EVIDENCE OF X-RAY SYNCHROTRON EMISSION FROM ELECTRONS ACCELERATED TO 40 TEV IN THE SUPERNOVA REMNANT CASSIOPEIA-A

We present the 2-60 keV spectrum of the supernova remnant Cassiopeia A measured using the Proportional Counter Array and the High Energy X-R ay Timing Experiment on the Rossi X-Ray Timing Explorer satellite. In addition to the previously reported strong emission-line features prod uced by thermal plasmas, the broadband spectrum has a high-energy ''ta il'' that extends to energies at least as high as 120 keV. This tail m ay be described by a broken power law that has photon indices of Gamma (1) = 1.8(-0.6)(+0.5) and Gamma(2) = 3.04(0.13)(0.15) and a break ener gy of E-b = 15.9(-0.4)(+0.3) keV. We argue that the high-energy compon ent, which dominates the spectrum above about 10 keV, is produced by s ynchrotron radiation from electrons that have energies up to at least 40 TeV. This conclusion supports the hypothesis that Galactic cosmic r ays are accelerated predominantly in supernova remnants.