Gaia Celestial Reference Frame

The Gaia Celestial Reference Frame , abbreviated GCRF or Gaia-CRF, is a catalog of quasars obtained from the data of the Gaia mission . So far, two versions of the GCRF have been published as sub-catalogs of the Gaia catalogs, the third version is due to appear at the end of 2020. Future releases will always contain updated versions.

Quasars are very bright and very far away, so that the proper motion and parallax of these objects can no longer be measured and is therefore assumed to be close to zero. Thus they form a non-rotating frame of reference for the measurements of the star positions . The zero point for this measuring system is the barycenter of the solar system, the axis of rotation is fixed in relation to these distant objects and corresponds to that of the ICRF . The position data from ICRF comes from radio sources that have been measured very precisely using Very Long Baseline Interferometry with radio telescopes in various frequency bands. Gaia, on the other hand, records these objects in the optical range. Based on the photometric data, Gaia can clearly identify quasars and differentiate them from other point light sources.

GCRF1

Gaia DR1 from September 14, 2016 contains the positions of 2152 ICRF2 quasars in the sub -catalog Gaia Celestial Reference Frame 1 or GCRF1 or Gaia-CRF1 . These quasars formed an initial frame of reference for the Gaia mission. For this catalog, equivalents of the ICRF catalog have been selected. Only objects from the ICRF catalogs that are bright enough not only in the radio and infrared range but also in the sensitivity range of the Gaia sensors could be included in Gaia DR1.

The G magnitudes of these quasars range from 12.4 to 21.0, with the vast majority between 17 and 20. The optical positions achieve an accuracy of ~ 0.25 mas at G <17 and grow with increasing magnitude to a few mas at G = 20. Overall, a very good correspondence between the radio positions and the optical positions was found. The deviation is <1 mas for 44% and <10 mas for 94% of the objects.

GCRF2

The Gaia Celestial Reference Frame 2 , abbreviated GCRF2 or Gaia-CRF2 is a result of the Gaia mission and is a sub- catalog of the Gaia DR2 catalog, which was published on April 25, 2018. GCRF2 contains the positions of 556,869 quasars with G magnitudes from ≃ 16 to 21.

For GCRF2 the frame of reference was compared with a preliminary version of ICRF3 . ICRRF3 was published in its final version a few months after Gaia DR2 on August 30, 2018. A subset of 2820 common objects matches well in position with the pre-release version of ICRF3. In the optical range, GCRF2 achieved an accuracy in the positions that is comparable to the objects of the ICRF3, but contains many more objects. GCRF2 is the first optical reference frame that meets the requirements of the International Celestial Reference System .

GCRF2 replaced the Hipparcos Celestial Reference Frame (HCRF), the most accurate optical reference frame to date supplied by the Hipparcos astronomy satellite .

The quasars of GCRF2 are relatively evenly spaced across the sky, but with about 10 to 50 quasars per square degree, more than 100 times denser than those of the ICRF. In the galactic plane, however, they are completely absent, since they are covered by the galactic disk in the optical area.

In contrast to the other objects, the quasars of Gaia DR2 have not yet been identified and compared using the photometric data, but using existing catalogs. For this adjustment, the pre-release version of ICRF3 and 1.4 million objects have been containing AllWISE AGN-catalog is used, the active galactic nuclei visible recorded in the middle infrared range up to a magnitude of the 26th So far, Gaia has been able to find an optical equivalent at least once for around half of the objects.

Uncertainties

G <18: 0.12 mas
G <20: 0.5 mas
G ≥ 20: 0.8
systematic uncertainty 20-30 μas

GCRF3

The GCRF3 catalog with around 1,500,000 objects is to be released together with the Gaia EDR3 catalog at the end of 2020 according to planning. Later versions of the Gaia Celestial Reference Frame should identify all quasars within the magnitude limit directly on the basis of their photometric data, so that the comparison with ICRF and the AllWISE-AGN catalog is not necessary.

Individual evidence

↑ F. Mignard, p Klioner, L. Lindegren, U. Bastian, A. Bombrun: Gaia Data Release 1: Reference frame and optical properties of ICRF sources . In: Astronomy & Astrophysics . tape 595 , November 2016, ISSN 0004-6361 , p. A5 , doi : 10.1051 / 0004-6361 / 201629534 ( aanda.org [accessed August 24, 2020]).
↑ ^a ^b ^c Gaia Collaboration: F. Mignard, S. Klioner, L. Lindegren, J. Hernandez, U. Bastian, A. Bombrun et al .: Gaia Data Release 2; The Celestial reference frame (Gaia-CRF2) . Ed .: A&A. April 25, 2018, arxiv : 1804.09377 ( arxiv.org [PDF]).
↑ ALLWISEAGN - AllWISE Catalog of Mid-IR AGNs. Retrieved July 7, 2018 .
↑ F. Mignard, SA Klioner, L. Lindegren, J. Hernández, U. Bastian: Gaia Data Release 2 - The celestial reference frame (Gaia CRF2) . In: Astronomy & Astrophysics . tape 616 , August 1, 2018, ISSN 0004-6361 , p. A14 , doi : 10.1051 / 0004-6361 / 201832916 ( aanda.org [accessed August 31, 2020]).

[1] F. Mignard, p Klioner, L. Lindegren, U. Bastian, A. Bombrun: Gaia Data Release 1: Reference frame and optical properties of ICRF sources . In: Astronomy & Astrophysics . tape 595 , November 2016, ISSN 0004-6361 , p. A5 , doi : 10.1051 / 0004-6361 / 201629534 ( aanda.org [accessed August 24, 2020]).

[:1-2] Gaia Collaboration: F. Mignard, S. Klioner, L. Lindegren, J. Hernandez, U. Bastian, A. Bombrun et al .: Gaia Data Release 2; The Celestial reference frame (Gaia-CRF2) . Ed .: A&A. April 25, 2018, arxiv : 1804.09377 ( arxiv.org [PDF]).

[3] ALLWISEAGN - AllWISE Catalog of Mid-IR AGNs. Retrieved July 7, 2018 .

[4] F. Mignard, SA Klioner, L. Lindegren, J. Hernández, U. Bastian: Gaia Data Release 2 - The celestial reference frame (Gaia CRF2) . In: Astronomy & Astrophysics . tape 616 , August 1, 2018, ISSN 0004-6361 , p. A14 , doi : 10.1051 / 0004-6361 / 201832916 ( aanda.org [accessed August 31, 2020]).