Soft X-ray properties of Seyfert galaxies in the ROSAT All-Sky Survey

We present the results of ROSAT All-Sky Survey observations of Seyfert and IR luminous galaxies from the extended 12 mu m galaxy sample and the optically selected CfA sample. Detections are available for 80% (44/55) of the Seyfert 1's and 34% (23/67) of the Seyfert 2's in the 12 pm sample, and for 76% (26/34) of the Seyfert 1's and 38% (6/16) of the Seyfert 2's in the CfA sample. Roughly half of the Seyfert galaxies (mostly Seyfert l's) have been fitted to an absorbed power-law model, yielding an average photon index of Gamma = 2.26 +/- 0.11 for 43 Seyfert 1's and Gamma = 2.45 +/- 0.18 for 10 Seyfert 2's, with both types having a median value of 2.3. The soft X-ray luminosity correlates with the 12 mu m luminosity, with Seyfert 1's having relatively more soft X-ray emission than Seyfert 2's of similar mid-IR luminosities by a factor of 1.6 +/- 0.3. Several physical interpretations of these results are discussed, including the standard unified model for Seyfert galaxies. Infrared luminous non-Seyferts are shown to have similar distributions of soft X-ray luminosity and X-ray-to-IR slope as Seyfert 2's, suggesting that some of them may barber obscured active nuclei (as has already been shown to be true for several objects) and/or that the soft X-rays from some Seyferts 2's may be nonnuclear. A soft X-ray luminosity function (XLF) is calculated for the 12 pm sample, which is described well by a single power law with a slope of -1.75. The normalization of this XLF agrees well with that of a hard X-ray selected sample. Several of our results, related to the XLF and the X-ray-to-IR relation, are shown to be consistent with the hard X-ray observations of the 12 mu m sample by Barcons et al.