  The broadband X-ray spectrum of the prototypical starburst galaxy M82 is very
complex. At least three spectral components are required to fit the combined
ROSAT and ASCA spectrum in the 0.1--10 keV range. The observed X-ray flux in
this band is dominated by a hard Gamma = 1.7, heavily absorbed power law
component which originates in the nucleus and near-nuclear disk of the galaxy.
Among the candidates for the origin of this hard X-ray emission, the most
plausible appears to be inverse-Compton scattered emission from the interaction
of M82's copious infrared photon flux with supernova-generated relativistic
electrons. The measured intrinsic luminosity of the power law component agrees
closely with calculations of the expected inverse-Compton luminosity. Moreover,
the radio and X-ray emission in the nucleus of M82 have the same spectral
slope, which should be the case if both types of emission are non- thermal and
are associated with a common population of electrons. The other two spectral
components, thermal plasmas with characteristic temperatures kT = 0.6 and 0.3
keV, are associated with the star formation and starburst-driven wind in M82.
The warmer thermal component is heavily absorbed as well and must also
originate in the central region of the galaxy. The softer thermal component,
however, is not absorbed, and is likely to represent the X-ray emission which
extends along M82's minor axis. The amount of absorption required in the
three-component model suggests that the intrinsic luminosity of M82 in the
0.1--10 keV band is about four times greater than its observed luminosity of 4
x 10(40) ergs/s.
