PeV particle acceleration and nonthermal emission in the minimalist model of the extended jets in W50/SS433

The W50 nebula around microquasar SS 433, powered by supercritical accretion, features two ``extended jets'' (tens of pc long and a few pc wide) from which polarized x-ray and very high-energy radiation above 100 TeV is detected. Here we present a model of very high-energy particle acceleration in these extended jets. In the ``minimalist'' model (discussed in Churazov, Khabibullin, and Bykov, 2024), a collimated outflow aligned with the rotation axis is propagating through a more isotropic wind produced by the accretion disk. The observed extended x-ray jets in this model are associated with the formation of strong recollimation magnetohydrodynamics shocks after the collision of the collimated outflow with the isotropic wind termination surface. The spectra of electrons and protons up to PeV energies are simulated with a nonlinear Monte Carlo model of diffusive shock acceleration with turbulent magnetic field amplification. The overall efficiency of the jets power transfer to accelerated protons in this model is above 10% and about 0.5% for electrons above 50 TeV. The magnetic field amplification by Bell's instability due to the electric current of cosmic rays escaping the accelerator produces highly anisotropic magnetic turbulence in the shock downstream. This results in the polarized synchrotron x-ray emission with the photon electric vector predominantly transverse to the jet direction and the degree of polarization above 20%. The model is able to reproduce the observed spectra and intensity profiles of nonthermal x-ray and gamma-ray emission, which are both dominated by the leptonic radiation.