Multi-zone Modeling of Blazar Jets: Constraints from GeV-Optical Correlation and Short-Timescale Variability
We have developed a multi-zone model of blazar jet emission, in which the emission region contains many cells with individual magnetic fields and electron energy distributions. Nonthermal emission from radio to $\gamma$-rays is generated by electrons accelerated by shocks passing through the region via synchrotron and inverse-Compton (IC) processes. The optical and GeV variability at days-to-months time-scale simulated from our model are strongly correlated with no significant time lag, as observed in most blazars and indicated by the standard shock-in-jet model. However, the mechanism of the shorter time-scale variability has been less explored, although such fluctuations at X-rays, $\gamma$-rays and optical bands have been observed regularly in recent years. In our model, the hr time-scale variability of the synchrotron radiation is due to the spatial fluctuation of the magnetic field in the emission region. We found that to reproduce the short-timescale variability of the observed synchrotron emission in blazars, the required fluctuations of the magnetic field are in the range $1-2\%$ to $25-30\%$. Similar variability of the IC emission, which does not depend on the magnetic field, may be reproduced in our model by implementing equipartition of energy between the magnetic field and particles. We found that orphan flares in the optical or GeV band, or optical-GeV correlation with a significant time delay, as observed occasionally, may be reproduced in certain special conditions related to the orientation of the magnetic field in the cells.