The optical-ultraviolet-γ-ray spectrum of 3C 279

We have obtained spectroscopy of the violently variable quasar 3C 279, simultaneous with γ-ray observations, in 1992 April. Our combined optical (McDonald Observatory and CTIO) and ultraviolet (HST) observations, made when the source was faint, show a very steep power-law continuum (Fν ∝ ν^{- 1.95}), and strong broad emission lines. This is the first time that the broad ultraviolet lines of this object have been measured, and we note several unusual properties of the spectrum. In particular, the profiles of C IV λ11549 and Mg II λ2798 are asymmetric, with very strong red wings, in contrast to the symmetric profile of Lyα C III] λ1909, and possibly Hβ. The observed asymmetry cannot be explained by a simple outflow associated with the eruption of the source. In addition, the C IV λU1549/Lyα and C III] λ1909/Lyα line intensity ratios are the largest we have observed in our HST sample of more than 30 radio-loud quasars, even though the C III] λ1909/C IV λ1549 ratio is quite typical. 3C 279 was observed in the γ-ray region by EGRET at the same time as our optical-ultraviolet observations. The extrapolated ultraviolet continuum falls nine orders of magnitude below the γ-ray point and we show that this, combined with the optical UV continuum slope, is enough to rule out several synchrotron-self-Compton models suggested to explain the multiwavelength spectra of blazars.