Ultraviolet and multiwavelength variability of the blazar 3C 279: Evidencefor thermal emission

The gamma-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT, and EGRET for 3 weeks between 1992 December and 1993 January. Durin g this period, the blazar was at a historical minimum at all wavelengths. H ere we present the UV data obtained during this multiwavelength campaign. A maximum UV variation of similar to 50% is detected while during the same p eriod the X-ray flux varied by no more than 13%. At the lowest UV flux leve l, the average spectrum in the 1230-2700 Angstrom interval is unusually fla t for this object ([alpha(UV)] similar to 1). The flattening could represen t the lo lowest energy tail of the inverse Compton component responsible fo r the X-ray emission, or it could be due to the presence of a thermal compo nent at similar to 20,000 K, possibly associated with an accretion disk. Th e presence of an accretion disk in this blazar object, likely observable on ly in very low states and otherwise hidden by the beamed, variable synchrot ron component, would be consistent with the scenario in which the seed phot ons for the inverse Compton mechanism producing the gamma-rays are external to the relativistic jet. We further discuss the long-term correlation of t he UV flux with the X-ray and gamma-ray fluxes obtained at various epochs. All UV archival data are included in the analysis. Both the X-ray and gamma -ray fluxes are generally well correlated with the UV flux, with approximat ely square root and quadratic dependences, respectively.