  It is well-known that the $\gamma$-ray emission in blazars originate in the
relativistic jet pointed at the observers. However, it is not clear whether the
exact location of the GeV emission is less than a pc from the central engine,
such that it may receive sufficient amount of photons from the broad line
region (BLR) or farther out at 1-100 pc range. The former assumption has been
successfully used to model the spectral energy distribution of many blazars.
However, simultaneous detection of TeV $\gamma$-rays along with GeV outbursts
in some cases indicate that the emission region must be outside the BLR. In
addition, GeV outbursts have sometimes been observed to be simultaneous with
the passing of a disturbance through the so called "VLBI core," which is
located tens of pc away from the central engine. Hence, the exact location of
$\gamma$-ray emission remains ambiguous. Here we present a method we have
developed to constrain the location of the emission region. We identify
simultaneous months-timescale GeV and optical outbursts in the light curves
spanning over 8 years of a sample of eleven blazars. Using theoretical jet
emission models we show that the energy ratio of simultaneous optical and GeV
outbursts is strongly dependent on the location of the emission region.
Comparing the energy dissipation of the observed multi-wavelength outbursts and
that of the simulated flares in our theoretical model we find that most of the
above outbursts originate beyond the BLR at approximately a few pc from the
central engine.

High Energy Astrophysical Phenomena