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It is well-known that the $\gamma$-ray emission in blazars originate in therelativistic jet pointed at the observers. However, it is not clear whether theexact 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 lineregion (BLR) or farther out at 1-100 pc range. The former assumption has beensuccessfully used to model the spectral energy distribution of many blazars.However, simultaneous detection of TeV $\gamma$-rays along with GeV outburstsin some cases indicate that the emission region must be outside the BLR. Inaddition, GeV outbursts have sometimes been observed to be simultaneous withthe passing of a disturbance through the so called "VLBI core," which islocated tens of pc away from the central engine. Hence, the exact location of$\gamma$-ray emission remains ambiguous. Here we present a method we havedeveloped to constrain the location of the emission region. We identifysimultaneous months-timescale GeV and optical outbursts in the light curvesspanning over 8 years of a sample of eleven blazars. Using theoretical jetemission models we show that the energy ratio of simultaneous optical and GeVoutbursts is strongly dependent on the location of the emission region.Comparing the energy dissipation of the observed multi-wavelength outbursts andthat of the simulated flares in our theoretical model we find that most of theabove outbursts originate beyond the BLR at approximately a few pc from thecentral engine.

Locating the GeV Emission Region in the Jets of Blazars from Months-Timescale Multi-Wavelength Outbursts