Fermi-VLBI associations


Fermi is the γ-ray space telescope that records emission in a range of 20 MeV to 300 GeV. To date, over 5000 point γ-ray sources have been detected. VLBI stands for very long baseline interferometry is a technique of using a network of radio telescopes that works as a gigantic instrument of the size comparable with Earth. VLBI records emission in a range of 100 MHz to over 100 GHz.

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.

Association of a γ-ray through VLBI detection

Association of a source detected with two techniques with slightly different positions can established using the likelihood ratio: the ratio of the probability that this the same source and the difference in positions is due to random position errors to the probability these are two sources are unrelated, and a background sources was detected in the error ellipse. The first probability is depends on position accuracy. The second probability in the number of background sources. Since the number of ultra-compact radio sources detectable with VLBI is rather, the method of association via VLBI is very efficient. Roughly speaking, if a radio with parsec-scale emission at 8 GHz greater 10 mJy is found within Fermi error ellipse, the radio source is associated with a γ-ray object.

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.

Results of association γ-ray and VLBI associations

Association between Fermi 4FGL, Version 19 and VLBI rfc 2019b catalogues.
The total number of Fermi sources in 4FGL catalogue:                            5066
The number of known galactic objects in 4FGL:                                    430
The number of extended soiurces in 4FGL:                                          76
The 4FGL objects except known galactic objects:                                 4560
The number of VLBI associations with extragalactic objects:                     2612  57%
The number of Gaia associations through VLBI association:                       2381  51%
The number of redshifts of gamma-ray loud AGNs found through VLBI association:  1394  27%
The number of Fermi sources without VLBI association:                           2454  49%
The number of double associations of Fermi sources:                               22  0.8%
The list of 2612 VLBI associations in 4FGL catalogue.

The list of 2454 Fermi sources in 4FGL catalogue without VLBI associations. Does not include 77 extended sources.

The list of 22 Fermi sources in 4FGL catalogue without two VLBI associations.

Results of recent projects of VLBI and Fermi associations

Low resolution radio images of the Fermi fields with ATCA

VLASS mages of Fermi sources considered as asscociated by the Fermi team that were either not observed or not detected with VLBI.


  1. L. Petrov, E. K. Mahony, P. G. Edwards, E. M. Sadler, F. K. Schinzel, "Australia Telescope Compact Array observations of Fermi unassociated sources", 2013, MNRAS, 432, 1294   ADS link.
  2. F. Schinzel, L. Petrov, G. Taylor, E. Mahony, P. Edwards, Y. Kovalev, "New Associations of Gamma-Ray Sources from the Fermi Second Source Catalog", Astrophysical Journal, Supplement Series, 2015, 217, 4S   ADS link.
  3. Schinzel, F.K., Petrov, L., Taylor, G.B., Edwards, P.G. "Radio Follow-up on all Unassociated Gamma-ray Sources from the Third Fermi Large Area Telescope Source Catalog", 2017, ApJ, 838, 139S   ADS link.

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This web page was prepared by Leonid Petrov ()
Last update: 2019.12.29_14:05:00