17 June 2021
The positions given by absolute astrometry (e.g., ICRF3 and Gaia) are currently available at 4 frequencies (8, 22, 32 GHz and optical) for a sample of a few hundred extragalactic radio sources thanks to the combined work of the Gaia consortium and the ICRF3 working group and in which SYRTE actively contributes. Recall that it is these celestial objects, otherwise called AGN or quasars, housing supermassive black holes and located up to billions of light years, that form the celestial reference points. They are therefore of a certain astrophysical interest and a proven geodesic interest.
The accuracies of the optical positions of the latest data release of Gaia (EDR3) are comparable to those at 8 GHz (median error of 0.2 mas on common sources). The optical and radio centroids, however, do not coincide exactly, and the question arises as to why. In a study with Niu Liu (who spent a collaborative year at SYRTE during his thesis at Nanjing University and currently in postdoc in Nanjing), we showed that positions are dependent on systematic errors, especially in VLBI and more particularly in K and Ka bands (22 and 32 GHz). These systematic errors are caused by the north-south asymmetry of the VLBI network and, in the case of the Ka-band, a network that is too weak. In a more recent study, we looked for intrinsic causes at the sources by comparing the radio-radio and radio-optical differences with the radio structures given by the VLBA as part of the American MOJAVE project. This study provided the following initial overview:
• The radio centroids X, K and Ka are close to one another (< 0.1 mas) and arranged in order of increasing frequencies towards the inside of the jet, i.e. towards the central black hole, and coherently with a frequency-dependent opacity effect;
• The optical centroids are "in the jet" generally outwards and significantly distant from the radio centroids (> 0.1 mas up to several mas);
• Sometimes these optical centroids coincide with a radio component in the jet. This component, often stationary, can possess a high degree of linear polarization in radio, testifying to a well-organized magnetic field typical of a synchrotron emission. A high degree of optical polarization was also recorded for these cases by Yuri Kovalev and colleagues in an independent study;
• Moreover, when the Gaia centroid is inside the jet vis-à-vis the radio, we rather have a weak optical polarization calling to think that we see the signature of the accretion disk.
The different regimes are illustrated in Fig. 1.
Figure 1. General diagram of an extragalactic radio source.
What are the consequences for celestial reference points that will now be multi-wavelength? If optical and radio are emitted by the same component and it corresponds to the core, near the base of the jet, without proper motion, the link is easy. If they are emitted by two different components but both accessible by radio, we will have a long (mas-scale) link but accessible by two techniques, by means of a structure map like those proposed by MOJAVE. In both cases, the links are quantifiable source by source through observations of structure and radio and optical polarization. This is what should be encouraged in observation programs such as MOJAVE or the USNO FRAMEx project.
Figure 2. On the left, the position of the optical and radio centroids relative to the X-band centroid and projected along or perpendicular to the direction of the radio jet (at the bottom, the median locations for each wavelength). On the right, three examples of observed configurations including one with coincidence between Gaia and a MOJAVE component in the jet. 0203-120 corresponds to configuration (1) in Figure 1 and 0346+800 in case (2). 0536+145 could be a case (3).
References :
- Lambert S, Niu L, Arias EF, Barache C, Souchay J, Taris F, Liu JC, Zhu Z 2021, Parsec-scale alignments of radio-optical offsets with jets in AGNs from multifrequency geodetic VLBI, Gaia EDR3 and the MOJAVE program, Astronomy and Astrophysics, in press
- Liu N, Lambert S, Charlot P, Zhu Z, Liu JC, Jian N, Wan XS, Ding CY 2021, Comparison of multifrequency positions of extragalactic sources from global geodetic VLBI monitoring program and Gaia EDR3, Astronomy and Astrophysics, in press