  We report here X-ray imaging spectroscopy observations of the northeastern
shell of the supernova remnant RCW 86 with Chandra and XMM-Newton. Along this
part of the shell the dominant X-ray radiation mechanism changes from thermal
to synchrotron emission. We argue that both the presence of X-ray synchrotron
radiation and the width of the synchrotron emitting region suggest a locally
higher shock velocity of V_s = 2700 km/s and a magnetic field of B = 24+/-5
microGauss. Moreover, we also show that a simple power law cosmic ray electron
spectrum with an exponential cut-off cannot explain the broad band synchrotron
emission. Instead a concave electron spectrum is needed, as predicted by
non-linear shock acceleration models. Finally, we show that the derived shock
velocity strengthens the case that RCW 86 is the remnant of SN 185.
