Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...450..501k&link_type=abstract
Astrophysical Journal v.450, p.501
Astronomy and Astrophysics
Astronomy
49
Accretion, Accretion Disks, Galaxies: Quasars: General, Polarization, Ultraviolet: Galaxies
Scientific paper
The standard paradigm of energy production in quasi-stellar objects (QSOs) 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 to the disk plane at most viewing angles. The polarization signature can be diagnostic of conditions in the disk atmosphere, where scattering imparts the polarization. Hence we have observed three intermediate-redshift (z = 0.5-1.6) QSOs with the HST/FOS spectropolarimeter to determine the amount of polarization on both sides of the Lyman limit, 912 Å. The QSOs were known to show broad, partial edges at the systemic redshifts and were not known to have narrow absorption lines corresponding to the edges; these are the expected properties for simple thin disks. According to theoretical model predictions the polarization of such a disk should rise with frequency in the optical/near-UV, but 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 striking polarization feature is a sharp rise in P below the Lyman edge, reaching values as high as ˜20% in PG 1630+377, a value never seen before in nonblazar active galaxies. A more subtle feature in the PG 1630+377 data is a rise in P across Lyα. A power-law fit to the polarized flux gives P × Fλ ∝ λ-9.9, which is much steeper than expected for Rayleigh scattering of a normal 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 polarized flux in PG 1630+377 appears to be blueshifted, while the polarized Lyα line is not, possibly indicating that just below the Lyman edge there is some absorption of the scattered continuum. It is also possible that Faraday depolarization and/or resonance scattering play roles.
Antonucci Robert R. J.
Bushouse Howard
Goodrich Robert W.
Kinney Anne L.
Koratkar Anuradha
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