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For refcode 1994ApJ...429...94F:
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1994ApJ...429...94F ROSAT PSPC OBSERVATIONS OF TWO X-RAY-FAINT EARLY-TYPE GALAXIES : NGC 4365 AND NGC 4382 G. FABBIANO Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 D.-W. KIM Chungnam National University, Daejeon 305-764, South Korea, and Harvard- Smithsonian Center for Astrophysics AND G. TRINCHIERI Osservatorio Astrofisico di Brera, Via Brera 28, Milano 20121, Italy, and Harvard-Smithsonian Center for Astrophysics Received 1993 September 3; accepted 1993 December 23 ABSTRACT We present the results of ROSAT PSPC observations of the two early- type galaxies NGC 4365 and NGC 4382. These galaxies are among those observed with Einstein to have the lowest X-ray to optical flux ratios of early-type galaxies. The PSPC data show that for radii r > 50" the radial distributions of the X-ray surface brightness are consistent with the optical distributions of King (1978). We also find that these galaxies have X-ray spectra significantly different from those observed in X-ray- bright ellipticals, with a relative excess of counts detected in the softest spectral channels. This confirms earlier Einstein results. The characteristics of the ROSAT PSPC do not allow us to discriminate between possible spectral models. If we adopt a two-component thermal model on the grounds of physical plausibility, we find that the spectral data can be fitted with a very soft optically thin component, with kT ~ 0.2 kev, and a hard component with kT > (1.0-1.5) keV. The hard component has a luminosity consistent with that expected from the integrated emission of a population of low-mass X-ray binaries in these galaxies; the nature of the very soft component is more speculative. Candidates include the coronal emission of late-type stars, supersoft X-ray sources, RS CVn, and perhaps a hot ISM. Alternatively, the spectral data may be fitted with a 0.6-1 keV bremsstrahlung spectrum (exponential plus Gaunt), and may suggest the presence of a totally new population of X-ray sources. Subject headings: galaxies: elliptical and lenticular, cD - galaxies: individual (NGC 4365, NGC 4382) - radiation mechanisms: nonthermal - radiation mechanisms: thermal - X-rays: galaxies
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