SOFT-X-RAY SPECTRA OF SEYFERT-2 GALAXIES

Soft X-ray spectra of three Seyfert 2 galaxies obtained with the ROSAT PSPC are presented. These spectra have the following properties: (1) For two of these objects, NGC 5506 and MCG -5-23-16, most of the flux occurs at energies greater than 1 keV and probably represents a low-en ergy extension of the source seen at higher (> 2 keV) energies. Both s ources suffer photoelectric absorption at approximately 1 keV, implyin g hydrogen column densities of several times 10(22) cm-2. The third so urce, Mrk 348, has very little flux in the ROSAT band, suggesting a ph otoelectric cutoff at energies above the ROSAT band, in agreement with Ginga observations. (2) In all three galaxies, X-rays are detected at energies below these photoelectric cutoffs with flux above that expec ted from a model in which the source has the same spectral index as at higher energies (2-10 keV) and is covered by a uniform, cold, photoel ectric absorber. This result indicates the presence of an additional c omponent at soft energies, which we refer to as a ''soft excess.'' (3) The monochromatic flux of the soft excess measured at 1 keV is approx imately 1%-2% of the directly viewed hard X-ray continuum in all three galaxies. (4) In MCG -5-23-16 and Mrk 348, the soft component is well -described by a power law with a steep spectrum (photon index, GAMMA = 4.8 and 2.4, respectively, with N approximately E(-GAMMA)) or a relat ively cool (kT < 1 keV) thermal bremsstrahlung or blackbody. The third object, NGC 5506, shows no strong evidence for spectral steepening at low energies, but additional absorption from gas in the disk of the e dge-on host galaxy may make the detection of such a component difficul t. A steeper continuum at low energies (GAMMA approximately 2.1-3.9) t han at high energies (GAMMA approximately 1.4-1.9) has also been found in some Seyfert 1 galaxies. This fact, combined with the ratio of flu x in the soft to hard continuum, argues that the soft excess may be do minated by electron-scattered nuclear radiation. The ratio of soft to hard continuum flux implies the optical depth to electron scattering i n the scattering zone is in the range 0.02-0.1 for these three galaxie s. Alternative explanations for the soft X-ray excesses, such as blend ed line emission, partial covering of the nuclear source and spatially extended X-ray emission, are, however, also viable. The PSPC data all ow an accurate measurement of the gas column density responsible for a bsorbing the hard X-ray continuum in NGC 5506 and MCG -5-23-16. In bot h cases, the PSPC value is in excellent agreement with previous measur ements. The lack of variation of these columns over the last 14 yr is most easily understood if a molecular torus, rather than a few broad-l ine region clouds, is responsible for this absorption.