APPLICATION OF THE DISK EVAPORATION MODEL TO ACTIVE GALACTIC NUCLEI

Journal Article · Thu Dec 10 00:00:00 EST 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/707/1/233· OSTI ID:21389318

Liu, B F ^[1]

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1. National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, P.O. Box 110, Kunming 650011 (China)

The disk corona evaporation model extensively developed for the interpretation of observational features of black hole X-ray binaries (BHXRBs) is applied to active galactic nuclei (AGNs). Since the evaporation of gas in the disk can lead to its truncation for accretion rates less than a maximal evaporation rate, the model can naturally account for the soft spectrum in high-luminosity AGNs and the hard spectrum in low-luminosity AGNs. The existence of two different luminosity levels describing transitions from the soft to hard state and from the hard to soft state in BHXRBs, when applied to AGNs, suggests that AGNs can be in either spectral state within a range of luminosities. For example, at a viscosity parameter, alpha, equal to 0.3, the Eddington ratio from the hard-to-soft transition and from the soft-to-hard transition occurs at 0.027 and 0.005, respectively. The differing Eddington ratios result from the importance of Compton cooling in the latter transition, in which the cooling associated with soft photons emitted by the optically thick inner disk in the soft spectral state inhibits evaporation. When the Eddington ratio of the AGN lies below the critical value corresponding to its evolutionary state, the disk is truncated. With decreasing Eddington ratios, the inner edge of the disk increases to greater distances from the black hole with a concomitant increase in the inner radius of the broad-line region, R {sub BLR}. The absence of an optically thick inner disk at low luminosities (L) gives rise to region in the R {sub BLR}-L plane for which the relation R {sub BLR} propor to L {sup 1/2} inferred at high luminosities is excluded. As a result, a lower limit to the accretion rate is predicted for the observability of broad emission lines, if the broad-line region is associated with an optically thick accretion disk. Thus, true Seyfert 2 galaxies may exist at very low accretion rates/luminosities. The differences between BHXRBs and AGNs in the framework of the disk corona model are discussed, and possible modifications to the model are briefly suggested.

• OSTI ID: 21389318
• Journal Information: Astrophysical Journal, Vol. 707, Issue 1; Other Information: DOI: 10.1088/0004-637X/707/1/233; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ACCRETION DISKS
BLACK HOLES
EMISSION
EVAPORATION MODEL
GALAXY NUCLEI
LUMINOSITY
PHOTONS
X RADIATION
X-RAY GALAXIES
BOSONS
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
GALAXIES
IONIZING RADIATIONS
MASSLESS PARTICLES
MATHEMATICAL MODELS
NUCLEAR MODELS
OPTICAL PROPERTIES
PHYSICAL PROPERTIES
RADIATIONS