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

TL;DR

This paper develops a detailed physical model of the microquasar V4641 Sgr, explaining its ultra-high-energy gamma-ray emission up to 0.8 PeV through stochastic electron acceleration and inverse Compton scattering, aligning with recent LHAASO observations.

Contribution

It introduces a novel Monte Carlo simulation framework for stochastic particle acceleration in microquasars, successfully reproducing observed UHE gamma-ray spectra and providing insights into particle energization mechanisms.

Findings

Model reproduces PeV electron acceleration and gamma-ray spectra.

Supports leptonic scenario with turbulence-driven second-order Fermi acceleration.

Highlights the need for further observations and theoretical refinement.

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) has recently reported five Galactic microquasars as Ultra-High-Energy (UHE) $\gamma$-ray emitters (> 100 TeV). Among these sources, the microquasar V4641 Sgr exhibits $\gamma$-ray emission up to $\sim$0.8 PeV, requiring the acceleration of particles to multi-PeV energies, as well as the hardest UHE spectrum. The mechanisms behind particle acceleration to such energies are not well understood. Furthermore, the limited multi-wavelength (MWL) information on this source appears contradictory, further complicating interpretation and suggesting that V4641 Sgr may represent a particularly unusual case. In this work, we present a detailed physical model of V4641 Sgr that combines first-principles simulations of stochastic (turbulent) particle acceleration with MWL emission modeling. We adopt a leptonic scenario in which electrons are accelerated via the second-order Fermi process driven by relativistic strong turbulence ($\delta B/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 $\gamma$-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 $\gamma$-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.