  We argue that the bright flare of the binary pulsar \object{PSR
B1259$-$63/LS2883} detected by the {\it Fermi} Large Area Telescope (LAT), is
due to the inverse Compton (IC) scattering of the unshocked electron-positron
pulsar wind with a Lorentz factor $\Gamma_0 \approx 10^4$. The combination of
two effects both linked to the circumstellar disk (CD), is a key element in the
proposed model. The first effect is related to the impact of the surrounding
medium on the termination of the pulsar wind. Inside the disk, the "early"
termination of the wind results in suppression of its gamma-ray luminosity.
When the pulsar escapes the disk, the conditions for termination of the wind
undergo significant changes. This would lead to a dramatic increase of the
pulsar wind zone, and thus to the proportional increase of the gamma-ray flux.
On the other hand, if the parts of the CD disturbed by the pulsar can supply
infrared photons of density high enough for efficient Comptonization of the
wind, almost the entire kinetic energy of the pulsar wind would be converted to
radiation, thus the gamma-ray luminosity of the wind could approach to the
level of the pulsar's spin-down luminosity as reported by the {\it Fermi}
collaboration.
