Other
Scientific paper
Oct 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.246..369l&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 246, NO. 3/OCT1, P. 369, 1990
Other
102
Scientific paper
This paper presents a detailed comparison of theoretical thin accretion disc models with the spectra of 101 bright AGN. The models have been described in Paper I and II of this series, they take into account all relativistic effects and use an improved approximation for the local spectrum. The comparison is made by fitting the observations with a two- component model, a thin 'bare' accretion disc and an underlying power law. The best fit model parameters are found by minimizing the fit χ^2^, and a χ^2^ test is used to deduce the significance level of the fit. The fit is found to be acceptable (i.e. cannot be rejected, at the 99 per cent significance level) in 79 per cent of the objects. However, the model parameters, black hole mass, accretion rate, and the disc inclination, are not well constrained. A range of a factor of 10 in each parameter is usually possible. The extrapolated IR power law underestimates the observed X-ray flux at 1 kev, in most objects. The observed soft X-ray excess can be fitted by the disc component in five out of 15 objects, but the disc continuum at these energies is typically much steeper than the one observed. The best-fitting models indicate that bright quasars accrete at about 0.1 the Eddington accretion rate, while lower luminosity objects have a larger range of accretion rates, with an average of about 0.03. Adopting the thin disc model implies the following. (1) The observed spectrum cannot be reddened by more than 0.05-0.15 in E(B-V) in most objects. (2) Models which include a thin disc and hot dust as the only continuum sources can be ruled out. There must be another continuum component which contributes around 1 micron, and its relative contribution is smaller in more luminous objects. (3) The continuum steepening at λ < 1216 A observed in some high-redshift quasars is not an intrinsic property. (4) The soft X-rays should be stronger in objects with a flat optical-UV slope and a medium to low luminosity.
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