Position accuracy of Fermi is in a range of 1–10 arcminutes. Bright γ-ray sources have week counterparts in optic range. Typically, there are tens of thousands of weak optic sources within the Fermi error ellipse.
Position accuracy of VLBI is in a range of 0.1–10 milliarcseconds (mas). This accuracy is more than enough to establish a counterpart in optic range. In fact, 60% VLBI detected sources have a counterpart in Gaia DR2 catalogue that is nearly complete up to 21 mag in G filter. VLBI is sensitive only to the radio emission from compact regions with angular size less than 1–10 mas. It is not sensitive to extended emission.
It was established that the majority of Fermi-detected γ-ray sources are active galaxy nuclea (AGNs). Since most of γ-ray loud AGNs are variable at scales from hours to years, its emission comes from parsec-scale regions of size less than 10 mas. There is a connection between parsec scale radio and γ-ray emission: γ-ray loud sources are in general radio-loud.
After association between Fermi and VLBI detections is established, the source is associated with an optical catalogue, such as Gaia. Association with the optical catalogue allows to find redshift in data archive or schedule redshift observations if not redshift information is found.
The total number of Fermi sources in 4FGL catalogue: 5098 The number of known galactic objects in 4FGL: 430 The 4FGL objects except known galactic objects: 4668 The number of VLBI associations with extragalactic objects: 2613 56% The number of VLBI associations not listed in the 4FGL: 144 3% The number of Gaia associations through VLBI association: 2380 51% The number of redshifts of gamma-ray loud AGNs found through VLBI association: 1394 27% The number of Fermi sources without VLBI association: 2485 49% The number of double associations of Fermi sources: 22 0.8% The probability of false association: 1.1%The list of 2613 VLBI associations in 4FGL catalogue.
The list of 2408 Fermi sources in 4FGL catalogue without VLBI associations. Does not include 77 extended sources.
The list of 144 Fermi sources in 4FGL catalogue with VLBI associations but listed as "not associated" in 4FGL.
Classification of 141 Fermi sources according to NED that are present in 4FGL catalogue with VLBI associations but listed as "not associated" in 4FGL (3 sources were not found in NED).
The list of 22 Fermi sources in 4FGL catalogue without two VLBI associations.