Oleg Titov

THE SECOND REALIZATION OF THE INTERNATIONAL CELESTIAL REFERENCE FRAME BY VERY LONG BASELINE INTERFEROMETRY

We present the second realization of the International Celestial Reference Frame (ICRF2) at radio wavelengths using nearly 30 years of Very Long Baseline Interferometry observations. ICRF2 contains precise positions of 3414 compact radio astronomical objects and has a positional noise floor of ∼40 μas and a directional stability of the frame axes of ∼10 μas. A set of 295 new “defining” sources was selected on the basis of positional stability and the lack of extensive intrinsic source structure. The positional stability of these 295 defining sources and their more uniform sky distribution eliminates the two greatest weaknesses of the first realization of the International Celestial Reference Frame (ICRF1). Alignment of ICRF2 with the International Celestial Reference System was made using 138 positionally stable sources common to both ICRF2 and ICRF1. The resulting ICRF2 was adopted by the International Astronomical Union as the new fundamental celestial reference frame, replacing ICRF1 as of 2010 January 1.

VLBI measurement of the secular aberration drift

Aims. While analyzing decades of very long baseline interferometry (VLBI) data, we detected the secular aberration drift of the extragalatic radio source proper motions caused by the rotation of the Solar System barycenter around the Galactic center. Our results agree with the predicted estimate to be 4–6 micro arcseconds per year (μas/yr) towards α = 266 ◦ and δ = −29 ◦ . In addition, we tried to detect the quadrupole systematics of the velocity field. Methods. The analysis method consisted of three steps. First, we analyzed geodetic and astrometric VLBI data to produce radio source coordinate time series. Second, we fitted proper motions of 555 sources with long observational histories over the period 1990–2010 to their respective coordinate time series. Finally, we fitted vector spherical harmonic components of degrees 1 and 2 to the proper motion field. Results. Within the error bars, the magnitude and the direction of the dipole component agree with predictions. The dipole vector has an amplitude of 6.4 ± 1.5 μas/yr and is directed towards equatorial coordinates α = 263 ◦ and δ = −20 ◦ . The quadrupole component has not been detected. The primordial gravitational wave density, integrated over a range of frequencies less than 10 −9 Hz, has a limit of 0.0042 h −2 where h is the normalized Hubble constant is H0/(100 km s −1 ).

The AuScope Geodetic VLBI Array

The AuScope geodetic Very Long Baseline Interferometry array consists of three new 12-m radio telescopes and a correlation facility in Australia. The telescopes at Hobart (Tasmania), Katherine (Northern Territory) and Yarragadee (Western Australia) are co-located with other space geodetic techniques including Global Navigation Satellite Systems (GNSS) and gravity infrastructure, and in the case of Yarragadee, satellite laser ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) facilities. The correlation facility is based in Perth (Western Australia). This new facility will make significant contributions to improving the densification of the International Celestial Reference Frame in the Southern Hemisphere, and subsequently enhance the International Terrestrial Reference Frame through the ability to detect and mitigate systematic error. This, combined with the simultaneous densification of the GNSS network across Australia, will enable the improved measurement of intraplate deformation across the Australian tectonic plate. In this paper, we present a description of this new infrastructure and present some initial results, including telescope performance measurements and positions of the telescopes in the International Terrestrial Reference Frame. We show that this array is already capable of achieving centimetre precision over typical long-baselines and that network and reference source systematic effects must be further improved to reach the ambitious goals of VLBI2010.

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

Context. Information on physical characteristics of astrometric radio sources, such as magnitude and redshift, is of great importance for many astronomical studies. However, data usually used in radio astrometry is often incomplete and outdated. Aims. Our purpose is to study the optical characteristics of more than 4000 radio sources observed by the astrometric VLBI technique since 1979. We also studied the effect of the asymmetry in the distribution of the reference radio sources on the correlation matrices between vector spherical harmonics of the first and second degrees. Methods. The radio source characteristics were mainly taken from the NASA/IPAC Extragalactic Database (NED). Characteristics of the gravitational lenses were checked with the CfA-Arizona Space Telescope LEns Survey. SIMBAD and HyperLeda databases were also used to clarify the characteristics of some objects. Also we simulated and investigated a list of 4000 radio sources evenly distributed around the celestial sphere. We estimated the correlation matrices between the vector spherical harmonics using the real as well as modelled distribution of the radio sources. Results. A new list OCARS (optical characteristics of astrometric radio sources) of 4261 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 the 667 common sources. Finally, we found that asymmetry in the radio source distribution between hemispheres could cause significant correlation between the vector spherical harmonics, especially in the case of sparse distribution of the sources with high redshift. We also identified radio sources having a many-year observation history and lack of redshift. These sources should be urgently observed with large optical telescopes. Conclusions. The list of optical characteristics created in this paper is recommended for use as a supplementary 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 optical wavelength.

On the Connection of the Apparent Proper Motion and the VLBI Structure of Compact Radio Sources

Many of the compact extragalactic radio sources that are used as fiducial points to define the celestial reference frame are known to have proper motions detectable with long-term geodetic/astrometric very long baseline interferometry (VLBI) measurements. These changes can be as high as several hundred microarcseconds per year for certain objects. When imaged with VLBI at milliarcsecond (mas) angular resolution, these sources (radio-loud active galactic nuclei) typically show structures dominated by a compact, often unresolved core and a one-sided jet. The positional instability of compact radio sources is believed to be connected with changes in their brightness distribution structure. For the first time, we test this assumption in a statistical sense on a large sample rather than on only individual objects. We investigate a sample of 62 radio sources for which reliable long-term time series of astrometric positions as well as detailed 8 GHz VLBI brightness distribution models are available. We compare the characteristic direction of their extended jet structure and the direction of their apparent proper motion. We present our data and analysis method, and conclude that there is indeed a correlation between the two characteristic directions. However, there are cases where the ~1-10?mas scale VLBI jet directions are significantly misaligned with respect to the apparent proper motion direction.

Improved VLBI measurement of the solar system acceleration

Aims. We propose new estimates of the secular aberration drift, which is mainly caused by the rotation of the solar system about the Galactic center, based on up-to-date VLBI observations and improved method of outlier elimination. Methods. We fitted degree-2 vector spherical harmonics to the extragalactic radio source proper motion field derived from geodetic VLBI observations during 1979‐2013. We paid particular attention to the outlier elimination procedure that removes outliers from (i) radio source coordinate time series and (ii) the proper motion sample. Results. We obtain more accurate values of the Solar system acceleration than in our previous paper. The acceleration vector is oriented towards the Galactic center within ∼7 ◦ . The component perpendicular to the Galactic plane is statistically insignificant. We show that an insufficient cleaning of the data set can lead to strong variations in the dipole amplitude and orientation, and hence to statistically biased results. The accelerated motion of the solar system in the Universe, which is mainly caused by its rotation about the Galactic center, induces an apparent proper motion of extragalactic objects of a few microseconds of arc per year (μas/yr), called secular aberration drift, in the direction of the acceleration vector. This effect can be detected in the systematic part of the proper motion field.

The proper motions of fundamental stars - I. 1535 stars from the Basic FK5

A direct combination of the positions given in the HIPPARCOS catalogue with astrometric ground-based catalogues having epochs later than 1939 allows us to obtain new proper motions for the 1535 stars of the Basic FK5. The results are presented as the catalogue Proper Motions of Fundamental Stars (PMFS), Part I. The median precision of the proper motions is 0.5 mas/year for

Optical Spectra of Candidate Southern Hemisphere International Celestial Reference Frame (ICRF) Radio Sources

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Apparent Proper Motions of Radio Sources from Geodetic VLBI Data

and 0.7 mas/year for

Recent VLBA/VERA/IVS Tests of General Relativity

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