  We report the discovery of fast variability of gamma-ray flares from blazar
OJ 287. This blazar is known to be powered by binary system of supermassive
black holes. The observed variability time scale T_var < 3-10 hr is much
shorter than the light crossing time of more massive (1.8x10^10 solar masses)
black hole and is comparable to the light crossing time of the less massive
(1.3x10^8 solar masses) black hole. This indicates that gamma-ray emission is
produced by relativistic jet ejected by the black hole of smaller mass.
Detection of gamma rays s with energies in excess of 10 GeV during the fast
variable flares constrains the Doppler factor of the jet to be larger than 4.
Possibility of the study of orbital modulation of emission from relativistic
jet makes OJ 287 a unique laboratory for the study of the mechanism(s) of
formation of jets by black holes, in particular, of the response of the jet
parameters to the changes of the parameters of the medium from which the black
hole accretes and into which the jet expands.
