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Broad absorption line QSOs (BALQSOs) comprise approximately 10\% of all optically-selected quasars, and as a class are the only such objects to exhibit strong ($>3$\%), static linear polarization. Observations of the emission line and continuum properties of BALQSOs suggest that all radio-quiet QSOs are potential BALQSOs, being observed as such if the line of sight intercepts an absorbing cloud. The unique insight which polarimetry lends to the geometry of a source of radiation therefore motivates intense study of the accessible BALQSO candidates.
Optical linear spectropolarimetry is presented of the high-redshift (z = 2.86) BALQSO 1524+5147, an object selected for its brightness (V = 17.0) and strong white-light polarization (P = 2.7\%). In the rest frame of the object (1070 - 1940\AA), the degree of continuum polarization is nearly constant with wavelength, and the position angle is undeviated through the spectrum. The broad emission lines (BELs) provide pure dilution of the polarized component, while BAL features primarily absorb this component.
These conditions indicate a single, nuclear source of polarized radiation, but constant polarimetric properties over a 3.5 yr baseline suggest that synchrotron emission is not responsible. Instead, we propose that the polarization arises in an asymmetric distribution of scatterers which are coincident with and/or interior to the BAL and BEL regions, and the latter two need not be distinct. The bluer spectrum of polarized vs. total flux suggests either that the scatterers are dust grains or that an additional, red component of unpolarized light contributes to the overall continuum. These properties are discussed in terms of competing models of the nuclear morphology of AGN.
This research is supported by NSF grant AST 91-14087.
