Multi-band Variability Analysis of Mrk 421
  We have assembled the historical variability data of Mrk 421 at radio 15 GHz,
X-ray and gamma-ray bands, spanning about 6.3, 10.3 and 7.5 yr, respective- ly.
We analyzed the variability by using three methods. The results indicated that
there is a period of 287.6 +- 4.4 days for 15 GHz, 309.5 +- 5.8 days for X- ray
and 283.4+- 4.7 days for gamma-ray, respectively. This period can be reasonably
explained by the nonballistic helical motion of the emitting material. The cor-
relation analysis suggested that the variabilities of radio 15 GHz, X-ray and
0-ray are remarkable correlated, and the emission of radio 15 GHz lags behind
that of X-ray, and the X-ray flux lags behind the gamma-ray. This suggests that
the gamma-ray derives from inverse Compton (IC) scattering of the synchrotron
photons, supporting the synchrotron self-Compton (SSC) model. Moreover, the
time delay between different wavebands could be explained by the shock-in-jet
models, in which a moving emission region produces the radio to gamma-ray
activity, implying that the emission region of gamma-ray is closer to the
center than ones of X-ray and radio emission.
