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Blazars exhibit flares across the electromagnetic spectrum. Many $\gamma$-rayflares are highly correlated with flares detected at optical wavelengths;however, a small subset appears to occur in isolation, with little or novariability detected at longer wavelengths. These "orphan" $\gamma$-ray flareschallenge current models of blazar variability, most of which are unable toreproduce this type of behavior. We present numerical calculations of thetime-variable emission of a blazar based on a proposal by Marscher et al.(2010) to explain such events. In this model, a plasmoid ("blob") propagatesrelativistically along the spine of a blazar jet and passes through asynchrotron-emitting ring of electrons representing a shocked portion of thejet sheath. This ring supplies a source of seed photons that areinverse-Compton scattered by the electrons in the moving blob. The modelincludes the effects of radiative cooling, a spatially varying magnetic field,and acceleration of the blob's bulk velocity. Synthetic light curves producedby our model are compared to the observed light curves from an orphan flarethat was coincident with the passage of a superluminal knot through the innerjet of the blazar PKS 1510$-$089. In addition, we present Very Long BaselineArray polarimetric observations that point to the existence of a jet sheath inPKS 1510$-$089, thus providing further observational support for theplausibility of our model. An estimate of the bolometric luminosity of thesheath within PKS 1510$-$089 is made, yielding $L_{\rm sh} \sim 3 \times10^{45} ~ \rm erg ~ \rm s^{-1}$. This indicates that the sheath within PKS1510$-$089 is potentially a very important source of seed photons.

Through The Ring Of Fire: $\gamma$-Ray Variability In Blazars By A Moving Plasmoid Passing A Local Source Of Seed Photons