QUASAR LYMAN EDGE REGIONS IN POLARIZED-LIGHT

The standard paradigm of energy production in quasi-stellar objects (Q SOs) includes an accretion disk surrounding a supermassive black hole. Two important consequences of such a disk are a feature, probably in absorption, at the Lyman edge, and significant polarization parallel t o the disk plane at most viewing angles. The polarization signature ca n be diagnostic of conditions in the disk atmosphere, where scattering imparts the polarization. Hence we have observed three intermediate-r edshift (z = 0.5-1.6) QSOs with the HST/FOS spectropolarimeter to dete rmine the amount of polarization on both sides of the Lyman limit, 912 Angstrom. The QSOs were known to show broad, partial edges at the sys temic redshifts and were not known to have narrow absorption lines cor responding to the edges; these are the expected properties for simple thin disks. According to theoretical model predictions the polarizatio n of such a disk should rise with frequency in the optical/near-UV, bu t drop above the Lyman edge. Our results for at least two of the three QSOs are quite different from these predictions. We see no rise in P with frequency in the region longward of the edge. Instead, the most s triking polarization feature is a sharp rise in P below the Lyman edge , reaching values as high as similar to 20% in PG 1630+377, a value ne ver seen before in nonblazar active galaxies. A more subtle feature in the PG 1630 + 377 data is a rise in P across Ly alpha. A power-law fi t to the polarized flux gives P x F-lambda proportional to lambda(-9.9 ), which is much steeper than expected for Rayleigh scattering of a no rmal QSO continuum. A more likely possibility seems to be that we are seeing the scattered light of a Lyman edge in emission. The edge in po larized flux in PG 1630+377 appears to be blueshifted, while the polar ized Ly alpha line is not, possibly indicating that just below the Lym an edge there is some absorption of the scattered continuum. It is als o possible that Faraday depolarization and/or resonance scattering pla y roles.