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Astron. Astrophys. 331, 884-890 (1998)

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BLR sizes and the X-ray spectrum in AGN

A. Wandel 1 and Th. Boller 2

1 Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
2 Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany

Received 18 November 1996 / Accepted 20 October 1997


Recent ROSAT studies of narrow-line Seyfert 1 galaxies revealed these objects (with FWHM [FORMULA]) generally show steeper soft X-ray spectra than broad line Seyfert 1's and that there are no AGN with broad lines and steep X-ray spectra. We derive a simple theoretical model which explains this observed correlation between the line width and the spectral index for Seyfert 1 galaxies. Assuming the line width is due to gravitational velocity dispersion, it is determined by the radius of the broad line region. Sources with steep X-ray spectra (for a given luminosity) have a stronger ionizing power than flat-spectrum sources with comparable luminosity, which implies that the BLR is formed at relatively larger distances from the central source, and hence has a smaller velocity dispersion and a smaller observed FWHM. We test the model over a hetrogeneous (normal and narrow-line) sample of some 50 AGN finding a good agreement with the data.

Key words: galaxies: active – galaxies: nuclei – X-rays: galaxies – black hole physics

Send offprint requests to: A. Wandel

© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998