Celestial Reference Frame Research Articles

THE CELESTIAL REFERENCE FRAME AT 24 AND 43 GHz. II. IMAGING

We have measured the sub-milli-arcsecond structure of 274 extragalactic sources at 24 and 43 GHz in order to assess their astrometric suitability for use in a high frequency celestial reference frame (CRF). Ten sessions of observations with the Very Long Baseline Array have been conducted over the course of $\sim$5 years, with a total of 1339 images produced for the 274 sources. There are several quantities that can be used to characterize the impact of intrinsic source structure on astrometric observations including the source flux density, the flux density variability, the source structure index, the source compactness, and the compactness variability. A detailed analysis of these imaging quantities shows that (1) our selection of compact sources from 8.4 GHz catalogs yielded sources with flux densities, averaged over the sessions in which each source was observed, of about 1 Jy at both 24 and 43 GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25% relative to their mean values, with variations in the session-to-session flux density scale being less than 10%, (3) sources were found to be more compact with less intrinsic structure at higher frequencies, and (4) variations of the core radio emission relative to the total flux density of the source are less than 8% on average at 24 GHz. We conclude that the reduction in the effects due to source structure gained by observing at higher frequencies will result in an improved CRF and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.

THE CELESTIAL REFERENCE FRAME AT 24 AND 43 GHz. I. ASTROMETRY

We present astrometric results for compact extragalactic objects observed with the Very Long Baseline Array at radio frequencies of 24 and 43 GHz. Data were obtained from ten 24 hr observing sessions made over a five-year period. These observations were motivated by the need to extend the International Celestial Reference Frame (ICRF) to higher radio frequencies to enable improved deep space navigation after 2016 and to improve state-of-the-art astrometry. Source coordinates for 268 sources were estimated at 24 GHz and for 131 sources at 43 GHz. The median formal uncertainties of right ascension and declination at 24 GHz are 0.08 and 0.15 mas, respectively. Median formal uncertainties at 43 GHz are 0.20 and 0.35 mas, respectively. Weighted root-mean-square differences between the 24 and 43 GHz positions and astrometric positions based on simultaneous 2.3 and 8.4 GHz Very Long Baseline Interferometry observations, such as the ICRF, are less than about 0.3 mas in both coordinates. With observations over five years we have achieved a precision at 24 GHz approaching that of the ICRF but unaccounted systematic errors limit the overall accuracy of the catalogs.

Optical counterpart positions of extragalactic radio sources and connecting optical and radio reference frames

We discuss the results of an investigation of astrometric positions of extragalactic radio sources from a list for the International Celestial Reference Frame. About 300 fields around extragalactic radio sources were observed during the years 2000-2003. The observations were performed mainly using two telescopes equipped with CCD cameras at TUG, Turkey (Russian-Turkish Telescope RTT150) and at YAO (1 m telescope), (Kunming, China). The mean accuracies of the measured positions are 38 mas in right ascension and 35 mas in declination. A comparison between the measured optical positions determined using the UCAC2 catalog and the radio positions from the current ICRF shows that the overall optical-minus-radio offsets are -4 and + 15 mas for right ascension and declination, respectively. The formal internal errors of these mean offsets are 4 mas. The results of optical positions with respect to the reference catalogue 2MASS are also given. A search for a relation between optical and radio reference frames indicates that the orientation angles are near zero within their accuracy of about 5 mas. The link accuracy becomes 3 mas when our observations are combined with other studies.

Future radio reference frames and implications for the Gaia link

Since January 1st, 2010, the IAU (International Astronomical Union) fundamental celestial reference frame has been the 2nd International Celestial Reference Frame (ICRF2), which is composed of Very Long Baseline Interferometry (VLBI) positions for more than 3000 extragalactic radio sources. This frame is constantly improving through joint efforts of the VLBI community. By surveying the whole sky up to magnitude 20, the European space astrometric mission Gaia will soon create its own celestial reference frame directly in the optical domain and with many more sources. By 2015–2020, the two frames will thus cohabit and it will be important to align these to the highest accuracy for consistency between optical and radio positions. In this paper, we present the various observational approaches that are undertaken to improve the VLBI frame in the future. These include extension to weaker sources for densification, extension to higher radio frequencies to take advantage of the more compact morphology of the sources at these frequencies, and further observations in the southern hemisphere for homogeneous sky coverage. We also elaborate on how such future radio frames should contribute to highly-precise alignment between the VLBI and Gaia frames within the next decade.

QSO Survey and Reference Frame with Gaia

The Gaia space astrometric mission will construct its own celestial reference frame based on thousands of QSO positions measured with an accuracy of a few tens of microarcseconds. For consistency with the current realization of the International Celestial Reference Frame, which is based on Very Long Baseline Interferometry (VLBI) positions for more than 3000 extragalactic radio sources, it is crucial that the two frames be aligned with the highest possible accuracy. This alignment requires a number of common objects with excellent optical and radio astrometric properties. Because of their physical properties, it is possible that the QSO optical and radio positions do not physically coincide at the 100 microarcsecond level. While such shifts may affect the accuracy in aligning the two frames, they may also help gaining insights into the physical conditions in the nuclear regions of these objects. This paper reviews the ongoing work to align the VLBI and Gaia frames along with the potential astrophysical outcome that may result from comparing/combining ultra-accurately these two frames.

Apparent close approaches between near-Earth asteroids and quasars

Aims. We investigate the link between the International Celestial Reference Frame (ICRF) and the dynamical reference frame realized by the ephemerides of the Solar System bodies. Methods. We propose a procedure that implies a selection of events for asteroids with accurately determined orbits crossing the CCD field containing selected quasars. Using a Bulirsch-Stoer numerical integrator, we constructed 8-years (2010-2018) ephemerides for a set of 836 numbered near-Earth asteroids (NEAs). We searched for close encounters (within a typical field of view of groundbased telescopes) between our selected set of asteroids and quasars with high-accuracy astrometric positions extracted from the Large Quasars Astrometric Catalog (LQAC). Results. In the designated period (2010-2018), we found a number of 2924, 14 257, and 6972 close approaches (within 10') between asteroids with a minimum solar elongation value of 60◦and quasars from the ICRF-Ext2, the Very Large Baseline Array Calibrator Survey (VLBA-CS), and the Very Large Array (VLA), respectively. This large number of close encounters provides the observational basis needed to investigate the link between the dynamical reference frame and the ICRF.

Pulsar timing array

AbstractSimultaneous timing of several pulsars distributed over the sky, so called Pulsar Timing Array (PTA), is used for a variety of metrological and astronomical applications. Three examples of PTA application are presented: link between celestial reference frames, ensemble pulsar time scale and detection of gravitational waves.

Instability of the celestial reference frame and effect on UT1

AbstractIt was shown that the ICRF radio sources including the defining sources have significant apparent motion that leads to rotation of the ICRF. This rotation is transformed to secular variations of EOP that is decreased or removed if motion of sources is took into account.

Celestial pole offsets from lunar laser ranging and comparison with VLBI

Context. The analysis of lunar laser ranging (LLR) observations is based on determining the round- trip travel times of light pulses between stations on the Earth and reflectors on the surface of the Moon. Several works have demonstrated that this technique is powerful in various domains including astronomy, geodynamics and gravitational physics. Aims. In the field of geodynamics, LLR contributes to the realization of a dynamical celestial reference frame, in contrast to very long baseline interferometry (VLBI) that determines a kinematical celestial reference frame. In this paper, we have determined corrections to the celestial pole coordinates, denoted X, Y, using LLR observations. This determination is of particular interest for comparison with the one obtained from VLBI observations. The main purpose is to study the benefits of LLR for the determination of the celestial pole coordinates and second how to best combine the time series obtained from both techniques. Methods. For these determinations, data acquired by LLR tracking stations since 1969 were analyzed and corrections to the nutation terms estimated using a weighted least square fit. Finally, LLR data were combined with the IVS combined VLBI series of 23-year duration. Results. We have demonstrated the possibility of determining the celestial pole offsets from LLR data even though the results are not as accurate nor as dense as those obtained with VLBI. This work provides some external constraints to the celestial pole coordinates derived from VLBI observations. Moreover, the LLR determination of the long periodic nutation terms shows an improvement with respect to previous studies. The combination of LLR and VLBI series may indicate that the combined series reveal details that do not appear in the VLBI series alone.

Effect of asymmetry of the radio source distribution on the apparent proper motion kinematic analysis

A new list of physical characteristics of 4261 astrometric radio sources, including all 717 ICRF-Ext.2 sources has been compiled. Comparison of our data of optical characteristics with the official International Earth Rotation and Reference Systems Service (IERS) list showed significant discrepancies for about half of 667 common sources. We also found that asymmetry in the radio sources distribution between hemispheres could cause significant correlation between the vector spherical harmonics, especially if the case of sparse distribution of the sources with high redshift. We identified radio sources having many-year observation history and lack redshift. This sources should be urgently observed at large optical telescopes. The list of optical characteristics created in this paper is recommended for use as a supplement material for the next International Celestial Reference Frame (ICRF) realization. It can be also effectively used for cosmological studies and planning of observing programs both in radio and optics.

THE ORBITS OF THE NEPTUNIAN SATELLITES AND THE ORIENTATION OF THE POLE OF NEPTUNE

This paper reports on an update to the orientation of Neptune's pole and to the orbits of the Neptunian satellites, Triton, Nereid, and Proteus. We determined the new pole and orbits in the International Celestial Reference Frame by fitting them to all available observations through the opposition of 2008. The new data in the fit are high-quality modern astrometry and constitute a 19 year extension of the previous data arc. We assess the accuracy of the orbits and compare them with our earlier orbits. We also provide mean elements as a geometrical description for the orbits.

The celestial reference frame stability and apparent motions of the radio sources

AbstractTime series of the coordinates of the ICRF radio sources were analyzed. It was shown that part of radio sources, including even the so-called “defining” sources, show a the significant apparent motion. Corrections for their a priori coordinates are time functions. The celestial reference frame stability is provided by the no-net-rotation condition applied to the selected subset of sources, which leads in our case to a rotation of the frame axes with time. Parameters of this rotation were calculated for different subsets of sources.To improve stability of the celestial reference frame new methods of selection of the extragalactic radio sources were suggested. The first one was called “cosmological” and the second one “kinematical”. It was shown that a selected subset of the ICRF sources, according to cosmological criteria, determines the most stable coordinate system during next decade.

On the Probability Distribution of Earth Orientation Parameters Data

On the Probability Distribution of Earth Orientation Parameters DataEarth Orientation Parameters (EOPs), i.e. pole coordinates (xp, yp), Universal Time (UT1-UTC), and celestial pole offsets (dX, dY), are the transformation parameters between the International Terrestrial Reference Frame (ITRF) and the International Celestial Reference Frame (ICRF). It is customarily assumed that each of the EOP time series follows the normal distribution. The normality assumption has been used specifically in EOP prediction studies. The objective of this paper is to investigate the normality hypothesis in detail. We analysed the daily time series of xp, yp, UT1-UTC, length-of-day (Δ), dX, and dY in the time interval from 01.01.1962 to 31.12.2008. The UT1-UTC data were transformed to UT1R-TAI by removing leap seconds and the tidal signal using the IERS model. The tidal effects δΔ were also removed from the Δ time series and Δ - δΔ data were obtained. Furthermore, we constructed the residuals of these time series using least-squares fit. We evaluated the skewness and kurtosis and tested their statistical significance by the D'Agostino and the Anscombe-Glynn tests, respectively. In addition, the Anderson-Darling test for the normal distribution was applied. It was found that the xp, yp time series and their residuals slightly depart

Expression of Cassini's third law for Callisto, and theory of its rotation

The rotation of the main natural satellites of the Solar System is widely assumed to be synchronous, because this corresponds to an equilibrium state. In the case of the Moon, 3 laws have been formulated by Cassini, assuming a spin–orbit resonance and a 1:1 nodal resonance. The recent gravitational data collected by the spacecrafts Galileo (in the jovian system) and Cassini (in the saturnian system) allows us to study the rotation of other natural satellites, and to check the universality of Cassini's laws. This paper deals with the rotation of the Galilean satellites of Jupiter J-4 Callisto. In this study we use both analytical (like Lie transforms) and numerical methods (numerical detection of chaos, numerical integration, frequency analysis) to first check the reliability of Cassini Laws for Callisto, and then to give a first theory of its rotation, Callisto's being considered as a rigid body. We first show that the Third Cassini Law (i.e. the nodal resonance), is not satisfied in every reference frame, in particular in the most natural one (i.e. the J2000 jovian equator). The difference of the nodes presents a chaotic-like behavior, that we prove to be just a geometrical illusion. Moreover, we give a mathematical condition ruling the choice of an inertial reference frame in which the Third Cassini Law is fulfilled. Secondly, we give a theory of Callisto's rotation in the International Celestial Reference Frame (ICRF). We highlight a small motion (i.e. <200 m) of its rotation axis about its body figure, a 11.86-yr periodicity in Callisto's length-of-day, and the proximity of a resonance that forces 182-yr librations in Callisto's obliquity.

DIVISION I / COMMISSION 8 / WORKING GROUP DENSIFICATION OF THE OPTICAL REFERENCE FRAME

A continuation of this WG was voted for at the IAU GA 2006 in Prague. The International Celestial Reference Frame (ICRF) is defined by the positions of 212 distant quasars at radio wavelengths. The primary, optical reference frame is the Hipparcos Celestial Reference Frame (HCRF), which is the Hipparcos Catalog without astrometric ‘problem’ stars (in: H. Rickman (ed.) 2001, Proceedings IAU XXIV General Assembly, Transactions IAU XXIVB (San Francisco: ASP), Resolution B1.2). The Tycho-2 catalog with its 2.5 million brightest stars forms the first step in the densification of the optical reference frame. However, the limiting magnitude of about V = 12 of the Tycho-2 catalog is not sufficient for most applications in astronomy and the goal of this IAU Working Group is to further extend the grid of highly accurate positions and motions toward more and fainter stars. The web site of this WG is at &lt;ad.usno.navy.mil/dens_wg/&gt;.

On radio source selection to define a stable celestial frame

Context. The most accurate realization of a quasi inertial reference frame, the International Celestial Reference Frame (ICRF), is made of 212 defining extragalactic radio sources whose coordinates are determined using VLBI observations. Recent studies demonstrated however that using other sets of sources could improve the frame stability. Aims. This study examines a simple radio source selection scheme to define celestial reference frame axes more stable than the ones as currently defined by the ICRF. Methods. After having derived source coordinate time series from 26 years of VLBI observations, we select the most suitable sources on the basis of their positional variability (rms and slope), and observational history. We determine the axis stability of the frame defined by the selection, as well as its suitability for global geodetic VLBI analyses, i.e., determination of Earth orientation parameters and source and station coordinates. Results. We select four frames made of 196, 200, 262, and 269 sources, respectively, showing a satisfactory sky coverage in both hemispheres. Our selections provide a frame stability improved by up to 40% with respect to the ICRF, and by 20% with respect to the frame as defined by the 247 stable sources of Feissel-Vernier et al. (2006, A&A, 452, 1107). Reanalysis of data with respect to this frame gives astrometric catalogues aligned to the ICRF-Ext.2 within 17 μas. Effects on the Earth orientation parameter estimates and terrestrial reference frame determination remain marginal. Conclusions. In view of the generation of the next ICRF, we recommend that such an algorithm be taken into consideration along with other criteria based on individual characteristics of the radio sources.

Astrometric suitability of optically-bright ICRF sources for the alignment with the future Gaia celestial reference frame

The ICRF, currently based on the position of 717 extragalactic radio sources observed by VLBI, is the fundamental celestial reference frame adopted by the IAU in 1997. Within the next 10 years, the European space astrometry mission Gaia, to be launched by 2011, will permit determination of the extragalactic reference frame directly in the visible for the first time. Aligning these two frames with the highest accuracy will therefore be very important in the future for ensuring consistency between the measured radio and optical positions. This paper is aimed at evaluating the current astrometric suitability of the individual ICRF radio sources which are considered appropriate for the alignment with the future Gaia frame. To this purpose, we cross-identified the ICRF and the optical catalog Veron-Cetty and Veron (2006), to identify the optically-bright ICRF sources that will be positioned with the highest accuracy with Gaia. Then we investigated the astrometric suitability of these sources by examining their VLBI brightness distribution. We identified 243 candidate ICRF sources for the alignment with the Gaia frame (with an optical counterpart brighter than the apparent magnitude 18), but only 70 of these (10% of the ICRF sources) are found to have the necessary high astrometric quality (i.e. a brightness distribution that is compact enough) for this link. Additionally, it was found that the QSOs that will have the most accurate positions in the Gaia frame tend to have less-accurate VLBI positions, most probably because of their physical structures. Altogether, this indicates that identifying other high-quality VLBI radio sources suitable for the alignment with the future Gaia frame is mandatory.

Celestial frame instability in VLBI analysis and its impact on geophysics

Context. Derivation of Earth's orientation parameters through VLBI analysis needs to handle the positions of the radio sources and the celestial reference frame correctly, so that radio source positional instabilities will not unduly perturb the nutation estimates or all the subsequent geophysical analysis. Aims. This work aims at estimating the magnitude of these perturbations. It also yields new determinations of the resonant frequencies associated with the Earth's fluid outer and inner cores. Methods. We generate several VLBI global solutions, analyzing VLBI delays accumulated from 1984 to 2007, and using different constraints on the radio source positions, to obtain nutation offset time series. Then we analyze the nutation time series obtained from each global solution, from which we extract some prominent spectral components reflecting the non rigid Earth's response to the external tidal potential. Finally, we deduce values of the outer and inner cores' resonant frequencies from each solution. Results. We show that the error propagated in VLBI analysis that is due to instability in the celestial reference frame can produce an additional error in the estimates of nutation spectral components of 15 μ as for the 18.6-yr term, and decreasing for shorter periods. This leads to an uncertainty of a few tenth of a day on the RFCN period and of less than 200 on its quality factor, and to an uncertainty of roughly 100 days on the FICN period and of 100 on its quality factor. The values yielded for the outer and inner cores' resonant frequencies are nevertheless close to the MHB values within the error bars.

Optical astrometric positions of 59 northern ICRF radio sources

Context.To investigate the link between the International Celestial Reference Frame (ICRF) and its optical Hipparcos-based representation on the northern hemisphere.

VLBI observations of weak extragalactic radio sources for the alignment of the future Gaia frame with the ICRF

AbstractThe space astrometry mission Gaia will construct a dense optical QSO-based celestial reference frame. For consistency between the optical and radio positions, it will be important to align the Gaia frame and the International Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is found that only 10% of the ICRF sources are suitable to establish this link, either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the situation, we have initiated a VLBI survey dedicated to finding additional high-quality radio sources for aligning the two frames. The sample consists of about 450 sources, typically 20 times weaker than the current ICRF sources, which have been selected by cross-correlating optical and radio catalogues. This paper presents the observing strategy and includes preliminary results of observation of 224 of these sources with the European VLBI Network in June 2007.