HST observations of some QSOs show a strong , abrupt increase in polarization at rest wavelength about 750 Å .
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 can not 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 912 Å .
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 _ { * } \equiv cJ / M ^ { 2 } = 0.5 and an observer viewing angle \mu _ { o } \equiv cos~ { } \theta _ { 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 .