Accretion Disks and the Lyman Continuum Polarization of QSOs

Abstract

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

Invited Review: The Ultraviolet and Optical Continuum Emission in Active Galactic Nuclei: The Status of Accretion Disks

“…It may in fact be that optical/UV radiation emerging from Shields et al (1998). The position angle of the accretion disk axis is unknown, so the overall offset in angle in the bottom figure is not significant.…”

“…In addition, they neglected the relativistic transfer function, which tends to smear and reduce the polarization rise (Agol 1997;Shields, Wobus, & Husfeld 1998), although this may not be a problem if the optically thick portion of the disk does not extend all the way down to the innermost stable orbit. Shields et al (1998) have produced a model that produces excellent quantitative agreement with the observations of steep polarization rises near the Lyman edge but is based on ad hoc assumptions. They assume that at every radius the disk produces a spectrum with a sharp Lyman edge in absorption, together with a large Lyman polarization edge in emission.…”

Non‐LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei. III. Integrated Spectra for Hydrogen‐Helium Disks

“…This is partly due to the smearing and rotation of the plane of polarization by the relativistic transfer function (cf. Shields, Wobus, & Husfeld 1998). Laor, Netzer, & Piran (1990).…”

The relationship between variable and polarized optical spectral components of luminous type 1 non-blazar quasars

“…Finally, it should also be noted that, although quantitative comparisons with model spectra are currently impossible, there is still a room to consider that the observed polarization properties in the four quasars with λ <4000 Å polarization dip may be explained by the intrinsic polarization imprinted in the ac-cretion disk atmosphere. By referring to the model calculations by Laor et al (1990) (see also Shields et al 1998;Hsu & Blaes 1998;Koratkar & Blaes 1999), Kishimoto et al (2003) and Antonucci et al (2004) stated that, in a certain accretion disk model parameter space, even though the total flux (and thus the variable component) of the accretion disk spectrum has no feature around Balmer edge spectral region, the polarization degree (and thus the polarized component) spectrum can show the decreasing feature at the blueward of Balmer edge due to the increase of the absorption opacity. 6 If this is true, our result of the discrepancy of the spectral shape between the variable component and polarized component in the four quasars can be explained so that the variable component spectrum is a scaled copy of the featureless accretion disk continuum, and the polarized component spectrum only represents the intrinsically polarized accretion disk continuum component.…”