Particle acceleration and non-thermal broadband spectrum of extended shocked outflows in black hole binary V4641 Sgr - a galactic pevatron

Abstract

Stellar mass black holes accreting at a super-Eddington rate produce strong outflows and non-thermal radiation up to PeV energy. Recently, very high energy (VHE) emission was discovered from the environment of the galactic microquasar V4641 Sgr, which is known to experience super-Eddington outbursts. We present MHD simulations supplemented with a nonlinear kinetic Monte Carlo model that reproduces the observed radio, X-ray, and gamma-ray emission from the region over tens of parsecs surrounding V4641 Sgr. The reported multi-wavelength non-thermal emission can be explained as produced by PeV electrons accelerated by a system of strong shock waves created by the interaction between a narrow trans-relativistic polar jet and a quasi-spherical accretion-disk wind. The shock positions are associated with bright spots of the gamma-ray and X-ray radiation, whose spatial profiles can be tested with dedicated observations. The model predicts that a sizable fraction of the jet's kinetic energy is channeled into TeV-PeV protons, suggesting that such systems might be powerful sources of PeV-range cosmic rays during the accelerator activity cycle. In this model, the bow-tie shape structure discovered by MeerKAT in V4641 Sgr is associated with the radio emission from the termination surface of the quasi-spherical accretion-disk wind. The newly presented (from the SRG/eROSITA all-sky survey) and archival (INTEGRAL and Swift BAT) X-ray data are quantitatively consistent with the model, and simultaneously reproduce VHE measurements with H.E.S.S., HAWC, and LHAASO. Deeper X-ray observations will provide a test of the intermittent operation of the accelerator, in contrast with more persistent sources such as SS433/W50.