We report the results of a multi-band observing campaign on the famous blazar 3C 279 conducted during a phase of increased activity from 2013 December to 2014 April, including first observations of it with NuSTAR. The $gamma$-ray emission of the sour
ce measured by Fermi-LAT showed multiple distinct flares reaching the highest flux level measured in this object since the beginning of the Fermi mission, with $F(E > 100,{rm MeV})$ of $10^{-5}$ photons cm$^{-2}$ s$^{-1}$, and with a flux doubling time scale as short as 2 hours. The $gamma$-ray spectrum during one of the flares was very hard, with an index of $Gamma_gamma = 1.7 pm 0.1$, which is rarely seen in flat spectrum radio quasars. The lack of concurrent optical variability implies a very high Compton dominance parameter $L_gamma/L_{rm syn} > 300$. Two 1-day NuSTAR observations with accompanying Swift pointings were separated by 2 weeks, probing different levels of source activity. While the 0.5$-$70 keV X-ray spectrum obtained during the first pointing, and fitted jointly with Swift-XRT is well-described by a simple power law, the second joint observation showed an unusual spectral structure: the spectrum softens by $DeltaGamma_{rm X} simeq 0.4$ at $sim$4 keV. Modeling the broad-band SED during this flare with the standard synchrotron plus inverse Compton model requires: (1) the location of the $gamma$-ray emitting region is comparable with the broad line region radius, (2) a very hard electron energy distribution index $p simeq 1$, (3) total jet power significantly exceeding the accretion disk luminosity $L_{rm j}/L_{rm d} gtrsim 10$, and (4) extremely low jet magnetization with $L_{rm B}/L_{rm j} lesssim 10^{-4}$. We also find that single-zone models that match the observed $gamma$-ray and optical spectra cannot satisfactorily explain the production of X-ray emission.
We present time-resolved broad-band observations of the quasar 3C 279 obtained from multi-wavelength campaigns conducted during the first two years of the Fermi Gamma-ray Space Telescope mission. While investigating the previously reported gamma-ray/
optical flare accompanied by a change in optical polarization, we found that the optical emission appears delayed with respect to the gamma-ray emission by about 10 days. X-ray observations reveal a pair of `isolated flares separated by ~90 days, with only weak gamma-ray/optical counterparts. The spectral structure measured by Spitzer reveals a synchrotron component peaking in the mid-infrared band with a sharp break at the far-infrared band during the gamma-ray flare, while the peak appears in the mm/sub-mm band in the low state. Selected spectral energy distributions are fitted with leptonic models including Comptonization of external radiation produced in a dusty torus or the broad-line region. Adopting the interpretation of the polarization swing involving propagation of the emitting region along a curved trajectory, we can explain the evolution of the broad-band spectra during the gamma-ray flaring event by a shift of its location from ~ 1 pc to ~ 4 pc from the central black hole. On the other hand, if the gamma-ray flare is generated instead at sub-pc distance from the central black hole, the far-infrared break can be explained by synchrotron self-absorption. We also model the low spectral state, dominated by the mm/sub-mm peaking synchrotron component, and suggest that the corresponding inverse-Compton component explains the steady X-ray emission.
Recent multi-wavelength observations of 3C454.3, in particular during its giant outburst in 2005, put severe constraints on the location of the blazar zone, its dissipative nature, and high energy radiation mechanisms. As the optical, X-ray, and mill
imeter light-curves indicate, significant fraction of the jet energy must be released in the vicinity of the millimeter-photosphere, i.e. at distances where, due to the lateral expansion, the jet becomes transparent at millimeter wavelengths. We conclude that this region is located at ~10 parsecs, the distance coinciding with the location of the hot dust region. This location is consistent with the high amplitude variations observed on ~10 day time scale, provided the Lorentz factor of a jet is ~20. We argue that dissipation is driven by reconfinement shock and demonstrate that X-rays and gamma-rays are likely to be produced via inverse Compton scattering of near/mid IR photons emitted by the hot dust. We also infer that the largest gamma-to-synchrotron luminosity ratio ever recorded in this object - having taken place during its lowest luminosity states - can be simply due to weaker magnetic fields carried by a less powerful jet.
