RAPID VARIABILITY OF BLAZAR 3C 279 DURING FLARING STATES IN 2013−2014 WITH JOINTFERMI-LAT,NuSTAR,SWIFT, AND GROUND-BASED MULTI-WAVELENGTH OBSERVATIONS

We report the results of a multiband 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 γ -ray emission of the source 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 of 10 −5 photons cm −2 s −1 , and with a flux-doubling time scale as short as 2 hr. The γ -ray spectrum during one of the flares was very hard, with an index of , which is rarely seen in flat-spectrum radio quasars. The lack of concurrent optical variability implies a very high Compton dominance parameter . 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 at ∼4 keV. Modeling the broadband spectral energy distribution during this flare with the standard synchrotron plus inverse-Compton model requires: (1) the location of the γ -ray emitting region is comparable with the broad-line region radius, (2) a very hard electron energy distribution index , (3) total jet power significantly exceeding the accretion-disk luminosity , and (4) extremely low jet magnetization with . We also find that single-zone models that match the observed γ -ray and optical spectra cannot satisfactorily explain the production of X-ray emission.