HST observations of some QSOs show a strong, abrupt increase in polarization at rest wavelength about 750 A, suggestive of a connection with the Lyman edge of hydrogen. Blaes and Agol (1996) have proposed an explanation in terms of stellar atmosphere effects in an accretion disk around a supermassive black hole. We have computed the polarized spectrum of a such a disk, including the effects of the relativistic transfer function. Relativistic effects add an additional blueshift of the polarization rise sufficient that the model cannot explain the observations. A good fit results if the emitted radiation is assumed to have a sharp increase in polarized flux at the Lyman edge in the rest frame of the orbiting gas. Relativistic effects then cause the observed polarization to rise sharply at a wavelength substantially less than 912 A. The blueshift depends on the angular momentum of the black hole and the inclination of the disk. A good fit to PG 1630+377 results from a simple model with a dimensionless angular momentum a = 0.5 and an observer viewing angle cos theta = 0.1. An intermediate value of a might result from coallescing black holes, successive accretion events, or electromagnetic extraction of angular momentum from the hole.Comment: 24 pages incl. 9 PostScript figures. Uses aaspp4.sty and flshrt.sty. To be published in The Astrophysical Journal, Vol. 496 (1988 April 1
HST observations of some QSOs show a strong, abrupt increase in polarization at rest wavelength about 750Å. The closeness of the polarization rise to the H I Lyman edge suggests a connection, but the displacement to shorter wavelengths, and the shape of the polarization rise require explanation.We have computed the polarized spectrum of a thermally emitting accretion disk around a supermassive black hole, including the effects of the relativistic transfer function. The local stellar atmosphere spectra show a blueshifted polarization rise in the Lyman continuum, as found by Blaes and Agol (1996). However, the relativistic transfer function adds an additional blueshift of sufficient magnitude that the model cannot explain the observations. We show that a good fit results if the emitted radiation is assumed to have a sharp increase in polarized flux at the Lyman edge in the rest frame of the orbiting gas. Relativistic effects then cause the observed polarization to rise sharply at a wavelength substantially less than 912Å. The blueshift depends on the angular momentum of the black hole and the inclination of the disk. A good fit to PG 1630+377 results from a simple model with a dimensionless angular momentum a * ≡ cJ/M 2 = 0.5 and an observer viewing angle µ o ≡ cos θ o = 0.1. Smaller values of a * give insufficient blueshifts, and values close to a * = 0.9982 require unrealistically large polarizations in the rest frame of the gas. An intermediate value of a * might result from coallescing black holes, successive accretion events, or electromagnetic extraction of angular momentum from the hole.
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