A Comprehensive Spectral and Variability Study of Narrow‐Line Seyfert 1 Galaxies Observed by ASCA . II. Spectral Analysis and Correlations

Abstract

I present a comprehensive and uniform analysis of 25 ASCA observations from 23 narrow-line Seyfert 1 galaxies (NLS1s). The spectral analysis and correlations are presented in this paper; the reduction and time series analysis are presented in the companion paper. A maximum likelihood analysis confirms that the hard X-ray photon index is significantly steeper at more than 90% confidence in this sample of NLS1s compared with a random sample of Seyfert galaxies with broad optical lines. Soft excess emission was detected in 17 of the 19 objects that had no significant absorption, a result that demonstrates that soft excesses appear considerably more frequently in NLS1s than in Seyfert 1 galaxies with broad optical lines. The strength of the soft excess, parameterized using a flux ratio obtained from the blackbody plus power-law model, has a wide range of values in these objects (a factor of 50). The photon index was found to be correlated with the Hβ FWHM, despite the small range of the latter parameter; however, neither parameter is correlated with the strength of the soft excess or ROSAT slope. Therefore, assuming that an excess of soft photons causes the steep photon index and narrow Hβ FWHM, that excess may lie primarily in the unobservable EUV. The strength of the soft excess is correlated with the variability parameters, so that objects with strong soft excesses show higher amplitude variability; this potentially important result is not easily explained. While a range of 2 orders of magnitude in luminosity is represented, the temperature of the soft excess is approximately consistent throughout the sample, in contrast with expectations of simple accretion disk models. The presence of ionized absorption was sought using a two-edge model. It was found that this component appears to be typically less common in NLS1s and some evidence was found that the typical ionization state is lower compared with broad-line Seyfert galaxies. This fact, plus evidence for a correlation between the presence of the warm absorber and significant optical polarization, may imply that the inner warm absorber is missing or is too highly ionized to be detected, and only the outer, dusty, less ionized warm absorber is present in many cases. The iron line equivalent width appears to be similar among narrow and broad-line Seyfert galaxies. This could mean that reprocessing occurs with similar geometry in both classes of objects; however, the detection of ionized iron lines in a few objects, implying possibly altered fluorescence yields, and poor statistics, makes this conclusion tentative. Constraints on physical processes and models based on extreme values of orientation and accretion rate for NLS1s are examined in light of the observational results.