Rapid high-amplitude gamma-ray variability in Blazar PKS 0507+17

We report a detailed analysis of $\gamma$-ray data of 2FGL J0509.9+1802 observed by the Large Area Telescope on board {\it Fermi} satellite, especially focusing on April 2013 when extraordinary $\gamma$-ray variability has been detected. Localization of $\gamma$-ray emission during this epoch suggests that the $\gamma$-ray source only associates with PKS 0507+17. The $\gamma$-ray emission of PKS 0507+17 is identified at the first time. The daily peak flux is over two orders of magnitude higher than the first two-year average flux, giving an isotropic $\gamma$-ray luminosity of $\simeq4\times10^{48}$ erg $\rm s^{-1}$. Rapid $\gamma$-ray variability with doubling time of 2-3 hours has been detected. Such a short doubling timescale has been detected for only a few bright blazars and indicates a location of $\gamma$-ray emission inside the broad line region. Together with the bluer-when-brighter $\gamma$-ray spectra, the variability phenomena could be well explained by the classic flat-spectrum radio quasar variability model that includes a fast injection of accelerated electrons and the external Compton cooling process.