Searcharxiv⌕ Search

arXiv subjects

Sergei A. Klioner

Publications and source records attributed to Sergei A. Klioner.

At least 19 recordsLinked to original sources
arXiv2026

The influence of standard relativistic effects on Gaia-like astrometric catalogs

With the observational accuracy of astrometric space telescopes such as Gaia, the Newtonian theoretical framework for positional astronomy is no longer appropriate. To model all effects relevant at the corresponding levels of accuracy, a more sophisticated description in the framework of General Relativity is required. It is generally known how various relativistic effects affect a given astrometric observation. In particular, the maximal light deflection of a given Solar System body can be easily estimated for a given observer. However, astrometric parameters for a given source are computed from a series of observations of that source. Depending on the relativistic effect, the fraction of observations affected, at some given level of accuracy, by that relativistic effect can vary and also be very small. Our goal is to investigate the impact of unaccounted relativistic effects on the astrometric parameters of a source observed by a Gaia-like astrometric telescope. The standard relativistic framework for Gaia is used to highlight such effects in some cases where they are most significant. Then, using the simulation tool AGISLab, we fit a set of Gaia-like observations of a given number of sources to the standard Gaia-like astrometric model for astrometric source parameters, but using different incomplete relativistic models. In this way we obtain the errors of the astrometric parameters if one were to drop different elements of the relativity model. A simplified analytical description of the errors is also provided and discussed. We demonstrate that, due to averaging over many observations per source, the errors in the astrometric parameters are drastically smaller than the corresponding relativistic effects in individual observations. This opens a way to potentially simplify the relativistic model for future astrometric projects aiming for higher accuracy than Gaia.

astro-ph.IM↗
arXiv2026

Relativistic time scales in the Solar system

This paper summarizes theoretical definitions of the relativistic coordinate time scales introduced by the IAU 2000 framework as well as practical aspects of their use. It is argued that the IAU framework already defines relativistic local GCRS-like reference systems and the corresponding TCG-like coordinates times for each body of the Solar system. The interrelations between the coordinate times and the proper time of an observer are discussed. The arguments put forward that any scaling of the local coordinate times like TCL for the Moon is unreasonable. Practical recipes of the transformations between TCB and the local coordinate time scales (TCG, TCL, etc) are then discussed. Time ephemerides giving the transformation between TCB and the local coordinate times at the center of mass of the corresponding body are computed for all major bodies of the Solar system using INPOP19a. Those time ephemerides represented as a standard set of Chebyshev polynomials are available online.

astro-ph.IM↗
arXiv2026

Post-Newtonian accelerations of a Mercury orbiter

We investigate the relativistic modeling of spacecraft motion in Mercury's post-Newtonian local coordinates. This investigation is motivated by the fact that Mercury's post-Newtonian gravitational field (as well as that of any other planet) admits an expansion in terms of multipole moments, which are most appropriately defined in the local reference system. The equations of motion in the Mercury-centric local frame include relativistic local perturbations, given by the Schwarzschild term, Lense-Thirring precession, and the acceleration due to the quadrupole moment, and relativistic third-body perturbations, which are the gravito-electric and gravito-magnetic accelerations, along with a coupling term between Mercury and other solar system bodies. The relativistic third-body perturbations are usually neglected in all practical applications. In this study, we analyze the magnitude of the post-Newtonian terms of the equations of motion formulated in the Mercury-centric frame, evaluating them along the trajectories of the two BepiColombo spacecrafts. Based on this analysis, we provide a practical approach for constructing a high-accuracy relativistic orbital model suitable for a Mercury orbiter.

gr-qc↗
arXiv2025

Gaia GraL: Gaia gravitational lens systems IX. Using XGBoost to explore the Gaia Focused Product Release GravLens catalogue

Aims. Quasar strong gravitational lenses are important tools for putting constraints on the dark matter distribution, dark energy contribution, and the Hubble-Lemaitre parameter. We aim to present a new supervised machine learning-based method to identify these lenses in large astrometric surveys. The Gaia Focused Product Release (FPR) GravLens catalogue is designed for the identification of multiply imaged quasars, as it provides astrometry and photometry of all sources in the field of 4.7 million quasars. Methods. Our new approach for automatically identifying four-image lens configurations in large catalogues is based on the eXtreme Gradient Boosting classification algorithm. To train this supervised algorithm, we performed realistic simulations of lenses with four images that account for the statistical distribution of the morphology of the deflecting halos as measured in the EAGLE simulation. We identified the parameters discriminant for the classification and performed two different trainings, namely, with and without distance information. Results. The performances of this method on the simulated data are quite good, with a true positive rate and a true negative rate of about 99.99% and 99.84%, respectively. Our validation of the method on a small set of known quasar lenses demonstrates its efficiency, with 75% of known lenses being correctly identified. We applied our algorithm (both trainings) to more than 0.9 million quadruplets selected from the Gaia FPR GravLens catalogue. We derived a list of 1127 candidates with at least one score larger than 0.75, where each candidate has two scores -- one from the model trained with distance information and one from the model trained without distance information -- and including 201 very good candidates with both high scores.

astro-ph.GA↗
arXiv2024

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $μ$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $μ$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

astro-ph.IM↗
arXiv2023

Gaia GraL: Gaia DR2 Gravitational Lens Systems. VIII. A radio census of lensed systems

We present radio observations of 24 confirmed and candidate strongly lensed quasars identified by the Gaia Gravitational Lenses (GraL) working group. We detect radio emission from 8 systems in 5.5 and 9 GHz observations with the Australia Telescope Compact Array (ATCA), and 12 systems in 6 GHz observations with the Karl G. Jansky Very Large Array (VLA). The resolution of our ATCA observations is insufficient to resolve the radio emission into multiple lensed images, but we do detect multiple images from 11 VLA targets. We have analysed these systems using our observations in conjunction with existing optical measurements, including measuring offsets between the radio and optical positions, for each image and building updated lens models. These observations significantly expand the existing sample of lensed radio quasars, suggest that most lensed systems are detectable at radio wavelengths with targeted observations, and demonstrate the feasibility of population studies with high resolution radio imaging.

astro-ph.GA↗
arXiv2023

Gaia Data Release 3: All-sky classification of 12.4 million variable sources into 25 classes

Gaia DR3 contains 1.8 billion sources with G-band photometry, 1.5 billion of which with BP and RP photometry, complemented by positions on the sky, parallax, and proper motion. The median number of field-of-view transits in the three photometric bands is between 40 and 44 measurements per source and covers 34 months of data collection. We pursue a classification of Galactic and extra-galactic objects that are detected as variable by Gaia across the whole sky. Supervised machine learning (eXtreme Gradient Boosting and Random Forest) was employed to generate multi-class, binary, and meta-classifiers that classified variable objects with photometric time series in the G, BP, and RP bands. Classification results comprise 12.4 million sources (selected from a much larger set of potential variable objects) and include about 9 million variable stars classified into 22 variability types in the Milky Way and nearby galaxies such as the Magellanic Clouds and Andromeda, plus thousands of supernova explosions in distant galaxies, 1 million active galactic nuclei, and almost 2.5 million galaxies. The identification of galaxies was made possible by the artificial variability of extended objects as detected by Gaia, so they were published in the galaxy_candidates table of the Gaia DR3 archive, separate from the classifications of genuine variability (in the vari_classifier_result table). The latter contains 24 variability classes or class groups of periodic and non-periodic variables (pulsating, eclipsing, rotating, eruptive, cataclysmic, stochastic, and microlensing), with amplitudes from a few milli-magnitudes to several magnitudes.

astro-ph.GA↗
arXiv2021

Gaia GraL: Gaia DR2 Gravitational Lens Systems. VII. XMM-Newton Observations of Lensed Quasars

We present XMM-Newton X-ray observations of nine confirmed lensed quasars at $1 \lesssim z \lesssim 3$ identified by the Gaia Gravitational Lens program. Eight systems are strongly detected, with 0.3--8.0 keV fluxes $F_{0.3-8.0} \gtrsim 5 \times 10^{-14}\ {\rm erg}\ {\rm cm}^{-2}\ {\rm s}^{-1}$. Modeling the X-ray spectra with an absorbed power law, we derive power law photon indices and 2--10 keV luminosities for the eight detected quasars. In addition to presenting sample properties for larger quasar population studies and for use in planning for future caustic crossing events, we also identify three quasars of interest: a quasar that shows evidence of flux variability from previous ROSAT observations, the most closely-separated individual lensed sources resolved by XMM-Newton, and one of the X-ray brightest quasars known at $z>3$. These sources represent the tip of discovery that will be enabled by SRG/eROSITA.

astro-ph.GA↗
arXiv2017

Gaia-like astrometry and gravitational waves

This paper discusses the effects of gravitational waves on high-accuracy astrometric observations such as those delivered by Gaia. Depending on the frequency of gravitational waves, two regimes for the influence of gravitational waves on astrometric data are identified: the regime when the effects of gravitational waves directly influence the derived proper motions of astrometric sources and the regime when those effects mostly appear in the residuals of the standard astrometric solution. The paper is focused on the second regime while the known results for the first regime are briefly summarized. The deflection of light due to a plane gravitational wave is then discussed. Starting from a model for the deflection we derive the corresponding partial derivatives and summarize some ideas for the search strategy of such signals in high-accuracy astrometric data. In order to reduce the dimensionality of the parameter space the use of vector spherical harmonics is suggested and explained. The explicit formulas for the VSH expansion of the astrometric signal of a plain gravitational wave are derived. Finally, potential sensitivity of Gaia astrometric data is discussed. Potential astrophysical sources of gravitational waves that can be interesting for astrometric detection are identified.

astro-ph.HE↗
arXiv2017

Impact of basic angle variations on the parallax zero point for a scanning astrometric satellite

Determination of absolute parallaxes by means of a scanning astrometric satellite such as Hipparcos or Gaia relies on the short-term stability of the so-called basic angle between the two viewing directions. Uncalibrated variations of the basic angle may produce systematic errors in the computed parallaxes. We examine the coupling between a global parallax shift and specific variations of the basic angle, namely those related to the satellite attitude with respect to the Sun. The changes in observables produced by small perturbations of the basic angle, attitude, and parallaxes are calculated analytically. We then look for a combination of perturbations that has no net effect on the observables. In the approximation of infinitely small fields of view, it is shown that certain perturbations of the basic angle are observationally indistinguishable from a global shift of the parallaxes. If such perturbations exist, they cannot be calibrated from the astrometric observations but will produce a global parallax bias. Numerical simulations of the astrometric solution, using both direct and iterative methods, confirm this theoretical result. For a given amplitude of the basic angle perturbation, the parallax bias is smaller for a larger basic angle and a larger solar aspect angle. In both these respects Gaia has a more favourable geometry than Hipparcos. In the case of Gaia, internal metrology is used to monitor basic angle variations. Additionally, Gaia has the advantage of detecting numerous quasars, which can be used to verify the parallax zero point.

astro-ph.IM↗
arXiv2016

Basic Celestial Mechanics

These are lecture notes of the lecture on elementary celestial mechanics read at Technische Universitaet Dresden, Dresden, Germany from 2000 till 2011.

astro-ph.IM↗
arXiv2016

Parametrized post-Newtonian equations of motion of N mass monopoles with the SEP violation

Post-Newtonian equations of motion of a system of $N$ mass monopoles in the framework of the PPN formalism with two parameters $β$ and $γ$ are derived for the case when the Strong Equivalence Principle can be violated. The derivation is based on the previously published general framework. The multipole moments of each body are defined as a PPN-generalization of the corresponding Blanchet-Damour multiple moments in a relevant local reference system defined for that body. The classical ten integrals of the derived equations of motion are given. A special version of the equations of motion, for which seven of these integrals are exact, is discussed. The derived equations of motion can be used to test the Strong Equivalence Principle in various solar system experiments.

gr-qc↗
arXiv2010

On the efficient computation of the quadrupole light deflection

Although the formulas for the light deflection due to quadrupole gravitational field of deflecting bodies are well known, the formulas are rather complicated, so that massive computations of quadrupole light deflection (e.g., in the framework of astrometric survey missions like Gaia) are time-consuming. Considering an observer situated within a few million kilometers from the Earth (clearly the most practical case), we derive the simplest possible form of the relevant formulas still having numerical accuracy of 1 micro-arcsecond. This form leads to simple upper estimates for the quadrupole light deflection in various cases allowing one to relate the magnitude of the actual quadrupole deflection with the corresponding monopole deflection due to the same body. These upper estimates can be used to decide if, for a given configuration, the actual quadrupole deflection should be computed for a given accuracy goal.

astro-ph.IM↗
arXiv2010

Efficient computation of the quadrupole light deflection

Efficient computation of the quadrupole light deflection for quasars/quasars and solar system objects within the framework of the baseline Gaia relativity model (GREM) is discussed. Two refinements have been achieved with the goal to improve the performance of the model: First, the quadrupole deflection formulas for both cases are simplified as much as possible considering the Gaia nominal orbit (only approximate minimal distances between Gaia and the giant planets were used here), physical parameters of the giant planets and the envisaged accuracy of 1 microarcsecond for individual systematic effects. The recommended formulas are given by Eq.(40) for stars/quasars and by Eq.(81) for solar system objects. Second, simple expressions for the upper estimate of the quadrupole light deflection have been found allowing, with a few additional arithmetical operations, to judge a priori if the quadrupole light deflection should be computed or not for a given source and for a given requested accuracy. The recommended criteria are given by Eq.(45) for stars/quasars and by Eq.(92) for solar system objects. Additionally, the quadrupole Shapiro effect for solar system objects is reconsidered. A strict upper bound of quadrupole Shapiro effect for solar system objects is given in Eq.(109).

astro-ph.EP↗
arXiv2010

Testing Local Lorentz Invariance with high-accuracy astrometric observations

This paper summarizes the analysis of the consequences of the violation of the Local Lorentz Invariance (LLI) on astrometric observations. We demonstrate that from the point of view of the LLI astrometric observations represent an experiment of Michelson-Morley type. The future high-accuracy astrometric projects (e.g., Gaia) will be used to test the LLI.

astro-ph.CO↗
arXiv2009

Using a coordinate-independent impact parameter in the post-post-Newtonian light deflection formulas

In [2] we have seen that among the post-post-Newtonian terms in the light deflection formulas there are "enhanced" ones that may become much larger than the other post-post-Newtonian terms. It is demonstrated here that these "enhanced" terms result from an inadequate choice of the impact parameter. Introducing another impact parameter, that can be considered as coordinate-independent, we demonstrate that all "enhanced" terms disappear from the formulas.

astro-ph.GA↗
arXiv2009

Analytical solution for light propagation in Schwarzschild field having an accuracy of 1 micro-arcsecond

Numerical integration of the differential equations of light propagation in the Schwarzschild metric shows that in some extreme situations relevant for practical observations (e.g. for Gaia) the well-known standard post-Newtonian formula for the boundary problem has an error up to 16 \muas. The aim of this note is to identify the reason for this error and to derive an extended formula accurate at the level of 1 \muas as needed e.g. for Gaia. The analytical parametrized post-post-Newtonian solution for light propagation derived by \citet{report1} gives the solution for the boundary problem with all analytical terms of order $\OO4$ taken into account. Giving an analytical upper estimates of each term we investigate which post-post-Newtonian terms may play a role for an observer in the solar system at the level of 1 \muas. We conclude that only one post-post-Newtonian term remains important for this numerical accuracy and derive a simplified analytical solution for the boundary problem for light propagation containing all the terms that are indeed relevant at the level of 1 \muas. The derived analytical solution has been verified using the results of a high-accuracy numerical integration of differential equations of light propagation and found to be correct at the level well below 1 \muas for arbitrary observer situated within the solar system.

astro-ph.IM↗