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.,
Bernloehr K., Boettcher 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., Delgado Giler A.G., Devin J., Djannati-Atai A.,
Dmytriiev A., Egberts K., Egg K., Ernenwein J.-P., Escanuela 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., Katarzynski K., Kerszberg D., Khelifi B., Komin N.,
Kosack K., Kostunin D., Lang R.G., Lazarevic S., Lemiere 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., Puehlhofer 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., Schuessler F., Shapopi J.N.S., Si Said W., Sol H.,
Stawarz L., 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.,
H.E.S.S.Collaboration, Takekawa S.
<Astron. Astrophys. 706, A8 (2026)>
=2026A&A...706A...8A 2026A&A...706A...8A (SIMBAD/NED BibCode)
ADC_Keywords: QSOs ; Gamma rays
Keywords: acceleration of particles - radiation mechanisms: non-thermal -
binaries: general - stars: jets - gamma rays: stars
Abstract:
Microquasars have emerged as promising candidates to explain the
cosmic-ray flux at PeV energies. LHAASO observations reveal V4641 Sgr
as the most extreme example so far, with a gamma-ray spectrum
extending up to 800∼TeV, requiring multi-PeV energy particles. The TeV
emission is highly extended, challenging expectations given the
reported low-inclination angle of the V4641 Sgr jets. We aim to
spatially and spectrally resolve the gamma-ray emission from V4641 Sgr
and investigate particle acceleration in the system. Using ∼100h of
H.E.S.S. data we perform a spectro-morphological study of the
gamma-ray emission around V4641 Sgr. We employ HI and dedicated CO
observations of the region to infer the presence of target material
for cosmic-ray interactions. Multi-TeV emission around V4641 Sgr is
detected with high significance. The emission region is elongated with
major and minor axes: 0.34°±0.04stat±0.01syst and
0.06°±0.01stat±0.01syst, respectively. A power-law
spectrum with index ∼1.8 is found and, together with results by other
gamma-ray instruments, reveals for the first time a spectral energy
distribution (SED) peaking at energies of ∼100TeV. We find indications
(3σ) of a two-component morphology, with indistinguishable
spectral properties. The position of V4641 Sgr is inconsistent with
both the best-fit position of the single-component model and the dip
between the two components. We find no significant evidence of
energy-dependent morphology. No dense gas is found at any distance
towards V4641 Sgr, placing an upper limit of ngas≲0.2cm-3 within
the gamma-ray emission region. The peak of the SED at ∼100TeV
identifies V4641 Sgr as a candidate cosmic-ray accelerator beyond the
Knee. However, the absence of dense target gas places stringent
energetic constraints on hadronic interpretations. The H.E.S.S.
measurement requires an unusually hard (∼1.5) spectral index for
protons. A leptonic scenario faces fewer obstacles, provided particle
transport is fast enough to avoid losses and reproduce the observed
energy-independent morphology, though the absence of bright X-ray
emission across the gamma-ray emission region requires a magnetic
field strength ≲3muG. Our findings favour a leptonic
origin of the gamma-ray emission. This conclusion does not exclude
hadron acceleration in the V4641 Sgr system.
Description:
FITS file with the sky map corresponding to Figure 1 in the paper.
Objects:
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RA (2000) DE Designation(s)
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18 19 21.63 -25 24 25.85 V4641 Sgr = XTE J1819-254
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 69 1 Information of fits map
fits/* . 1 Individual fits map
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1 A1 --- --- [G]
2- 10 F9.5 deg GLON Galactic longitude of center (J2000)
11- 19 F9.5 deg GLAT Galactic latitude of center (J2000)
21- 23 I3 --- Nx Number of pixels along X-axis
25- 27 I3 --- Ny Number of pixels along Y-axis
29- 31 I3 Kibyte size Size of FITS file
33- 52 A20 --- FileName Name of FITS file, in subdirectory fits
54- 69 A16 --- Title Title of the FITS file
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Acknowledgements:
Laura Olivera-Nieto, laura.olivera-nieto(at)mpi-hd.mpg.de
H.E.S.S. Collaboration, contact.hess(at)hess-experiment.eu
(End) Laura Olivera-Nieto [Univ. Amsterdam], Patricia Vannier [CDS] 14-Nov-2025