Wide-band X-ray spectra and images of the starburst galaxy M82

The ASCA results of the starburst galaxy M82 are presented. The X-rays in the 0.5-10 keV band exhibit a thin thermal spectrum with emission lines from highly ionized magnesium, silicon, and sulfur, as well as a hard tail extending to higher than 10 keV energy. The soft X-rays are spatially extended, while the hard X-rays show an unresolved point-like structure with possible a long-term flux variability. The flux ratio of the emission lines and the spatially extended structure in the low-energy band indicate that at least two-temperature thin thermal plasmas are present. The abundances of the oxygen, neon, magnesium, silicon, sulfur, and iron in the thin thermal plasmas are found to be significantly lower than the cosmic value. Neither type-Ia nor type-II supernova explosions can reproduce the observed abundance ratio. The origin of the unresolved hard X-rays is uncertain, but is probably an obscured low-luminosity AGN.