Other
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.7107b&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #71.07; Bulletin of the American Astronomical Society, Vol. 24, p.1234
Other
Scientific paper
The current favorite model for the origin of the BBB in AGNs is the geometrically thin, optically thick accretion disk picture. But this model appears to disagree with the observations on several counts. In general it predicts the wrong polarization, the wrong variability timescale, and the wrong behavior at the Lyman edge. It also requires an extension of a steep infrared power-law underlying the disk emission, for which there is no evidence - in fact, there is much to suggest that the infrared is dust emission. Finally, the brightness temperature (~ 3times 10(5) K) inferred from microlensing variations in one source is too high to be produced by an accretion disk surrounding a massive black hole (Rauch and Blandford 1991, ApJ 381, L39). An alternative hypothesis, first proposed by Antonucci and Barvainis (1988, ApJ 332, L13) and further discussed by Ferland, Korista, and Peterson (1990, ApJ 363, L21), is that the BBB is optically thin thermal emission, i.e. predominantly free-free. This model has a number of points in its favor, including: 1) it predicts the correct mean optical/UV spectral index of quasars (alpha ~ -0.3, f_ν ~ nu (alpha ) ); 2) it is compatible with thermal dust emission in the infrared; 3) it predicts the correct angle of linear polarization (parallel to the radio source axis, if the source is flattened perpendicular to the axis) 4) it's physical temperature, and brightness temperature, can be high (> 10(5) K); 5) the presence of optically thin thermal emission has been directly inferred in sources with constant, ultrabroad line wings; 6) optical/UV variations are simultaneous and broad band, as expected from a broad band mechanism like free-free; 7) reprocessed X-rays in NGC 5548 appear as optical/UV emission with the spectral shape and variablility characteristics of optically thin rather than optically thick gas; 8) it predicts a very weak Lyman edge for sources with T ~ 10(6) K (evidence suggests that luminous quasars may be this hot or hotter). These points and others are discussed in this paper, including free-free source models comprised of multiple small clouds which are optically thin to absorption but may be thick or thin to electron scattering.
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