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SS 433 is a binary system containing a supergiant star that is overflowingits Roche lobe with matter accreting onto a compact object (either a black holeor neutron star). Two jets of ionized matter with a bulk velocity of$\sim0.26c$ extend from the binary, perpendicular to the line of sight, andterminate inside W50, a supernova remnant that is being distorted by the jets.SS 433 differs from other microquasars in that the accretion is believed to besuper-Eddington, and the luminosity of the system is $\sim10^{40}$ ergs$^{-1}$. The lobes of W50 in which the jets terminate, about 40 pc from thecentral source, are expected to accelerate charged particles, and indeed radioand X-ray emission consistent with electron synchrotron emission in a magneticfield have been observed. At higher energies (>100 GeV), the particle fluxes of$\gamma$ rays from X-ray hotspots around SS 433 have been reported as fluxupper limits. In this energy regime, it has been unclear whether the emissionis dominated by electrons that are interacting with photons from the cosmicmicrowave background through inverse-Compton scattering or by protonsinteracting with the ambient gas. Here we report TeV $\gamma$-ray observationsof the SS 433/W50 system where the lobes are spatially resolved. The TeVemission is localized to structures in the lobes, far from the center of thesystem where the jets are formed. We have measured photon energies of at least25 TeV, and these are certainly not Doppler boosted, because of the viewinggeometry. We conclude that the emission from radio to TeV energies isconsistent with a single population of electrons with energies extending to atleast hundreds of TeV in a magnetic field of $\sim16$~micro-Gauss.

Very high energy particle acceleration powered by the jets of the microquasar SS 433