The third Fermi Large Area Telescope (LAT) $\gamma$-ray source catalog (3FGL) contains over 1000 objects for which there is no known counterpart at other wavelengths. The physical origin of the $\gamma$-ray emission of those objects is unknown. Such objects are commonly referred to as unassociated and mostly do not exhibit significant $\gamma$-ray flux variability. We performed a survey of all unassociated $\gamma$-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 $\gamma$-ray sources. The follow-up with very long baseline interferometry for a subset of those candidates yielded 142 new AGN associations with $\gamma$-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 $\gamma$-ray sources we did not find a single compact radio source above 2 mJy within 3$\sigma$ of their $\gamma$-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 $\gamma$-ray object, 19 of which are most likely supernova remnants or HII regions, whereas 17 could be radio galaxies.
F. Schinzel, L. Petrov, G. Taylor, et. al.
Fri, 24 Feb 17
Comments: 14 pages, 9 figures, 6 tables, 5 machine readable tables, accepted for publication in ApJS