Niu Liu

Nanjing University | NJU · Department of Astronomy

Aims. This study aims to investigate the systematic errors in planetary ephemeris reference frames through pulsar timing observations. Methods. We used the published data sets from several pulsar timing arrays and performed timing analyses for each pulsar using different planetary ephemerides retrieved from the Jet Propulsion Laboratory’s Developme...

This study aims to investigate the systematics in planetary ephemeris reference frames through pulsar timing observations. We used the published data sets from several pulsar timing arrays and performed timing analyses for each pulsar using different planetary ephemerides retrieved from the Jet Propulsion Laboratory's Development Ephemeris (DE), Ep...

Context. Pulsars are special objects whose positions can be determined independently from timing, radio interferometric, and Gaia astrometry at sub-milliarcsecond (mas) precision; thus, they provide a unique way to monitor the link between dynamical and kinematic reference frames. Aims. We aim to assess the orientation consistency between the dynam...

Pulsars are special objects whose positions can be determined independently from timing, radio interferometric, and Gaia astrometry at sub-milliarcsecond (mas) precision; thus, they provide a unique way to monitor the link between dynamical and kinematic reference frames. We aimed to assess the orientation consistency between the dynamical referenc...

Aims. At the beginning of 2019 the third realization of the International Celestial Reference Frame (ICRF3) replaced the previous version (ICRF2). In the current International Celestial Reference Frame (ICRF), a major improvement is the inclusion of the Galactic aberration (GA) effect. The Earth orientation parameters (EOP), which link the celestia...

We present an updated study on assessing the axes stability of the third generation of the International Celestial Reference Frame (ICRF3) in terms of linear drift and scatter based on the extragalactic source position time series from analyses of archival very long baseline interferometry observations. Our results show that the axes of the ICRF3 a...

Context. The celestial reference frame is established on the basis of the absolute positions of extragalactic sources that are assumed to be fixed in space. The fixing of the axes is one of the crucial points for the concept behind the International Celestial Reference System (ICRS). However, due to various effects such as its intrinsic activity, t...

The celestial reference frame is realized by absolute positions of extragalactic sources that are assumed to be fixed in the space. The fixing of the axes is one of the crucial points for the International Celestial Reference System (ICRS) concept. However, due to various effects such as its intrinsic activity, the apparent position of the extragal...

For detecting exoplanets with high precision, using the angular distance between the two stars to detect the periodic motion of the star will be a better choice. This approach can avoid importing the position error of the reference catalog in the process that using the traditional photographic plate to derive the star position. At the precision lev...

In the near future, two radio telescopes will be installed in the Indonesian region. These telescopes are proposed to be integrated into the existing Very Long Baseline Interferometry (VLBI) network both for astronomical and geodetical purposes. Here we simulate the impact of the inclusion of the future Indonesian antennas to the estimates of Earth...

Context. Comparisons of optical positions derived from the Gaia mission and radio positions measured by very long baseline interferometry (VLBI) probe the structure of active galactic nuclei (AGN) on the milliarcsecond scale. So far, these comparisons have focused on using the S ∕ X -band (2/8 GHz) radio positions, but did not take advantage of the...

We extend previous works by considering two additional radio frequencies (K band and X/Ka band) with the aim to study the frequency dependence of the source positions and its potential connection with the physical properties of the underlying AGN. We compared the absolute source positions measured at four different wavelengths, that is, the optical...

Aims. We aim to study the relative positions of quasar emission centers at different wavelengths in order to help link the various realizations of the International Celestial Reference System (ICRS), and to unveil systematic uncertainties and individual source behavior at different wavelengths. Methods. We based our study on four catalogs represent...

Aims. In order to check the astrometric solution quality, dependences of parallaxes and proper motions on precision, reliability, and consistency of sample solutions are studied for the quasars in the celestial reference frame of the second release of Gaia data (Gaia-CRF2). Methods. Astrometric statistics (the number of visibility periods, the semi...

Context. The third generation of the ICRF – ICRF3 – was published in 2018. This new fundamental catalog provides radio source positions measured independently at three bands: S / X , K , and X / Ka , representing three independent radio celestial frames which altogether constitute a multi-frequency ICRF. Aims. We aim to investigate the overall prop...

We aim to investigate the overall properties of the ICRF3 with the help of the Gaia Data release 2 (Gaia DR2). This could serve as an external check of the quality of the ICRF3. The radio source positions of the ICRF3 catalog were compared with the Gaia DR2 positions of their optical counterparts at G < 18.7. Their properties were analyzed in terms...

The variation of Earth orientation that is characterized by Earth orientation parameters (EOP) is of interest in astronomy and geophysics since the study of EOP opens a window to Earth’s deep interior. In addition, EOP connects the celestial reference frame (CRF) and terrestrial reference frame (TRF) and should be in accordance with realizations of...

Aims. We propose to estimate the accuracy of current VLBI catalogs. Methods. The difference of source position estimated from two decimation solutions was analyzed to estimate the scale factor and noise floor for the formal error of radio source positions by two different methods. In one method, we investigated the weighted rootsquare-mean (wrms)...

The link between the International Celestial Reference Frame at radio wavelength and the forthcoming Gaia optical reference frame is a mandatory task after the completion of the Gaia mission. Starting from the provisional reference frame in which Gaia astrometric solutions were obtained, we discuss the ways to correct the residual rotation and acce...

Nutation is measured by VLBI since the early 1980s. Its accuracy improved drastically in the 1990s. Comparison of observed nutation with the reference model of Mathews et al. (2002; MHB 2000) leaves residuals of about 0.2 mas in rms that are dominated by the signature of the free core nutation (FCN) whose amplitude and phase are still unpredictable...

Aims. In order to investigate the systematic errors in the very long baseline interferometry (VLBI) positions of extragalactic sources (quasars) and the global differences between Gaia and VLBI catalogs, we use the first data release of Gaia ( Gaia DR1) quasar positions as the reference and study the positional offsets of the second realization of...

We compare quasar positions of the auxiliary quasar solution with ICRF2 sources using different samples and evaluate the influence on the {\it Gaia} DR1 reference frame owing to the Galactic aberration effect over the J2000.0-J20015.0 period. Then we estimate the global rotation between TGAS with {\it Tycho}-2 proper motion systems to investigate t...

We aim to evaluate the possibility of improving the ICRS realization starting from the ICRF2 catalogue by investigating the coordinate time series of radio sources observed by VLBI between 1979 and 2016. Sources with long observational history are selected as the candidates and the least squares fits with special handling of the weights are perform...