We present the first in a series of papers describing the X-ray properties of a sample of 18 Seyfert 1 galaxies, using data obtained by ASCA. The imaging data reveal a number of serendipitous hard X-ray sources in some source fields, but none contribute significantly to the hard X-ray flux of the AGN. All but one of the Seyferts show evidence for variability on timescales of minutes-hours, with the amplitude anti-correlated with the source luminosity, confirming previous results. In at least 8 sources, there is evidence that the variability amplitude below 2 keV is greater than that in the hard X-ray band, perhaps indicating variable components other than the power-law in the soft band. Ultra-rapid variability, implying significant power at frequencies > 10 −3 Hz is detected in at least 5 sources, but is difficult to detect in most cases, due to the sampling and signal-to-noise ratio. In Mrk 766 and MCG-6-30-15 there is also an indication that the high-frequency power-spectra are variable in shape and/or intensity. There is similar evidence in NGC 4151, but on longer time scales.

We present the results from a detailed analysis of the 0.6 -10 keV spectra of 23 ASCA observations of 18 objects. We find that in most cases the underlying continuum can be well-represented by a powerlaw with a photon index Γ ∼ 2. However we find strong evidence for photoionized gas in the line-of-sight to 13/18 objects. We present detailed modelling of this gas based upon the ION photoionization code. Other studies have been made of the 'warm absorber' phenomenon but this paper contains the first consideration of the importance of the covering-fraction of the ionized gas and a direct comparison between models of attenuation by ionized versus neutral material.We find the X-ray ionization parameter for the ionized material is strongly peaked at U X ∼ 0.1. The column densities of ionized material are typically in the range N H,z ∼ 10 21 -10 23 cm −2 , although highly ionized (and hence psuedo-transparent) column densities up to 10 24 cm −2 cannot be excluded in some cases. We also investigate the importance of the emission-spectrum from the ionized gas, finding that it significantly improves the fit to many sources with an intensity consistent with material subtending a large solid angle at the central source. Allowing a fraction of the continuum to be observed without attenuation also improves the fit to many sources, and is definitely required in the case of NGC 4151. A deficit of counts is observed at ∼ 1 keV in the sources exhibiting the strongest absorption features. We suggest this is likely 6 H 0 = 50 km s −1 Mpc −1 and q 0 = 0.5 are used throughout 7 using sispi(v1.0) and the gain-history file released on 1994 Jul 28 in the case of the SIS data 8 where, in the case of the SIS we used the Gaussian 'fudge', but not the filter 'fudge' 10 for up-to-date information see http://heasarc.gsfc.nasa.gov/docs/asca/ascagof.html 11 See the note to Table 6 for an explanation of the apparent increase in χ 2 for the observations of Fairall 9, 3C 120 & IC 4392A. 12 The exception being Mrk 841(1), which falls outside our formal limit of P Bii ≤ 0.95 by just ∆P Bii = 0.001

We present ASCA spatial, temporal, and spectral data for a sample of 26 observations of 25 type 2 active galactic nuclei (AGNs), composed of 17 Seyfert 2 galaxies and eight narrow emission line galaxies (NELGs). Twenty-four of the 25 sources were detected.The ASCA images are generally consistent with emission from point sources at energies above D3 keV. We use archival ROSAT data to examine each Ðeld at high spatial resolution and to check for the presence of sources that would contaminate the ASCA data.Of the Ðve sources bright enough for 128 s temporal analysis, three are variable at [99% conÐdence, with characteristics consistent with those observed in Seyfert 1 galaxies. Analysis on a timescale of 5760 s reveals six sources variable at [99% conÐdence, and comparison with previous X-ray results shows most of the sample to be variable in hard X-ray Ñux on timescales of years.Simple continuum models are Ðtted to the sample spectra to characterize the variety of spectral forms and hence to determine the fundamental nature of the X-ray spectrum of each source. No single spectral model provides an adequate Ðt to all the sample sources. Thirty-six percent of the sample cannot be adequately Ðtted by any of our test models (all rejected at [95% conÐdence). Approximately half of the sample have an iron Ka line with an equivalent width consistent with an origin in the line-of-sight absorber ; the remaining lines must be produced in material out of the line of sight. Absorbing columns up to 1024 atoms cm~2 are detected, and even larger columns are inferred for some sources. The mean underlying hard X-ray power-law index is ! D 2.Many X-ray emission lines were detected at high levels of conÐdence. The iron K-shell regime is dominated by emission from "" neutral ÏÏ material. Many data sets also show evidence for complexity in the iron Ka proÐle, which may be interpreted as evidence for broad-line proÐles, including Ñux both redward and blueward of the line peak, and/or for the existence of hydrogen-like and helium-like iron K lines.Hydrogen-like and helium-like lines are detected from Fe, Ne, Si, S, and Ar in addition to Mg lines. While almost half of the sample have an estimated starburst contribution of [30% in the 0.5È4.5 keV bandpass, the soft X-ray emission lines are not solely associated with a strong starburst component.