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The X-ray binary SS 433, embedded in the W50 nebula (or supernova remnantW50), shows bipolar jets that are ejected with mildly relativistic velocities,and extend toward the east and west out to scales of tens of parsecs. PreviousX-ray observations revealed twin lobes along the jet precession axis thatcontain compact bright knots dominated by synchrotron radiation, which provideevidence of electron acceleration in this system. Particle acceleration in thissystem is substantiated by the recently detected gamma rays with energies up toat least 25 TeV. To further elucidate the origin of the knots and particleacceleration sites in SS 433/W50, we report here on detailed, spatiallyresolved X-ray spectroscopy of its western lobe with Chandra. We detectsynchrotron emission along the jet precession axis, as well as optically thinthermal emission that is more spatially extended. Between the two previouslyknown knots, w1 and w2, we discover another synchrotron knot, which we callw1.5. We find no significant synchrotron emission between SS 433 and theinnermost X-ray knot (w1), suggesting that electrons only begin to beaccelerated at w1. The X-ray spectra become gradually steeper from w1 to w2,and then rapidly so immediately outside of w2. Comparing with a model takinginto account electron transport and cooling along the jet, this resultindicates that the magnetic field in w2 is substantially enhanced, which alsoexplains its brightness. We discuss possible origins of the enhanced magneticfield of w2 as well as scenarios to explain the other two knots.

Spatially resolved study of the SS 433/W50 west region with Chandra: X-ray structure and spectral variation of non-thermal emission