ROSAT POSITION-SENSITIVE PROPORTIONAL COUNTER SPECTRA OF 6 SEYFERT-1 GALAXIES

We present the results from ROSAT Position Sensitive Proportional Counter (PSPC) observations of six Seyfert 1 galaxies in the soft (0.1-2.0 keV) X-ray band. The sources (Mrk 335, ESO 198-G24, ESO 141-G55, Mrk 509, NGC 7469, and MCG-2-58-22) were chosen to have low absorbing column densities (N(H) approximately 10(20) cm-2) along the line of sight. As expected, it is found that all the sources possess significantly steeper spectra below approximately 1 keV than observed at higher X-ray energies. Assuming a simple absorbed power-law spectral model, the mean (photon) spectral index for the sample is GAMMA = 2.38 +/- 0.25, compared to the canonical GAMMA approximately 1.7 typically observed in the 2-10 keV band. Furthermore, we find strong evidence for soft X-ray spectral features in half the sources. In NGC 7469 and ESO 198-G24, we find that the addition of a narrow emission line or an absorption edge to the underlying continuum is a significant improvement to the parameterization of the spectra. Mrk 335 also shows evidence for spectral complexity, but from these data it is not possible to unambiguously distinguish between an absorption edge and a steepening of the spectrum at low energies. We examine these results in the light of the accuracy of the PSPC spectral calibration. We discuss the implications of these results on physical models for the X-ray emission of Seyfert 1 galaxies as a whole. In particular, we suggest that Seyfert 1 galaxies may make a major contribution to the soft X-ray background. Since these sources make only a minor contribution to the hard X-ray (2-40 keV) background, these data therefore provide evidence that the sources which dominate the background differ as a function of energy.