Modeling Particle Acceleration and MWL Emission of a PeVatron Microquasar V4641 Sgr

TL;DR

This work addresses how V4641 Sgr achieves PeV particle acceleration and UHE gamma-ray emission. It develops a leptonic model where electrons are energized by second-order Fermi acceleration in strong relativistic turbulence within extended gamma-ray bubbles, simulated with the STRIPE Monte Carlo framework. The model computes synchrotron and IC emission, including KN effects and gamma-gamma attenuation, and compares to HESS/HAWC/LHAASO and X-ray data to infer best-fit turbulence parameters. The findings show electrons can reach tens of PeV and reproduce the hard TeV–PeV spectrum, but reveal tensions with the full MWL picture and bubble extent, signaling the need for deeper MWL observations and more comprehensive parameter studies.

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) has recently reported five Galactic microquasars as Ultra-High-Energy (UHE) $γ$-ray emitters (> 100 TeV). Among these sources, the microquasar V4641 Sgr exhibits $γ$-ray emission up to $\sim$0.8 PeV, requiring the acceleration of particles to multi-PeV energies, as well as the hardest UHE spectrum. The mechanisms behind particle acceleration to such energies are not well understood. Furthermore, the limited multi-wavelength (MWL) information on this source appears contradictory, further complicating interpretation and suggesting that V4641 Sgr may represent a particularly unusual case. In this work, we present a detailed physical model of V4641 Sgr that combines first-principles simulations of stochastic (turbulent) particle acceleration with MWL emission modeling. We adopt a leptonic scenario in which electrons are accelerated via the second-order Fermi process driven by relativistic strong turbulence ($δB/B \sim 1$). The particle energization is simulated using a dedicated Monte Carlo framework STRIPE that incorporates the effects of intermittent energy gains and radiative losses. The resulting accelerated electrons produce UHE $γ$-rays through inverse Compton scattering on both the cosmic microwave background (CMB) and the interstellar radiation fields (ISRF). Our model is capable of reproducing key observational characteristics of the system, including particle acceleration to energies of tens of PeV, as well as the TeV-PeV $γ$-ray spectrum and the hard spectral index measured by LHAASO. Nonetheless, several aspects remain unresolved, highlighting the need for deeper observational coverage and further theoretical refinement.

Figures (1)

• Figure 1: Top: simulated electron spectra resulting from strong turbulent re-acceleration for different $t_{\rm esc}$. The successful SED fit is achieved using the electron spectrum with $t_{\rm esc} = 6 \ l_c/c$. Bottom: composite SED fit of TeV-PeV $\gamma$-ray and X-ray data of V4641 Sgr. The H.E.S.S., HAWC, LHAASO and Fermi-LAT data is taken from neronov2025, X-ray flux measurement from xrism, and the X-ray flux upper limit is derived based on wan2025.