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We propose a model to explain the ultra-bright GeV gamma-ray flares observedfrom the blazar 3C454.3. The model is based on the concept of a relativisticjet interacting with compact gas condensations produced when a star (red giant)crosses the jet close to the central black hole. The study includes ananalytical treatment of the evolution of the envelop lost by the star withinthe jet, and calculations of the related high-energy radiation. The modelreadily explains the day-long, variable on timescales of hours, GeV gamma-rayflare from 3C454.3, observed during November 2010 on top of a weeks-longplateau. In the proposed scenario, the plateau state is caused by a strong windgenerated by the heating of the star atmosphere by nonthermal particlesaccelerated at the jet-star interaction region. The flare itself could beproduced by a few clouds of matter lost by the red giant after the initialimpact of the jet. In the framework of the proposed scenario, the observationsconstrain the key model parameters of the source, including the mass of thecentral black hole: $M_{\rm BH}\simeq 10^9 M_{\odot}$, the total jet power:$L_{\rm j}\simeq 10^{48}\,\rm erg\,s^{-1}$, and the Doppler factor of thegamma-ray emitting clouds, $\delta\simeq 20$. Whereas we do not specify theparticle acceleration mechanisms, the potential gamma-ray production processesare discussed and compared in the context of the proposed model. We argue thatsynchrotron radiation of protons has certain advantages compared to otherradiation channels of directly accelerated electrons.

Star-Jet Interactions and Gamma-Ray Outbursts from 3C454.3