The Ultraviolet Peak of the Energy Distribution in 3C 273: Evidence for an Accretion Disk and Hot Corona around a Massive Black Hole

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Abstract
We present absolutely calibrated far-ultraviolet spectrophotometry of the quasar 3C 273 covering the 900-1800 Å range. Our ~3 Å resolution spectra were obtained with the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 mission in 1990 December and during the Astro-2 mission in 1995 March. Both spectra exhibit a change in slope near the Lyman limit in the quasar rest frame. At longer UV wavelengths, the continuum has a power-law index of 0.5-0.7, while shortward of the Lyman limit it is 1.2-1.7. The energy distribution in νfν, therefore, peaks close to the quasar Lyman limit. The short-wavelength UV power law extrapolates well to match the soft X-ray excess seen in simultaneous observations with the Broad Band X-Ray Telescope (BBXRT) and nearly simultaneous ROSAT observations. The general shape of the broadband spectrum of 3C 273 is consistent with that of an optically thick accretion disk whose emergent spectrum has been Comptonized by a hot medium. Our UV spectrum is well described by a Schwarzschild black hole of 7 × 108 M☉ accreting matter at a rate of 13 M☉ yr-1 through a disk inclined at 60°. Superposed on the intrinsic disk spectrum is an empirically determined Lyman edge of optical depth 0.5. The Comptonizing medium has a Compton parameter y ≈ 1, obtained with an optical depth to electron scattering of unity and a temperature of 4 × 108 K. This overall shape is the same as that found by Zheng et al. and Laor et al. in their UV and X-ray composite spectra for quasars, giving physical validity to the composite spectrum approach. When combined with those results, we find that the generic ionizing continuum shape for quasars is a power law of energy index 1.7-2.2, extending from the Lyman limit to ~1 keV. The observational gap in the extreme ultraviolet for these combined data describing the quasar continuum shape is now only half a decade in frequency.
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