BLR sizes and the X-ray spectrum in AGN

Recent ROSAT studies of narrow-line Seyfert 1 galaxies revealed these objects (with FWHM Hβ ≤ 2000 km s−1) generally show steeper soft X-ray spectra than broad line Seyfert 1’s and that there are noAGNwith 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.