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

Abstract

The γ-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT and EGRET for three weeks between December 1992 and January 1993. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during the above 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 1230-2700Å interval is unusually flat for this object ( α U V ∼ 1). The flattening could represent the lowest energy tail of the inverse Compton component responsible for the X-ray emission, or could be due to the presence of a thermal component at ∼20000 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-and γ-ray fluxes are generally well correlated with the UV flux, approximately with square root and quadratic dependences, respectively.

Spectral Energy Distributions of 3C 279 Revisited:BeppoSAXObservations and Variability Models

“…estimated in Celotti, Padovani, & Ghisellini (1997) from the observed emission lines (see also Francis et al 1991) as ergs s~1. This value corresponds to D30% L BLR \ 6 ] 1044 of the accretion disk luminosity estimated by Pian et al (1999) from the UV observations during the 1993 campaign. Applying the model to the 1993 state, the one with lowest emission, we found that the best reproduction was obtained for an energy density ergs cm~3 ; this correu BLR D 10~4 sponds to a BLR radius cm, consistent R BLR \ 4 ] 1018 with the value obtained from the relation found R BLR -L BLR by Kaspi et al (2000).…”

WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007–2008

“…This lower limit to the temperature (and consequently to the luminosity) is constrained by the break of the SED in the optical-UV transition, but much hotter and hence more luminous discs are possible. For comparison, the thermal disc fitted by Pian et al (1999) to the UV data of 3C 279 has a temperature of 20 000 K and a luminosity of 2 × 10 45 erg s −1 .…”

The WEBT Campaign on the Blazar 3C 279 in 2006

“…The most extreme variability patterns include intraday variability with flux decays of P0.1 mag hr À1 ( Kartaltepe & Balonek 2007). Observations with the International Ultraviolet Explorer in the very low activity state of the source in 1992 DecemberY1993 January revealed the existence of a thermal emission component, possibly related to an accretion disk, with a luminosity of L UV $ 2 ; 10 46 erg s À1 if this component is assumed to be emitting isotropically (Pian et al 1999). Pian et al (1999) also identified an X-ray spectral variability trend in archival ROSAT data, indicating a lag of $2Y3 days of the soft X-ray spectral hardening behind a flux increase.…”