A. Neronov, Ie. Vovk

We notice that Galactic sources of PeV gamma-rays are generally expected to be surrounded by multi-degree-scale extended emission produced as a result of injection of electron-positron pairs in the interstellar medium from interactions of PeV gamma-rays with microwave photons. The surface brightness of such emission may be variable in time because of variability of the PeV emission power of its parent source. A flare of the parent source produces an ``echo'' spreading across the extended emission region that may be detectable on the time scales from decades to millennia. We consider a possibility that the very extended PeV source in the Cygnus region is the produced by the Cygnus X-3 PeV gamma-ray source, possibly being an echo of its past flare.