Variations on timescales of minutes, hours and one day were observed in
our observations. The apparent efficiency for conversion of accreted matter
into energy is 
if 
within 
minutes
as on 1994 November 7 (Guibin et al. 1998). If 
mag in 
minutes as on 1994 November 8, the 
. No matter what
the exact value of 
is, there must be relativistic beaming to explain
microvariability, since is always larger than 0.1. The size of the
emitting region corresponding to the fastest variation (in a timescale of
minutes) is about 
cm in the frame of the blazar if a
Doppler factor 5 is assumed (Valtaoja et al. 1994). On the basis of the
smallest observed periods, the mass of central black hole is less than 
, and the radius of the event horizon of central
black hole is less than 1013 cm, according to the formula given by
Wiita (1985). The size of the emitting region corresponding to the fastest
variations is larger than the radius of event horizon of the black hole.
Therefore, the rapid variability might result from vortices and magnetic
flux tubes in the accretion disk, which facilitate the escape of collimated
radiation from deep within the hot disk, producing rapid flux variability
(Abramowicz 1992). The variations could also come from shocks within the
jet. Shocks in turbulent jets will lead to variations of local emissivity
and produce outbursts, due to compression of magnetic field in the shock
(Hughes et al. 1985; K
nigl & Choudhuri 1985;
Jones 1988); fluctuations of injection of particles and acceleration process may give
rise to rapid variability.