  Blazars radiate from radio through gamma-ray frequencies thereby being ideal
targets for multifrequency studies. Such studies allow constraining the
properties of the emitting jet. 3C 279 is among the most notable blazars and
therefore subject to extensive multifrequency campaigns. We report the results
of a campaign ranging from near-IR to gamma-ray energies of an outburst of 3C
279 in June 2015. The overall spectral energy distribution from near-IR to
gamma rays can be well represented by either a leptonic or a lepto-hadronic
radiation transfer model. Even though the data are equally well represented by
the two models, their inferred parameters challenge the physical conditions in
the jet. In fact, the leptonic model requires parameters with a magnetic field
far below equipartition with the relativistic particle energy density. On the
contrary, equipartition may be achieved with the lepto-hadronic model, which
however implies an extreme total jet power close to Eddington luminosity.
