  Supernova remnants may exhibit both thermal and nonthermal X-ray emission.
Such remnants can be distinguished by the weakness of their X-ray lines,
because of the presence of a strong nonthermal X-ray continuum. RCW 86 is a
remnant with weak lines, resulting in low and peculiar abundances when thermal
models alone are used to interpret its X-ray spectrum. This indicates the
presence of a strong nonthermal synchrotron continuum. We analyze ASCA X-ray
spectra of RCW 86 with the help of both nonequilibrium ionization thermal
models and nonthermal synchrotron models. A two-temperature thermal model and a
simple nonthermal model with an exponential cutoff (plus interstellar
absorption) give reasonable results. We obtain blast wave velocity of 800 km/s,
the shock ionization age of 1-3x10^11 s/cm^3, and the break in nonthermal
spectra at 2-4x10^16 Hz. The strength of nonthermal continuum correlates well
with the radio brightness in the bright SW section of the remnant. This is
convincing evidence for X-ray synchrotron emission in RCW 86.
