Observational Evidence for Balmer, Lyman, and He II Edges from the Accretion Disks in Active Galactic Nuclei

Details Observational Evidence for Balmer, Lyman, and He II Edges from the Accretion Disks in Active Galactic Nuclei Observational Evidence for Balmer, Lyman, and He II Edges from the Accretion Disks in Active Galactic Nuclei

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21334208g&link_type=abstract

American Astronomical Society, AAS Meeting #213, #342.08; Bulletin of the American Astronomical Society, Vol. 41, p.477

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Bound-free edges from the accretion disks in AGNs have long been expected, but observational detection has proved to be difficult. The best evidence for bound-free edges is the detection of the (hydrogen) Balmer edge in absorption in polarized flux by Kishimoto, Antonucci, & Blaes (2003) and Kishimoto et al. (2004). I show that their polarized spectra are in good agreement with the broadening of Balmer absorption edges from a 10,000 K annulus in the disk viewed obliquely from the location of the scattering electrons. The range of broadening in objects is small. Because the radial dependence of temperature in an accretion disk is relatively well understood, the broadening of the hydrogen and helium Lyman edges can readily be calculated from the broadening of the Balmer edge. Spectropolarimetric observations of the hydrogen Lyman edge (Koratkar et al. 1995, 1998) suggest that the Lyman edge can be in emission. The He II Lyman edge is unobservable, but Mathews & Ferland (1987) have argued from the relative strength of He II line emission that the spectrum of AGNs must turn down sharply in the EUV. I show that the relatively small object-to-object variation of the He II/Hbeta ratio requires most AGNs to have very similar spectral energy distributions around 0.1 keV. Since the He II/Hbeta ratio is highly sensitive to the number of photons with energies greater than 4 Rydbergs, fine tuning of the spectral shape is required. This is incompatible with approximating the disk emission with black bodies. I show that the fine tuning can naturally be provided by broadening of the He II Lyman absorption edge.
This research has been supported by NASA grant NNH-08CC03C and NSF grant AST 08-03883

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