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We report the results of a multi-band observing campaign on the famous blazar3C 279 conducted during a phase of increased activity from 2013 December to2014 April, including first observations of it with NuSTAR. The $\gamma$-rayemission of the source measured by Fermi-LAT showed multiple distinct flaresreaching the highest flux level measured in this object since the beginning ofthe 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 radioquasars. The lack of concurrent optical variability implies a very high Comptondominance parameter $L_\gamma/L_{\rm syn} > 300$. Two 1-day NuSTAR observationswith accompanying Swift pointings were separated by 2 weeks, probing differentlevels of source activity. While the 0.5$-$70 keV X-ray spectrum obtainedduring the first pointing, and fitted jointly with Swift-XRT is well-describedby a simple power law, the second joint observation showed an unusual spectralstructure: the spectrum softens by $\Delta\Gamma_{\rm X} \simeq 0.4$ at $\sim$4keV. Modeling the broad-band SED during this flare with the standardsynchrotron plus inverse Compton model requires: (1) the location of the$\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 jetpower significantly exceeding the accretion disk luminosity $L_{\rm j}/L_{\rmd} \gtrsim 10$, and (4) extremely low jet magnetization with $L_{\rm B}/L_{\rmj} \lesssim 10^{-4}$. We also find that single-zone models that match theobserved $\gamma$-ray and optical spectra cannot satisfactorily explain theproduction of X-ray emission.

astrophysical journal/the astrophysical journal

Rapid Variability of Blazar 3C 279 during Flaring States in 2013-2014  with Joint Fermi-LAT, NuSTAR, Swift, and Ground-Based Multi-wavelength  Observations