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تسجيل مستخدم جديدFirst detection in gamma-rays of a young radio galaxy: Fermi-LAT observations of the Compact Symmetric Object PKS 1718-649
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We report the $gamma$-ray detection of a young radio galaxy, PKS 1718$-$649, belonging to the class of Compact Symmetric Objects (CSOs), with the Large Area Telescope (LAT) on board the {it Fermi} satellite. The third {it Fermi} Gamma-ray LAT catalog (3FGL) includes an unassociated $gamma$-ray source, 3FGL J1728.0$-$6446, located close to PKS 1718$-$649. Using the latest Pass 8 calibration, we confirm that the best fit $1 sigma$ position of the $gamma$-ray source is compatible with the radio location of PKS 1718$-$649. Cross-matching of the $gamma$-ray source position with the positions of blazar sources from several catalogs yields negative results. Thus, we conclude that PKS 1718$-$649 is the most likely counterpart to the unassociated LAT source. We obtain a detection test statistics TS$sim 36$ ($>$5$sigma$) with a best fit photon spectral index $Gamma=$2.9$pm$0.3 and a 0.1-100 GeV photon flux density $F_{rm 0.1-100GeV}=$(11.5$pm$0.3)$times{rm 10^{-9}}$ ph cm$^{-2}$ s$^{-1}$. We argue that the linear size ($sim$2 pc), the kinematic age ($sim$100 years), and the source distance ($z=0.014$) make PKS 1718$-$649 an ideal candidate for $gamma$-ray detection in the framework of the model proposing that the most compact and the youngest CSOs can efficiently produce GeV radiation via inverse-Compton scattering of the ambient photon fields by the radio lobe non-thermal electrons. Thus, our detection of the source in $gamma$-rays establishes young radio galaxies as a distinct class of extragalactic high-energy emitters, and yields an unique insight on the physical conditions in compact radio lobes interacting with the interstellar medium of the host galaxy.
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