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-energy extension of the source seen at higher (>2 keV) energies. Both sources suffer photoelectric absorption at ~1 keV, implying hydrogen column densities of several times 10^22^ cm^-2^. The third source, Mrk 348, has very little flux in the ROSAT band, suggesting a photoelectric 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 expected 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, photoelectric absorber. This result indicates the presence of an additional component at soft energies, which we refer to as a "soft excess." (3) The monochromatic flux of the soft excess measured at 1 keV is ~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~ E^-{GAMMA}^) or a relatively 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 edge-on host galaxy may make the detection of such a component difficult. A steeper continuum at low energies ({GAMMA} ~ 2.1-3.9) than at high energies ({GAMMA} ~ 1.4-1.9) has also been found in some Seyfert 1 galaxies. This fact, combined with the ratio of flux in the soft to hard continuum, argues that the soft excess may be dominated by electron- scattered nuclear radiation. The ratio of soft to hard continuum flux implies the optical depth to electron scattering in the scattering zone is in the range 0.02-0.1 for these three galaxies. Alternative explanations for the soft X-ray excesses, such as blended line emission, partial covering of the nuclear source and spatially extended X-ray emission, are, however, also viable. The PSPC data allow an accurate measurement of the gas column density responsible for absorbing the hard X-ray continuum in NGC 5506 and MCG - 5-23-16. In both cases, the PSPC value is in excellent agreement with previous measurements. 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-line region clouds, is responsible for this absorption.