Parameter constraints in a near-equipartition model with multifrequency NuSTAR, Swift, and Fermi-LAT data from 3C 279

Precise spectra of 3C 279 in the 0.5–70 keV range, obtained during two epochs of Swift and NuSTAR observations, are analysed using a near-equipartition model. We apply a one-zone leptonic model with a three-parameter log-parabola electron energy distribution to fit the Swift and NuSTAR X-ray data, as well as simultaneous optical and Fermi-LAT gamma-ray data. The Markov chain Monte Carlo 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 fitted in near-equipartition conditions, defined by the ratio of the energy density of relativistic electrons to magnetic field ζe 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 (∼1 G), but the Doppler factor in the high state is larger than that in the low state (∼28 versus ∼18). We derive that the gamma-ray emission site takes place outside the broad-line region, at ≳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.