Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........24l&link_type=abstract
Thesis (PH.D.)--THE JOHNS HOPKINS UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 57-01, Section: B, page
Physics
4
Lyman Edges, Continuum Shapes
Scientific paper
To produce physically realistic accretion disk spectra, especially in the vicinity of the Lyman edge, we solve the radiative transfer equation for the radiation pressure dominated inner region of the massive thin accretion disks which might exist in Active Galactic Nuclei (AGN). We analytically solve the vertical structure of the accretion disk by assuming an adiabatic polytrope, and then numerically solve the radiation transfer equation in local thermodynamic equilibrium. We also consider three possibilities which can change the spectral shape of the Lyman-limit region: relativistic effects produced by the strong gravitational field near the black hole, Comptonization of the emitted disk spectrum by a surrounding hot corona, and the effects of X-ray illumination. The principle result is that the Lyman edge features may be in emission or in absorption, depending on the dominant sources of opacity in the disk atmosphere at various radii. The net contrast in an integrated spectrum is small for a wide range of reasonable parameters, and strong Lyman edges are predicted only in objects that have low luminosity relative to the Eddington limit. We compare the predicted Lyman edge features with the Hopkins Ultraviolet Telescope (HUT) observations of the quasar 3C 273 and of the Seyfert 1 galaxy Mrk 335 and the HUT and the Faint Object Spectrograph of the Hubble Space Telescope (HST/FOS) observations of the quasar 1E1821+643 to test the observational signatures of accretion disks in AGN. The HUT spectra of 3C 273 and Mrk 335 show a distinct break in the continuum slope just longward of the redshifted Lyman edge. The continuum shape of the two objects is empirically well characterized by a broken power law in Flambda. A physically realistic spectrum of the accretion disk can also fit the observed continuum shape nearly as well as the best-fit broken power law. The HUT and the HST/FOS spectra of 1E1821+643, however, do not show any distinct features in the continuum around the redshifted Lyman edge. The continuum shape of 1E1821+643 can be modeled by the spectrum of a simple black body accretion disk without any definite Lyman edge features. Finally we study the connection between the shape of a specific accretion disk spectrum for the broad band continuum and the measured intensity ratios among selected emission lines. According to photoionization models for emission lines in AGN, the intensity ratios among Ly alpha, C scIV, O scVI, and He scII emission lines can be used to indicate the shape of the ionizing continuum. We calculate the predicted line intensity ratios using the accretion disk spectrum for the optical/UV/soft X-ray continuum as an ionizing radiation in CLOUDY model and compare them with the measured line intensity ratios. Our single zone modeling of the broad emission line regions (BLR) of 3C 273, Mrk 335, and 1E1821+643, however, could not clearly indicate a favored ionizing continuum by the measured line intensity ratios. We conclude that the BLR of these AGN are likely to be stratified in ionization parameters, even for these highly ionized lines. (Abstract shortened by UMI.).
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