THE COMPLEX BROAD-BAND X-RAY-SPECTRUM OF THE STARBURST GALAXY M82

The broadband X-ray spectrum of the prototypical starburst galaxg M82 is very complex. At least three spectral components are required to fi t the combined ROSAT and ASCA spectrum in the 0.1-10 keV range. The ob served X-ray flux in this band is dominated by a hard Gamma=1.7, heavi ly absorbed power-law component which originates in the nucleus and ne ar-nuclear disk of the galaxy. Among the candidates for the origin of this hard X-ray emission, the most plausible appears to be inverse-Com pton scattered emission from the interaction of the copious infrared p hoton flux of M82 with supernova-generated relativistic electrons. The measured intrinsic luminosity of the power-law component agrees close ly with calculations of the expected inverse-Compton luminosity, Moreo ver, 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 nonthermal and ape associated with a common population of electrons. The other two spectral components, thermal plasmas with characteristic temperatures kT approximate to 0.6 and 0.3 keV, are associated with t he 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, i s not absorbed, and is likely to represent the X-ray emission that ex; ends along tie minor axis of M82. The amount of absorption required in the three-component model suggests that the intrinsic luminosity of M 82 in the 0.1-10 keV band is about four times greater than its observe d luminosity of 4 x 10(40) ergs s(-1).