Ultraviolet and Multiwavelength Variability of the Blazar 3C 279: Evidence for Thermal Emission

The γ-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. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during this multiwavelength campaign. A maximum UV variation of ~50% is detected, while during the same period the X-ray flux varied by no more than 13%. At the lowest UV flux level, the average spectrum in the 1230-2700 Å interval is unusually flat for this object (⟨αUV⟩ ~ 1). The flattening could represent the lowest energy tail of the inverse Compton component responsible for the X-ray emission, or it could be due to the presence of a thermal component at ~20,000 K, possibly associated with an accretion disk. The presence of an accretion disk in this blazar object, likely observable only in very low states and otherwise hidden by the beamed, variable synchrotron component, would be consistent with the scenario in which the seed photons for the inverse Compton mechanism producing the γ-rays are external to the relativistic jet. We further discuss the long-term correlation of the UV flux with the X-ray and γ-ray fluxes obtained at various epochs. All UV archival data are included in the analysis. Both the X-ray and γ-ray fluxes are generally well correlated with the UV flux, with approximately square root and quadratic dependences, respectively.