[ITAL]ORFEUS II[/ITAL] Far-Ultraviolet Observations of 3C 273: The Intrinsic Spectrum

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Abstract
Using the Berkeley spectrometer of the ORFEUS-SPAS II mission, we observed the spectrum of the bright, low-redshift QSO 3C 273 in the wavelength range 900-1200 Å (780-1040 Å in the QSO's rest frame). The QSO's spectrum is dominated by broad emission blends of the resonance lines of O VI, C III, N III, and S VI. Only relatively weak emission is detected at the wavelengths of the higher Lyman lines, and no significant Lyman discontinuity is present at the QSO's redshift. The reddening-corrected underlying smooth continuum shows (for the epoch of the ORFEUS observations) a turnover of the νFν spectrum at about 2.5×1015 Hz. While this turnover frequency is in the general range expected for active galactic nucleus accretion disks, it is lower than predicted for QSOs as luminous as 3C 273 by standard accretion disk models.