FIRST DETECTION IN GAMMA-RAYS OF A YOUNG RADIO GALAXY: FERMI-LAT OBSERVATIONS OF THE COMPACT SYMMETRIC OBJECT PKS 1718−649

Abstract

We report the γ-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 Fermi satellite. The third Fermi Gamma-ray LAT catalog (3FGL) includes an unassociated γ-ray source, 3FGL J1728.0−6446, located close to PKS 1718−649. Using the latest Pass 8 calibration, we confirm that the best-fit 1σ position of the γ-ray source is compatible with the radio location of PKS 1718−649. Cross-matching of the γ-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 ~ 36 (>5σ) with a best-fit photon spectral index Γ = 2.9 ± 0.3 and a 0.1–100 GeV photon flux density F _{0.1−100 GeV} = (11.5 ± 0.3) × 10⁻⁹ ph cm⁻² s⁻¹. We argue that the linear size (~2 pc), the kinematic age (~100 years), and the source distance (z = 0.014) make PKS 1718−649 an ideal candidate for γ-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 γ-rays establishes young radio galaxies as a distinct class of extragalactic high-energy emitters and yields a unique insight on the physical conditions in compact radio lobes interacting with the interstellar medium of the host galaxy.