ASCA Observations of Type 2 Seyfert Galaxies. I. Data Analysis Results

Abstract

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 domi-nated 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.