Description du sujet de thèse

Context, motivations, and scientific goals

The extragalactic celestial reference frame (CRF) is a practical realization of a quasi-inertial system based on precise positions of compact distant sources (quasars) with crucial applications in astronomy and geodesy. Today, two fundamental CRF realizations coexist: the ICRF3 realized by geodetic very long baseline interferometry (VLBI), and the optical ESA's Gaia-CRF3 of ~1.6 million objects both with median precisions of ~0.25 mas.
There are several conflicts in searching for the best reference sources, stable and with a short optical-radio link. First, quasar centroids as seen by VLBI are “wandering” over time scales of few months to decades by up to a mas/yr due the evolution of the radio jet. In addition, the common ICRF3/Gaia sources exhibit significant optical-radio offsets of few 0.1 mas to several mas, preventing from realizing a wavelength-independent CRF.
In this thesis, we propose to, firstly, revise the VLBI analysis methods currently in use to better understand the propagation of systematic errors, particularly those arising from the CRF instabilities. Then, the student will investigate the links between various source properties, especially the trends between photometric variability and astrometry suggested by recent studies in our team to, finally, be able to select the best reference sources on the basis of their intrinsic properties and apply the selections to operational VLBI analysis.

Work

As there is no unique way of analyzing VLBI delays, we propose first to focus on the VLBI analysis configuration, study the possible variants, especially in terms of handling the reference points, and quantify their impacts in the various VLBI products. One question generally eluded is how the wandering of quasars can impact the solution in terms of noise and systematic errors and whether some analysis configurations can minimize this impact. This step is particularly motivated by the additional 7 years since the ICRF3 production and by the increasing rate of data that justify to reconsider the classical versus alternative analysis configurations.
The next step will be a thorough study of the characteristics of the population of sources common to VLBI and Gaia in the light of all recent studies. The student will search for trends between the astrometric signatures, photometric properties and variability that arises from internal processes and influenced by the geometry of disk-jet system, the redshift, the known properties of the central engine, the spectral properties and classifications, the parsec-scale radio structure. He/she will determine whether the 'good' astrometric sources, i.e., suitable to define stable reference points with short multiwavelength link between radio and optical bands can be selected on the basis of photometric properties and possibly of their intrinsic properties. Conversely, the astrophotometric signatures measured by VLBI and Gaia could also allow to predict some of these intrinsic properties. This step can be approached using classical correlations or more innovative methods of multivariate analysis and we anticipate here an opportunity of using machine learning that could help in predicting properties or classifying sources.
A last step will be to make explicit selections of the best astrometric sources and test their efficiency in real VLBI analysis. Following our preliminary tests, the selection of adapted sources could improve by a factor of 2 to 10 the alignment between optical and radio frames and the radio frame stability which would constitute a serious achievement.

Contexte de travail

He/she will work alongside the members of the REFAG team at LTE under the supervision of an assistant astronomer and will also coordinate his/her work with a postdoctoral fellow during part of the thesis. The LTE is a laboratory specialising in the definition and implementation of time scales and spatial landmarks. It is located on the Paris site of the Paris Observatory (XIVth arrondissement). The REFAG team is specialized in the processing of space geodesy and VLBI astrometry data and their interpretation in terms of geodynamics and AGN physics. It also hosts several components of international services related to astrometry and geodesy.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.