First detection in gamma-rays of a young radio galaxy: Fermi-LAT observations of the Compact Symmetric Object PKS 1718-649Report as inadecuate

First detection in gamma-rays of a young radio galaxy: Fermi-LAT observations of the Compact Symmetric Object PKS 1718-649 - Download this document for free, or read online. Document in PDF available to download.

1 Laboratoire AIM 2 CfA - Harvard-Smithsonian Center for Astrophysics 3 USN - Unité Scientifique de la Station de Nançay 4 Astronomical Observatory 5 Dipartimento di Fisica

Abstract : 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 { m 0.1-100GeV}=$11.5$\pm$0.3$\times{ m 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.

Keywords : gamma-rays: galaxies galaxies: active-galaxies: individual PKS 1718-649 galaxies: jets radiation mechanisms: non-thermal

Author: Giulia Migliori - Aneta Siemiginowska - Malgorzata Sobolewska - Alan Loh - Stéphane Corbel - Łukasz Stawarz - Luisa Ostorero -



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