Voyager and IUE Observations of 3C 273: Detection of Possible Lyman Continuum Absorption in a Nearby (z = 0.158) Quasar

Abstract

We report results of near-simultaneous IUE and Voyager ultraviolet spectrometer (UVS) observations of 3C 273, obtained during the period of 1985 late July-early August. The data cover a total combined wavelength range 500-3200 A (observed frame). This is the first time that observations shortward of 1200 A have been obtained for any extragalactic object. The source was significantly detected for all observed wavelengths above 912 A, at a level (for λ_obs_ > 1060 A) consistent with previous optical and IUE observations. The UVS data show a significant drop in flux for observed wavelengths below 1060 A. This may be due to Lyman continuum absorption in the rest frame of 3C 273. If so, then the lack of associated Lymanα absorption severely constrains the properties of the absorbing medium. The most plausible explanation may therefore be that the edge is due to absorption in an accretion disk within the nucleus of 3C 273. Alternatively, models in which the drop in flux is due to a turnover in the underlying continuum cannot be excluded; this interpretation raises serious questions concerning the total energy budget in the broad-line regions. In either case, there can be no reddening of the source of continuum emission other than that due to our own Galaxy.