arXiv subjects

J. -C. Liu

Publications and source records attributed to J. -C. Liu.

7 recordsLinked to original sources

arXiv2026

Consistency and precision of very long baseline interferometry source coordinate time series

Coordinate time series of extragalactic radio sources from very long baseline interferometry observations are widely used to evaluate source positional stability, select stable sources for celestial reference frames, assess reference-frame axis stability, and investigate source-related astrometric variability. Their external consistency, realistic errors, and relation to processing strategies and analysis configurations remain to be investigated. We aim to compare coordinate time series solutions from different analysis centers and investigate how processing strategies and configurations contribute to their differences. We collected eight solutions from seven analysis centers and selected 496 common sources after data selection, reference-frame alignment, and extraction of common observing sessions. Inter-solution differences were characterized with pairwise positional offsets and correlation analysis, and realistic positional errors with the N-cornered-hat (NCH) method and bootstrap resampling. Most solutions have median weighted root mean square (WRMS) values of a few hundred microarcseconds $(\mathrm{μas})$, and the median NCH-derived precision is about 200-300 $\mathrm{μas}$ in right ascension and 250-400 $\mathrm{μas}$ in declination. Solutions with similar strategies, software, or source constraints show higher consistency, with more similar WRMS values, stronger correlations, and closer precision estimates. Consistency is affected not only by the global or independent mode distinction but also by detailed analysis configuration. NCH-derived precision depends on declination, being poorer in the southern sky, and stabilizes with more observing sessions. Processing strategies and configurations introduce marginal differences; solutions remain generally consistent. Inter-solution comparisons independently estimate realistic stochastic errors beyond formal errors.

astro-ph.IM↗

arXiv2026

Astrometric properties of reference frame sources as a function of redshift

Previous studies based on the latest realisation of the International Celestial Reference Frame (ICRF3) have suggested a correlation between astrometric properties (such as the radio-optical offset) and redshift for active galactic nuclei (AGNs). We extend these investigations by using a large, all-sky sample of approximately 22,000 compact radio sources from the Radio Fundamental Catalogue (RFC) to examine this relationship in a systematic and statistically robust manner. We compiled redshifts for about 10,000 RFC sources over the range 0 < z < 5 by combining data from the Dark Energy Spectroscopic Instrument Data Release 1 and the Sloan Digital Sky Survey Data Release 17/19 with additional datasets from the NASA/IPAC Extragalactic Database. Cross-matching with Gaia Data Release 3 yielded a sample of 4,068 RFC objects with reliable spectroscopic redshifts and classifications, including galaxies and quasi-stellar objects (QSOs). We analysed the redshift dependence of their radio astrometric properties from very long baseline interferometry (VLBI) and their optical astrometric properties from Gaia. We find that the VLBI astrometric properties show no significant dependence on redshift within the achieved level of precision. In contrast, several optical astrometric quantities exhibit clear redshift-dependent behaviour. The median absolute radio-optical offsets decrease markedly over 0 < z < 0.5, where galaxies dominate the sample, decline more gradually over 0.5 < z < 1.3, and exhibit a mild increase at z > 1.3, where QSOs dominate. Similar behaviour is observed for several Gaia astrometric quantities, including astrometric uncertainties, proper motions, and G magnitudes. These behaviours can be largely explained by the dependence of Gaia astrometric performance on G magnitude and by the evolution of the G magnitude with redshift.

astro-ph.IM↗

arXiv2025

Evidence of mini-jet emission in a large emission zone from a magnetically-dominated gamma-ray burst jet

The second brightest GRB in history, GRB230307A, provides an ideal laboratory to study the mechanism of GRB prompt emission thanks to its extraordinarily high photon statistics and its single episode activity. Here we demonstrate that the rapidly variable components of its prompt emission compose an overall broad single pulse-like profile. Although these individual rapid components are aligned in time across all energy bands, this overall profile conspires to show a well-defined energy-dependent behavior which is typically seen in single GRB pulses. Such a feature demonstrates that the prompt emission of this burst is from many individual emitting units that are casually linked in a emission site at a large distance from the central engine. Such a scenario is in natural consistency with the internal-collision-induced magnetic reconnection and turbulence framework, which invokes many mini-jets due to local magnetic reconnection that constantly appear and disappear in a global magnetically-dominated jet.

astro-ph.HE↗

arXiv2024

Magnetar emergence in a peculiar gamma-ray burst from a compact star merger

The central engine that powers gamma-ray bursts (GRBs), the most powerful explosions in the universe, is still not identified. Besides hyper-accreting black holes, rapidly spinning and highly magnetized neutron stars, known as millisecond magnetars, have been suggested to power both long and short GRBs. The presence of a magnetar engine following compact star mergers is of particular interest as it would provide essential constraints on the poorly understood equation of state for neutron stars. Indirect indications of a magnetar engine in these merger sources have been observed in the form of plateau features present in the X-ray afterglow light curves of some short GRBs. Additionally, some X-ray transients lacking gamma-ray bursts (GRB-less) have been identified as potential magnetar candidates originating from compact star mergers. Nevertheless, smoking gun evidence is still lacking for a magnetar engine in short GRBs, and the associated theoretical challenges have been raised. Here we present a comprehensive analysis of the broad-band prompt emission data of a peculiar, very bright GRB 230307A. Despite its apparently long duration, the prompt emission and host galaxy properties are consistent with a compact star merger origin, as suggested by its association with a kilonova. Intriguingly, an extended X-ray emission component shows up as the $γ$-ray emission dies out, signifying the likely emergence of a magnetar central engine. We also identify an achromatic temporal break in the high-energy band during the prompt emission phase, which was never observed in previous bursts and reveals a narrow jet with half opening angle of approximately $\sim 3.4^\circ (R_{GRB}/10^{15}~{cm})^{-1/2}$, where $R_{GRB}$ is the GRB prompt emission radius.

astro-ph.HE↗

arXiv2024

Microwave interference from a spin ensemble and its mirror image in waveguide magnonics

We investigate microwave interference from a spin ensemble and its mirror image in a one-dimensional waveguide. Away from the mirror, the resonance frequencies of the Kittel mode (KM) inside a ferrimagnetic spin ensemble have sinusoidal shifts as the normalized distance between the spin ensemble and the mirror increases compared to the setup without the mirror. These shifts are a consequence of the KM's interaction with its own image. Furthermore, the variation of the magnon radiative decay into the waveguide shows a cosine squared oscillation and is enhanced twofold when the KM sits at the magnetic antinode of the corresponding eigenmode. We can finely tune the KM to achieve the maximum adsorption of the input photons at the critical coupling point. Moreover, by placing the KM in proximity to the node of the resonance field, its lifetime is extended to more than eight times compared to its positioning near the antinode.

physics.app-ph↗

arXiv2017

Overall properties of the Gaia DR1 reference frame

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 the property of the {\it Gaia} DR1 reference frame. Finally, the Galactic kinematics analysis using the K-M giant proper motions is performed to understand the property of {\it Gaia} DR1 reference frame. The positional comparison between the auxiliary quasar solution and ICRF2 shows negligible orientation and validates the declination bias of $\sim$$-0.1$\mas~in {\it Gaia} quasar positions with respect to ICRF2. Galactic aberration effect is thought to cause an offset $\sim$$0.01$\mas~of the $Z$ axis direction of {\it Gaia} DR1 reference frame. The global rotation between TGAS and {\it Tycho}-2 proper motion systems, obtained by different samples, shows a much smaller value than the claimed value $0.24$\masyr. For the Galactic kinematics analysis of the TGAS K-M giants, we find possible non-zero Galactic rotation components beyond the classical Oort constants: the rigid part $ω_{Y_G} = -0.38 \pm 0.15$\masyr~and the differential part $ω^\prime_{Y_G} = -0.29 \pm 0.19$\masyr~around the $Y_G$ axis of Galactic coordinates, which indicates possible residual rotation in {\it Gaia} DR1 reference frame or problems in the current Galactic kinematical model.

astro-ph.IM↗

arXiv2016

Test of source selection for constructing a more stable and uniform celestial reference frame

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 performed to derive the linear drifts of the source coordinates. Then the sources are sorted based on the normalized linear drift (i) over the whole sky and (ii) in four homolographic areas divided by declinations. The axial stability of the reference system and sky distribution defined by the selected sources are evaluated, which are acted as the criterion for the final source lists. With our improved source selection scheme, two groups of sources are proposed and considered suitable for defining a more stable and homogeneous celestial reference system compared to the current ICRF2. The number of sources in the final lists are 323 and 294, respectively, and the global rotation of the axes derived from apparent motion of the sources are about two times better than the ICRF2.

astro-ph.GA↗