X-RAY OBSERVATIONS OF NGC-253 AND M83 WITH GINGA

X-ray spectra of two starburst galaxies NGC 253 and M83 in 2-20 keV have been obtained with the X-ray astronomy satellite Ginga. The observed spectra can be described by thermal bremsstrahlung models with temperatures of 6-7 keV for both galaxies, with no significant absorption. The estimated luminosities in 2-10 keV are ∼1 × 1040 ergs s-1 and ∼8 × 1039 ergs s-1 for NGC 253 and M83, respectively, consistent with IPC measurements in a lower energy band. No significant iron K-emission line was detected from either galaxy with 90% upper limit on the equivalent width ∼400 eV, suggesting a significant depletion of iron if most of the continuum emission is of thermal origin. The spectral shape and the enhanced X-ray luminosity of these two galaxies, compared with M31 or the Galaxy, suggest that a major fraction of the X-ray emission may originate from hot gas. However, the lack of an iron emission line remains a puzzle.