Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000nva..conf..143b&link_type=abstract
NEW VISTAS IN ASTROPHYSICS. Proceedings of the International School of Cosmic Ray Astrophysics 20th Anniversary, 11th Course. He
Astronomy and Astrophysics
Astrophysics
Scientific paper
Recent X-ray observations have greatly advanced the study of ultrasoft Narrow-Line Seyfert 1 galaxies and have revealed strikingly clear correlations between optical emission line properties and the shape of the X-ray continuum. Narrow-Line Seyfert 1 galaxies are generally characterized by steep soft X-ray spectra with photon indices for simple power law fits up to about 5. Detailed spectral modeling shows that Narrow-Line Seyfert 1 galaxies often have very strong soft X-ray excess components compared to their hard X-ray tails. A clear anticorrelation is found between the ROSAT spectral softness and the Hβ full-width at half-maximum intensity (FWHM) in type 1 Seyferts. This is remarkable as the X-ray spectra of most Seyfert 1 type galaxies are formed predominantly within ≈ 50 Schwarzschild radii of their black holes, while Seyfert optical permitted lines are formed in a separate and significantly larger region. It appears that the anticorrelation between Hβ FWHM and ROSAT spectral softness is due to a primary underlying physical parameter. We develop a model in which Narrow-Line Seyfert 1 galaxies are those Seyfert 1's which are accreting at relatively high fractions of the Eddington rate compared to their black hole mass. Narrow-Line Seyfert 1 galaxies can also show remarkably rapid and large-amplitude X-ray variability. One spectacular object, the radio-quiet, ultrasoft Narrow-Line Seyfert 1 galaxy IRAS 13224-3809, shows persistent giant-amplitude variability events by factors of 35-60 on time scales of just a few days, most likely due to strong relativistic effects. The ROSAT HRI light curve of IRAS 13224-3809 is nonlinear in character, suggesting that the X-ray emission regions on the accretion disk interact nonlinearly or are affected by nonlinear flux amplification. Dramatic flux variability is also apparent in the highly-luminous Seyfert 1 galaxy PHL 1092. Analysis of the X-ray light curve of PHL 1092 also suggests the presence of relativistic flux boosting.
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