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We obtained far-ultraviolet spectra of the quasar 3C 273 using the Hopkins Ultraviolet Telescope (HUT) during the flight of Astro-1 aboard the space Shuttle Columbia in December 1990. The HUT spectra cover the 830-1850 \AA\ wavelength range with a resolution of $\sim$3 \AA. In two successive orbits we accumulated 3389 s of high quality data. Approximately 750 s of the total were obtained during orbital night. Our spectra extend well past the Lyman edge down to a wavelength of 787 \AA\ in the rest frame of this z=0.158 quasar.
The HUT spectrum shows a steeply rising continuum in the far ultraviolet with a distinct break to a flatter slope at 1128 $\pm$ 3.5 \AA\, just longward of the redshifted Lyman edge of 3C 273. The continuum shape is empirically well characterized by a broken power law in $F_\lambda$ with a spectral index of 1.6 at wavelengths longer than 1128 \AA\ and 0.2 at shorter wavelengths. We propose that the break and the flattening of the continuum may be due to Lyman edge features in the spectrum of an accretion disk. We model the spectral shape in the Lyman edge region 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 modest amount of Comptonization can describe the continuum shape nearly as well as the best-fit broken power law.
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