Leptonic and Hadronic Models of High-energy Nebula Around V4641 Sgr

TL;DR

This work presents a coherent treatment of the extended high-energy nebula around V4641 Sgr by evaluating purely leptonic, purely hadronic, and leptohadronic scenarios. By combining spectral fits with morphology constrained by propagation along the Galactic magnetic field and by incorporating local gas densities from HI data, the authors show that pure leptonic models struggle to explain the observed extended emission without fine-tuned transport, while purely hadronic models require an enormous proton reservoir and clash with XRISM X-ray measurements. Leptohadronic models—either a hadronic flash with a continuous leptonic component or a hadronic flash coupled to sustained leptonic emission—best reproduce both the spectrum and the morphology, and they make testable predictions for X-ray and neutrino signals. Discriminating between these scenarios will rely on future multi-messenger and high-resolution X-ray observations, as well as next-generation neutrino detectors, with X-ray measurements at the nebula tips and neutrino flux constraints offering the most straightforward handles on the relative contributions of leptonic and hadronic processes.

Abstract

A prominent, 200-pc-scale high-energy nebula surrounding the microquasar V4641~Sgr is the brightest known gamma-ray source in the Southern sky at $E > 100\,\mathrm{TeV}$. In this paper, we develop self-consistent leptonic, hadronic, and leptohadronic models that reproduce both the observed spectrum and morphology of the source. Purely leptonic models are energetically more favorable yet they require rather specific morphological assumptions. The gamma-ray morphology of the source can be better explained within a hadronic scenario based on the identification of cold gas structures spatially correlated with the observed gamma-ray emission. However, a purely hadronic model for the source emission requires a substantial energy reservoir in protons and fails to reproduce the extended X-ray emission recently detected by XRISM. We show that emission including a combination of leptonic and hadronic components can reproduce both the spectral and morphological properties of the source. We provide predictions for the X-ray and neutrino spectra of~the~nebula that can discriminate the hadronic and leptonic contributions to the overall source signal.

Figures (12)

• Figure 1: Position of V4641 Sgr in the Milky Way relative to the Galactic bar, which consists of the Galactic Bulge (rich green) and the Long bar (pale green) BlandHawthorn2016. The upper panel shows the top view in Galactic coordinates, and the lower panel presents the side view. Velocity vectors are shown for the Sun and for V4641 Sgr: the black vectors correspond to velocities relative to the Galactic center, the gray vector shows the motion of V4641 Sgr relative to the Sun GaiaDR3Lindstrom2005, and the red vector represents the binary’s velocity in the rotating frame of the bar. The approximate positions of the spiral arms are indicated in pale gray Drimmel2001.
• Figure 2: Combined image of H.E.S.S. significance contours HESS-2024 (cyan), HAWC contours (magenta) and HAWC $>100\mathrm{~TeV}$ TS map (blue-green) HAWC2024, and LHAASO TS map (blue-red-yellow) LHAASO2024. The circles on the bottom left correspond to the point-spread-function (PSF) size of the observations HESS-AA2025HAWC_PSF_Crab. The XRISM X-ray measurement region boundaries are shown in white Suzuki2025.
• Figure 3: H.E.S.S. total flux above $0.8\mathrm{~TeV}$ measurement along the major axis of the source HESS-AA2025 fitted by flash-like (solid red) and continuous profiles (dashed green).
• Figure 4: A continuous leptonic emission model fitting the deabsorbed gamma-ray and X-ray emission measurements. Left panel: the TS map of the source as observed by H.E.S.S. HESS-AA2025, HAWC HAWC2024, and LHAASO LHAASO2024 in equatorial frame with the XRISM detection region Suzuki2025 (red) and two proposed measurement regions (green and purple). Right panel: photon spectrum, produced by continuous leptonic emission at the source for $B=3\mathrm{~\mu G}$, $l_\parallel = 110\mathrm{~pc},$ and $D_\parallel/D_\perp = 13$. Total spectrum (IC and Synchrotron emission) is shown in blue, the red synchrotron curve represents the flux from the electrons inside the XRISM region, the green and the purple curves correspond to the expected synchrotron flux at the edges of the nebula. The color of the dashed curves correspond to the color of the regions in the left panel.
• Figure 5: Two leptonic scenarios explaining asymmetry of the nebula. The $x$-axis coincides with the line-of-sight; $z$-axis is orthogonal and corresponds to the projection of the nebula major axis onto the celestial sphere. In all plates, the source is marked as a blue star in the coordinates origin. Left column: top and side views of the nebula in the "bent GMF line" case, the curvature radius is $200\mathrm{~pc}$, the curvature center is marked with a white cross. Right column: top and side view of the nebula in the "ballistic-diffusive transition" scenario. In the top plate, the projection angle is $45^\circ$, the particles coming towards the observer are presented in yellow-red colormap, the ones propagating from the observer are marked in gray. In the lower plate, only the particles coming towards the observer are shown.