THE OPTICAL-ULTRAVIOLET-GAMMA-RAY SPECTRUM OF 3C 279

We have obtained spectroscopy of the violently variable quasar 3C 279, simultaneous with gamma-ray observations, in 1992 April. Our combined optical (McDonald Observatory and CTIO) and ultraviolet (HST) observa tions, made when the source was faint, show a very steep power-law con tinuum (F(nu) is-proportional-to nu-1.95), and strong broad emission l ines. This is the first time that the broad ultraviolet lines of this object have been measured, and we note several unusual properties of t he spectrum. In particular, the profiles of C IV lambda1549 and Mg II lambda2798 are asymmetric, with very strong red wings, in contrast to the symmetric profile of Lyalpha C III] lambda1909, and possibly Hbeta . The observed asymmetry cannot be explained by a simple outflow assoc iated with the eruption of the source. In addition, the C IV lambda154 9/Lyalpha and C III] lambda1909/Lyalpha 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] lambda1909/C IV lambda1549 ratio is qu ite typical. 3C 279 was observed in the gamma-ray region by EGRET at t he same time as our optical-ultraviolet observations. The extrapolated ultraviolet continuum falls nine orders of magnitude below the gamma- ray point and we show that this, combined with the optical UV continuu m slope, is enough to rule out several synchrotron-self-Compton models suggested to explain the multiwavelength spectra of blazars. blazars.