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Session 39 - AGNs, QSOs, Active Galaxies.
Display session, Wednesday, June 11
South Main Hall,

[39.11] Lyman Continuum Polarization in QSOs: Evidence for a Moderately Rotating Black Hole?

G. A. Shields, L. Wobus (U. Texas Austin), D. Husfeld (Munich U. Observatory)

HST observations of some QSOs show a strong, abrupt increase in polarization at rest wavelength about 750 ÅImpey et al. 1995, Ap.J. 440, 80; Koratkar et al. 1995, Ap.J. 450, 501). The polarization rises from 1 to 2% at longer wavelengths to values as high as 20% at the shortest observed wavelengths. 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 (Laor et al. 1990, MNRAS 242, 560). We achieve a good fit to the observed polarization rise in a model in which the locally emitted radiation at the disk surface is assumed to have a sharp increase in polarized flux at the Lyman edge in the rest frame of the gas. Relativistic effects cause the observed polarization to rise fairly sharply at a wavelength substantially less than 912 ÅThe blueshift depends on the radius at which the bulk of the polarized Lyman continuum is emitted, which in turn depends on the angular momentum of the black hole. A dimensionless angular momentum a = 0.5 agrees with the observed polarization feature in PG 1630+377. An intermediate value of black hole angular momentum might result from successive accretion events involving material orbiting in different planes.

This material is based in part upon work supported by the Texas Advanced Research Program under Grant No. 003658-015.

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Program listing for Wednesday