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Session 51 - Active Galactic Nuclei & Quasars.
Oral session, Thursday, June 12
North Main Hall A,

[51.02] ASCA observations of ultrasoft narrow-line Seyfert 1 galaxies

W. N. Brandt, S. Mathur, M. Elvis (CfA), T. Boller (MPE)

Many narrow-line Seyfert 1 galaxies are known to have extremely strong soft X-ray excess components, but their harder (>2 keV) X-ray spectra have not been well studied. We have recently used ASCA data to perform the first systematic investigation of the 2--10 keV X-ray continua of these galaxies. Analysis of the ASCA data for over 30 Seyfert 1s suggests that soft ROSAT narrow-line Seyfert 1s have generally steeper 2--10 keV ASCA power-law slopes than are seen in more `typical' Seyfert 1s. Some narrow-line Seyfert 1s have remarkably steep 2--10 keV continua with ASCA power-law photon indices of 2.2--2.6. Steep 2--10 keV ASCA spectra may be due to Compton cooling of the accretion disk corona by soft photons, or they may be due to significant curvature of the intrinsic 2--10 keV continuum. Either possibility has important consequences. We also discuss spectral complexity in narrow-line Seyfert 1s. We present the results of a detailed ASCA study of the narrow-line Seyfert 1 type quasar PKS 0558--504 in which strong continuum curvature is apparent without significant evidence for Compton reflection. Amazing X-ray variability has been seen from some narrow-line Seyfert 1s, and we discuss this variability and its implications. In particular, we describe the persistent, giant-amplitude (over a factor of 30) variability events seen from IRAS 13224--3809 by ASCA and ROSAT. The variability appears to be highly nonlinear and may be due to unusually strong relativistic effects.

Program listing for Thursday