Previous
abstract 
Next
abstract
We study the characteristics of the ultraviolet continuum of the quasar PKS 0405$-$123 using the archival HST/FOS spectrum. The spectrum from 1150 to 3300 \AA\ shows a steeply rising continuum in $F_{\lambda}$ with a strong absorption feature $\sim$ 100 \AA\ wide around the intrinsic Lyman limit of this z=0.574 quasar. The spectrum also shows Lyman absorption line systems in the wavelength range of the broad absorption feature. A Ly$\alpha$ absorption line whose corresponding Lyman limit could contribute to the broad absorption feature is also identified. We investigate the possibility that the broad absorption feature may be due to the sum of the contributions from each Lyman absorption system. The estimated opacity due to the Lyman absorption systems in the region of the broad absorption feature, however, is not high enough to completely account for it. We thus propose that a significant part of the continuum drop in the broad absorption feature may be due to a broadened Lyman edge in the spectrum of an accretion disk.
We model the ultraviolet continuum using an $\alpha$-disk with an adiabatic vertical structure. We compute the emitted spectrum by solving the radiative transfer numerically. The observed spectrum is corrected for relativistic effects assuming a Schwarzschild metric, and we also consider the effect of Comptonization by a surrounding hot corona on the observed spectrum. A realistic disk spectrum with a significant amount of Comptonization describes the steep continuum shape and the broad Lyman edge feature, and it is consistent with the X-ray flux observed with EINSTEIN observatory IPC.
\noindent This work was supported by NASA Grant NAG 5-1630 to the FOS team and NASA contract NAS 5-27000 to the Johns Hopkins University.
