  We test the synchrotron emission scenario for the very bright gamma-ray flare
of blazar 3C 279 observed in 2015 June using time-dependent numerical
simulations. A bulk Lorentz factor as high as 100 can bring the synchrotron
maximum energy above the GeV energy range. We find two possible solutions for
the X-ray to gamma-ray spectrum. One is a prompt electron injection model with
a hard power-law index as magnetic reconnection models suggest. A too strong
magnetic field yields a too bright synchrotron X-ray flux due to secondary
electron--positron pairs. Even in the prompt electron injection model, the
Poynting flux luminosity is at most comparable to the gamma-ray or electron
luminosity. Another model is the stochastic acceleration model, which leads to
a very unique picture accompanying the electromagnetic cascade and
re-acceleration of the secondary electron--positron pairs. In this model, the
energy budget of the magnetic field is very low compared to gamma rays and
electrons.
