2006ApJ...648L..33V

We report X-ray imaging spectroscopy observations of the northeastern shell of the supernova remnant RCW 86 using 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 µG. Moreover, we also show that a simple power-law cosmic-ray electron spectrum with an exponential cutoff cannot explain the broadband synchrotron emission. Instead, a concave electron spectrum is needed, as predicted by nonlinear shock acceleration models. Finally, we show that the derived shock velocity strengthens the case that RCW 86 is the remnant of SN 185.