Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...281....7p&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 281, no. 1, p. 7-14
Astronomy and Astrophysics
Astronomy
10
Accretion Disks, Light Emission, Luminosity, Seyfert Galaxies, Ultraviolet Astronomy, Ultraviolet Emission, X Ray Astronomy, X Ray Sources, Active Galactic Nuclei, Black Holes (Astronomy), Spectrum Analysis, Ultraviolet Spectra, Visible Spectrum, X Ray Spectra
Scientific paper
The variable optical (O) and ultraviolet (UV) fluxes in NGC 4151 are well correlated with each other at all levels, with an upper limit on their phase difference which is inconsistent with that expected in the standard accretion disk picture. At low and intermediate levels the O and UV fluxes are also well correlated with the 2-10 keV flux, while the correlation breaks down at high levels. The hypothesis that the O and UV emission might be due to reprocessing in a thin accretion disk of harder (X-ray) photons from a central source is quantitatively investigated. The luminosity L0 and radius R0 of a spherical and optically thick central source are derived, taking into account the upper limit cos i less than or equal to 0.3 on the disk inclination implied by the upper limit on the Compton diffusion component in the X-ray spectrum of this galaxy. The luminosity L0 is at least one order of magnitude larger than L(2-10 keV). From a comparison with the available observational data it appears that, if the bulk of L0 is emitted above 10 keV, at least 50% of it should be emitted above 0.5 MeV, and on the basis of its first published measurement it is argued that the Oriented Scintillation Spectrometer Experiment (OSSE) experiment on the Compton Observatory can readily test this hypothesis, provided that more observations are carried out simultaneously with those in the O, UV and 2-10 keV X-rays. Alternatively L0 could be emitted in the form of a soft X-ray excess, and it is argued that its presence may be related to the excess flux in the 1-4 keV range attributed so far to a leaky absorber. In either case the correlation between L0 and L(2-10 keV) should break down when the UV flux reaches high levels. If it is assumed that L0 is equal to the disk accretion (viscous) luminosity, an upper limit on the black hole mass, hence on the Eddington luminosity LE, is obtained: it is suggested that the break down of the correlation occurs when L0 approaches LE. This upper limit, however, is smaller than an estimate based on the emission line widths: were this discrepancy confirmed by more sophisticated evaluations of both estimates, one would be forced to conclude that L0 is much greater than the ordinary accretion disk luminosity.
Perola Giuseppe Cesare
Piro Luigi
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