Accretion Disks and the Nature and Origin of AGN Continuum Variability
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| Format: | Artículo científico |
| Sprache: | en |
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Universidad Nacional Autónoma de México
2008
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| _version_ | 1876429464907284480 |
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| author | C. Martin Gaskell |
| author_facet | C. Martin Gaskell |
| contents | Accretion Disks and the Nature and Origin of AGN Continuum Variability C. Martin Gaskell Física, Astronomía y Matemáticas Accretion accretion disks galaxies: active quasars: general galaxies: Seyfert Theory and observations of the dominant thermal continuum emission in AGN are examined. After correctionfor reddening, the steady state AGN optical-UV spectral energy distributions (SEDs) are very similar. The SEDs are dominated energetically by the "big blue bump" (BBB), but this bump never shows the ν +1/3spectrum predicted for a standard thin accretion disk with a r -0.75 radial temperature gradient. Instead, theobserved optical-UV SED implies a temperature gradient of r -0.57 independent of the thickness of the disk.This means that there is some flow of heat outwards in the disk. The disk is large and the region emittingthe optical continuum is as large as the inner broad-line region (BLR). Because optical variability is seen inall AGN on the light-crossing time of the BLR, variations must propagate close to the speed of light, ratherthan on dynamical timescales. This argues that the energy-generation mechanism is electromagnetic ratherthat hydrodynamic. Since the velocities are near the speed of light, there can be significant local anisotropy in the emission. The large rapid variations of the BBB imply that the magnetohydrodynamic energy generation is fundamentally unstable. Because of the inevitable radial temperature gradient in the accreting material, different spectral regions come predominantly from different radii, and variations in different spectral regions correspond to variability at different radii. This explains the frequently observed independence of X-ray and optical variations, cases of variability at lower energies leading variability at higher energies, and rapid changesin emission-line reverberation lags. Some observational tests of the local variability hypothesis are proposed. 2008 artículo científico 0185-1101 https://www.redalyc.org/articulo.oa?id=57103203 en http://www.redalyc.org/revista.oa?id=571 Revista Mexicana de Astronomía y Astrofísica application/pdf Universidad Nacional Autónoma de México Revista Mexicana de Astronomía y Astrofísica (México) Vol.32 |
| format | Artículo científico |
| id | redalyc_57103203 |
| institution | Redalyc |
| language | en |
| publishDate | 2008 |
| publisher | Universidad Nacional Autónoma de México |
| spellingShingle | Accretion Disks and the Nature and Origin of AGN Continuum Variability C. Martin Gaskell Física, Astronomía y Matemáticas Accretion accretion disks galaxies: active quasars: general galaxies: Seyfert Accretion Disks and the Nature and Origin of AGN Continuum Variability C. Martin Gaskell Física, Astronomía y Matemáticas Accretion accretion disks galaxies: active quasars: general galaxies: Seyfert Theory and observations of the dominant thermal continuum emission in AGN are examined. After correctionfor reddening, the steady state AGN optical-UV spectral energy distributions (SEDs) are very similar. The SEDs are dominated energetically by the "big blue bump" (BBB), but this bump never shows the ν +1/3spectrum predicted for a standard thin accretion disk with a r -0.75 radial temperature gradient. Instead, theobserved optical-UV SED implies a temperature gradient of r -0.57 independent of the thickness of the disk.This means that there is some flow of heat outwards in the disk. The disk is large and the region emittingthe optical continuum is as large as the inner broad-line region (BLR). Because optical variability is seen inall AGN on the light-crossing time of the BLR, variations must propagate close to the speed of light, ratherthan on dynamical timescales. This argues that the energy-generation mechanism is electromagnetic ratherthat hydrodynamic. Since the velocities are near the speed of light, there can be significant local anisotropy in the emission. The large rapid variations of the BBB imply that the magnetohydrodynamic energy generation is fundamentally unstable. Because of the inevitable radial temperature gradient in the accreting material, different spectral regions come predominantly from different radii, and variations in different spectral regions correspond to variability at different radii. This explains the frequently observed independence of X-ray and optical variations, cases of variability at lower energies leading variability at higher energies, and rapid changesin emission-line reverberation lags. Some observational tests of the local variability hypothesis are proposed. 2008 artículo científico 0185-1101 https://www.redalyc.org/articulo.oa?id=57103203 en http://www.redalyc.org/revista.oa?id=571 Revista Mexicana de Astronomía y Astrofísica application/pdf Universidad Nacional Autónoma de México Revista Mexicana de Astronomía y Astrofísica (México) Vol.32 |
| title | Accretion Disks and the Nature and Origin of AGN Continuum Variability |
| topic | Física, Astronomía y Matemáticas Accretion accretion disks galaxies: active quasars: general galaxies: Seyfert |
| url | https://www.redalyc.org/articulo.oa?id=57103203 |