  Multiwavelength monitoring of the blazar 3C 279 observed a very bright,
12-hour, orphan gamma-ray flare on 20 Dec 2013 with a uniquely hard Fermi-LAT
spectrum and high Compton dominance. We work with a one-zone, leptonic model
with both first- and second-order Fermi acceleration, which now reproduces the
unique flaring behavior. We present a simplified analytic electron energy
distribution to provide intuition about how particle acceleration shapes
multi-wavelength blazar jet emission spectra. The contributions of individual
processes in relativistic jets is fundamental to understanding the particle
energy budget in the formation and propagation of astrophysical jets. We show
that first- and second-order Fermi acceleration are sufficient to explain the
flare, and that magnetic reconnection is not needed. Our analysis suggests that
the flare is initiated by an increase in the particle energies due to shock
acceleration, which also increases the stochastic acceleration. The higher
energy particle preferentially occupy the outer jet, along the sheath, which
decreases the apparent magnetic field and synchrotron radiation, while
increasing electron exposure to the broad line region photon fields, driving up
the external Compton emission.
