The complex X-ray spectra of M82 and NGC 253

We present the results of the first imaging x-ray observations in the 0.4-10.0 keV bandpass of the nearby starburst galaxies M82 and NGC 253. The ASCA spectra of both M82 and NGC 253 are complex with strong line emission from O, Ne, Fe, Mg, S, and Si, allowing elemental abundances to be estimated in the x-ray band for the first time in these sources. Two components are required to fit the spectra of both galaxies, with a ''soft'' component well described by a thermal model with a temperature of similar to 10(6-7) K and a ''hard'' component well described by either a thermal model (T-hard similar to 10(8) K) or a power-law model (Gamma similar to 1.8-2.0). We find that different models (with different continua) yield absolute abundances that differ by more than an order of magnitude, while relative abundances are more robust and suggest an underabundance of Fe (inferred from the Fe-L complex) relative to alpha-burning elements. Dust depletion may be responsible for the low relative abundance of Fe inferred from the soft-component fits. We set the most reliable limits to date on the Fe-K emission line equivalent width (EW), with an upper limit in NGC 253 of EW<180 eV while a line at similar to 6.6 keV is marginally significant (at the 3 sigma level) with EW similar to 100 eV in M82. The low Fe-K line emission EW limits suggest either significantly sub-solar abundances in the material producing the hard component (if thermal) or that there is a significant non-thermal or non-equilibrium contribution to the hard component. Most of the soft flux (which originates mostly within the central kpc of M82 and NGC 253) is consistent with starburst models of supernovae-heated ISM and, to a lesser extent, starburst-driven superwind emission and the direct emission from supernova (SN). The hard component in both galaxies may have some contribution from similar to 10(8) K superwind emission or individual SN, although most of the emission probably originates in point sources (most likely blackhole candidates or mini-AGN with M(Edd)> 3 - 20M.) and, possibly, inverse-Compton scattering of IR photons. The similarity of the spectral x-ray characteristics of NGC 253 and M82 to some LINERs and low-luminosity AGN suggests a link between AGN and starbursts (e.g., both may contain an accretion-driven emission component). (C) 1997 American Astronomical Society.