A Comprehensive Spectral and Variability Study of Narrow‐Line Seyfert 1 Galaxies Observed by ASCA . I. Observations and Time Series Analysis

Abstract

I present a comprehensive and uniform analysis of 25 ASCA observations from 23 narrow-line Seyfert 1 galaxies. The time series analysis is presented in this paper, Part 1, and the spectral analysis and correlations are presented in the companion paper, Part 2. Time series analysis shows that the excess variance from the NLS1 light curves is inversely correlated with their X-ray luminosity. However, with a logarithmic slope of ~-0.3, the dependence of the excess variance on luminosity is flat compared with broad-line objects and the expected value of -1 from simple models. At a particular X-ray luminosity, the excess variance is typically an order of magnitude larger for NLS1s than for Seyfert 1 with broad optical lines. There is, however, a large scatter, and a few objects show an even larger excess variance. The excess variance can be interpreted as a timescale if the shape of the variability power spectrum, the length of the observation, and the window function are the same for all observations, and the properties of the sample objects are shown to be roughly consistent with this requirement. In particular, no strong evidence for changes during an observation in the shape or normalization of the power spectrum was found once the systematic errors due to the 1/f nature of the power spectrum was accounted for properly. Some of the more variable light curves are shown to be inconsistent with a linear, Gaussian process, implying that the process is non-Gaussian. It is possible that the process is nonlinear, but while the distinction between these possibilities is very important for differentiating between models, such a distinction cannot be made using these data. The enhanced excess variance exhibited by NLS1s can be interpreted as evidence that they are scaled-down versions of broad-line objects, having black hole masses roughly an order of magnitude smaller and requiring an accretion rate an order of magnitude higher. Alternatively, NLS1s may exhibit an inherently different type of variability, characterized by high-amplitude flares, in which case a smaller black hole mass would not be required.