The Complex Broadband X‐Ray Spectrum of the Starburst Galaxy M82

The broadband X-ray spectrum of the prototypical starburst galaxy M82 is verycomplex. At least three spectral components are required to fit the combinedROSAT and ASCA spectrum in the 0.1--10 keV range. The observed X-ray flux inthis band is dominated by a hard Gamma = 1.7, heavily absorbed power lawcomponent 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 mostplausible appears to be inverse-Compton scattered emission from the interactionof M82's copious infrared photon flux with supernova-generated relativisticelectrons. The measured intrinsic luminosity of the power law component agreesclosely with calculations of the expected inverse-Compton luminosity. Moreover,the radio and X-ray emission in the nucleus of M82 have the same spectralslope, which should be the case if both types of emission are non- thermal andare associated with a common population of electrons. The other two spectralcomponents, thermal plasmas with characteristic temperatures kT = 0.6 and 0.3keV, are associated with the star formation and starburst-driven wind in M82.The warmer thermal component is heavily absorbed as well and must alsooriginate in the central region of the galaxy. The softer thermal component,however, is not absorbed, and is likely to represent the X-ray emission whichextends along M82's minor axis. The amount of absorption required in thethree-component model suggests that the intrinsic luminosity of M82 in the0.1--10 keV band is about four times greater than its observed luminosity of 4x 10(40) ergs/s.Comment: 23 pages, TeX, seven postscript figures, three tables Accepted for the March 20, 1997 issue of Ap