Radio Follow-up on all Unassociated Gamma-ray Sources from the Third Fermi Large Area Telescope Source Catalog

Abstract

The third Fermi Large Area Telescope (LAT) $γ$-ray source catalog (3FGL) contains over 1000 objects for which there is no known counterpart at other wavelengths. The physical origin of the $γ$-ray emission of those objects is unknown. Such objects are commonly referred to as unassociated and mostly do not exhibit significant $γ$-ray flux variability. We performed a survey of all unassociated $γ$-ray sources found in 3FGL using the Australia Telescope Compact Array and Very Large Array in the range of 4.0-10.0 GHz. We found 2097 radio candidates for association with $γ$-ray sources. The follow-up with very long baseline interferometry for a subset of those candidates yielded 142 new AGN associations with $γ$-ray sources, provided alternative associations for 7 objects, and improved positions for another 144 known associations to the milliarcsecond level of accuracy. In addition, for 245 unassociated $γ$-ray sources we did not find a single compact radio source above 2 mJy within 3$σ$ of their $γ$-ray localization. A significant fraction of these empty fields, 39%, are located away from the galactic plane. We also found 36 extended radio sources that are candidates for association with a corresponding $γ$-ray object, 19 of which are most likely supernova remnants or HII regions, whereas 17 could be radio galaxies.