The Soft–x–ray Spectral Shape of X–ray–weak Seyferts

We present and analyze ROSAT–PSPC observations of eight Seyfert 2 galaxies, two Seyfert 1/QSOs, and one IR–luminous non–Seyfert. These targets were selected from the Extended 12 μm Galaxy Sample and, therefore, have different multiwavelength properties from most (optically or X–ray selected) Seyferts previously observed in the soft X–rays. The targets were also selected as having atypical X–ray fluxes among their respective classes, e.g. relatively X–ray strong Seyfert 2s and X–ray weak Seyfert 1/QSOs. Comparing our observations with those from the ROSAT All–Sky Survey, we find variability (of a factor of 1.5—2 in flux) in both of the Seyfert 1/QSOs, but in none of the Seyfert 2s. Both variable objects have steeper photon indices in the more luminous state, with the softest (<1.0 keV) flux varying the most. The timescales indicate that the variable component arises from a region less than a parsec in size. Fitting the spectra to an absorbed power–law model, we find that both the Seyfert 2s and the Seyfert 1/QSOs are best fit with a photon index of 3.1—3.2. This is in agreement with the average photon index of a sample of Markarian Seyfert 2s observed by Turner, Urry, & Mushotzky (1993), indicating that most Seyfert 2s, even those displaying a wide variety multiwavelength of characteristics, as well as some Seyfert 1/QSOs, have a photon index much steeper than the canonical (Seyfert 1) value of ∼1.7. One possible explanation is that these objects have a flatter continuum plus a soft (< 1.0 keV) excess in the form of high–EW iron and/or oxygen fluorescence lines, a black–body or even a thermal plasma. Alternatively, the underlying continuum may indeed be steep, powered by a different physical mechanism than that which produces the flat continua in other Seyfert 1s/QSOs. We imaged one Seyfert 2 (NGC 5005) with the ROSAT HRI, finding about 13% of the soft X–rays to come from an extended source. This object also has the most evidence from spectral fitting for an extra contribution to the soft–X–ray flux in addition to a power–law component, indicating that different components to the soft X–ray spectrum of this object (and likely of other X–ray–weak Seyferts) may come from spatially distinct regions. Subject headings: Galaxies: Active — Galaxies: Nuclei — Galaxies: Seyfert — X–Rays: Galaxies Accepted for publication in the 10 January 1996 issue of ApJ.