A Leptonic Interpretation of the UHE Gamma-ray Emission from V4641 Sgr

TL;DR

The paper investigates whether leptonic processes can explain the UHE gamma-ray emission from V4641 Sgr, observed up to $0.8\, \mathrm{PeV}$, by invoking inverse Compton scattering from electrons accelerated along a velocity-shear jet. It develops a joint stochastic and shear acceleration framework, solves a Fokker-Planck equation to obtain the electron spectrum, and computes IC and X-ray synchrotron emission using seed photon fields, incorporating KN effects.Through MCMC fitting to LHAASO/HAWC/H.E.S.S data, the study finds that SHA can reproduce the observed spectrum with magnetized jets of order $\mu$G and spine speeds around $0.6-0.7\,c$, with an electron kinetic luminosity near $10^{37}\rm\ erg\ s^{-1}$. XRISM X-ray limits favor a jet radius $R_{\rm jet}\gtrsim 1$ pc and predict an elongated X-ray/TeV structure that can be tested with future high-resolution instruments. The work also discusses robustness to the turbulence spectrum and jet–orbit misalignment, arguing that the leptonic SHA scenario provides a viable alternative to hadronic models for the extended UHE emission.

Abstract

Recently, the microquasar V4641 Sgr and its surrounding is detected at TeV-PeV gamma-ray band. Interestingly, the spectrum follows a power-law function continuing up to 0.8 PeV as reported by LHAASO, and the morphology of the emission appears a puzzling elongated structure. In this work, we propose that the elongated UHE emission from V4641 Sgr could originate from the inverse Compton radiation of electrons with a very hard spectrum, which may result from shear acceleration mechanism in the jets driven by V4641 Sgr. We also calculate the corresponding X-ray synchrotron emission from the same electron population, predicting the potential range of non-thermal X-ray flux of the source. The recent observation by XRISM toward the central part of the UHE source could pose a constraint on the model parameters. In the future, a full coverage of the source by sensitive X-ray instrument and high-resolution TeV-PeV gamma-ray instrument may provide a critical test of the model.

Figures (5)

• Figure 1: $\eta-B_0$ planes which show the potential parameter sets under two confinements: $c\tau_{\rm sc} \le R_{\rm jet}$ and $t_{\rm acc,\rm SHA} \le t_{\rm c}$, with $\beta_0$ = 0.7, q=5/3, $R_{\rm jet}$ = 5 pc and $\gamma$ = 0.8 ${\rm PeV}/\left({m_{\rm e} c^2}\right)$. The orange dashed line indicates the limit for the 1st-order resonance and the shaded area represents the allowed parameter regions when $\xi=1$.
• Figure 2: The fitting results for the UHE spectrum of V4641 Sgr, with data points from HAWC alfaro2024ultra, LHAASO lhaaso2024ultrahigh and H.E.S.S. neronov2025multimessenger. The upper limits in 1 - 300 GeV are extracted from zhao2025upper. The blue solid lines indicate the spectral energy distributions (SEDs) of total IC emission from three photon fields and the lightblue regions are composed of possible fitting results in 1$\sigma$ range of further $\chi^2$ tests.
• Figure 3: Timescales for different processes with the best-fit parameters in \ref{['subsec:UHE']}, derived from equation (\ref{['e12']}), (\ref{['e8']}), (\ref{['e17']}), (\ref{['e20']}) and (\ref{['e21']}). The red solid line represents the timescale for stochastic acceleration (STA) and the purple one represents the one for shear acceleration (SHA); The orange dotted line shows the timescale for radiative cooling and the blue dashed line is the scattering timescale for electrons in the jet; The dash-dotted lines indicate the extra limitations caused by the Hillas criterion ($\gamma_{\rm Hillas}$) and the particle mean free path (MFP) ($\gamma_{\rm MFP}$).
• Figure 4: The synchrotron emission produced by the same electron population responsible for the UHE gamma-ray emission. The red solid lines represent the emission under the best-fit parameters and the reddish regions are composed of predicted emission with possible spectral energy distributions (SEDs) in 1$\sigma$ range of $\chi^2$ tests, which is coherent with the ones shown in Figure \ref{['fig:IC']}. The black solid lines represent the total integrated flux inferred from the surface brightness obtained by XRISM suzuki2025detection from 2 to 10 keV.
• Figure 5: The fitting results for the UHE spectrum and predicted X-ray synchrotron flux, with different types of turbulence. The left-side panels show results from the Kraichnan-type ($q$ = $\rm 3/2$) while the right-side panels show that from the hard-sphere limit ($q$ = $\rm 2$). Both of the two cases can explain the observations, with only minor impacts on crucial parameters.