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
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
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
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
- 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
- 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
- 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
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This web page was prepared by Leonid Petrov
Last update: 2019.09.03_11:06:06