  Precise spectra of 3C 279 in the 0.5-70 keV range, obtained during two epochs
of
  \emph{Swift} and \emph{NuSTAR} observations, are analyzed using a
near-equipartition model. We apply a one-zone leptonic model with a
three-parameter log-parabola electron energy distribution (EED) to fit the
\emph{Swift} and \emph{NuSTAR} X-ray data, as well as simultaneous optical and
\emph{Fermi}-LAT $\gamma$-ray data. The Markov Chain Monte Carlo (MCMC)
technique is used to search the high-dimensional parameter space and evaluate
the uncertainties on model parameters. We show that the two spectra can be
successfully fit in near-equipartition conditions, defined by the ratio of the
energy density of relativistic electrons to magnetic field $\zeta_{\rm e}$
being close to unity. In both spectra, the observed X-rays are dominated by
synchrotron-self Compton photons, and the observed $\gamma$ rays are dominated
by Compton scattering of external infrared photons from a surrounding dusty
torus.
  Model parameters are well constrained. From the low state to the high state,
both the curvature of the log-parabola width parameter and the synchrotron peak
frequency significantly increase. The derived magnetic fields in the two states
are nearly identical ($\sim1$\ G), but the Doppler factor in the high state is
larger than that in the low state ($\sim$28 versus $\sim$18). We derive that
the gamma-ray emission site takes place outside the broad-line region, at
$\gtrsim$ 0.1 pc from the black hole, but within the dusty torus. Implications
for 3C 279 as a source of high-energy cosmic-rays are discussed.
