The Two Mini-lobes of the CSO OQ208—— VLBI Observations at 5 GHz and 8.4 GHz

We present the results of VLBI observations at 5 GHz and 8.4 GHz of the compact symmetric object (CSO) OQ208. Images taken on four epochs at 5GHz and one at 8.4GHz show that the parsec radio structure of the source consists of two mini-lobes, both of them are resolved into two hot-spots. We note that the component D is stronger than the component C in the south-west region at 5 GHz, indicating that component D is less free-free absorbed than C at low frequency. On the basis of the separation of components A and D, a proper motion of 0.032±0.02 mas yr~(-1) between the two mini-lobes is estimated. This value is about half the previous estimates based on the separation of components A and C with 8.4 GHz VLBI data. The reason for the decrease in the expansion velocity is discussed.     

A complete sample of young radio sources

We present the results of a VLBI study of a complete sample of Compact Symmetric Objects (CSO) or CSO candidates, which are currently thought to be the progenitors of the extended powerful extragalactic radio sources. The objects presented here are a subsample of the complete sample of bright GHz-Peaked-Spectrum radio sources selected by Stanghellini et al. (1998, A&A 131, 303).     

VLBI标准接口的发展

VLBI标准接口是近年来在VLBI技术中诞生的一个新概念,它旨在解决多年来各VLBI数据传输系统不兼容的问题．该文阐述了VLBI数据传输系统的发展历史,包括传统的记录／回放系统和新兴的网络数据传输两方面的内容;说明了VLBI标准接口的作用和意义;并分别介绍了VLBI标准接口规范3方面的内容:VLBI标准接口件规范(VSI-H)、VLBI标准接口软件规范(VSI-S)和VLBI标准接口网络规范(VSI-E)．     

VLBI spectral absorption in AGN

We present recent VLBI observations of HI and CO gas in AGN, emphasising the variety of phenomena probed by this technique. Broad (>100 km s⁻¹) HI absorption in ellipticals appears to be commonly associated with circumnuclear gas in a TORUS (Thick Obscuration Required by Unified Schemes) at radii of 50–500 pc from the central engine. Examples include Cygnus A and the CSO 4C31.04. In both of these sources recent HST imaging appears to confirm the presence of a TORUS. However, not all broad absorption HI is TORUS related. In 3C 236, for instance, it seems to be due to a jet-cloud interaction. Narrow <100 km s⁻¹ HI absorption lines also appear to be caused by a variety of different phenomena. For instance, VLBI observations show that the narrow, rapidly in-falling HI absorption gas in 4C31.04 is probably associated with High Velocity Clouds similar to those in-falling into our own Galaxy. In contrast, in 1413+135, the narrow HI and molecular absorption is probably due to a foreground molecular cloud in the host's galactic disk which fortuitously lies along the line of sight to the radio nucleus.     

空间VLBI的天体脉泽观测研究

在本文中较全面地述了天体脉泽源的ＶＬＢＩ观测方法，简单地提及ＶＬＢＩ观测研究的新进展，着重述评空间ＶＬＢＩ用脉泽的观测，并提及了地面ＶＬＢＩ的一些预备观测情况。     

星载氢钟VLBI观测实验及分析

中国计划于2025年左右建立月球轨道VLBI (Very Long Baseline Interferometer)测站,将会搭载被动型星载氢钟作为时间频率标准.由于是首次在VLBI观测中使用星载氢钟,需要研究和验证其可行性.因此,利用星载氢钟作为频率基准开展了VLBI观测.实验时,分别使用主动型地面氢钟和被动型星载氢钟作为频率基准,利用上海天文台佘山25 m射电望远镜和其他测站对我国火星探测器天问一号进行了交替VLBI观测.数据处理分析结果表明,基于地面氢钟与星载氢钟的VLBI残余群时延标准差均在0.5 ns以内,表明星载氢钟可满足深空探测VLBI测定轨的精度要求,验证了其作为月球VLBI测站频率基准的可行性.     

The Experiment and Analysis on VLBI Observations of Space Passive Hydrogen Maser

China plans to establish a lunar orbital VLBI (Very Long Baseline Interferometer) station around 2025, which will carry a space passive hydrogen maser as the time and frequency reference. Since it is the first time to use a space passive hydrogen maser for VLBI observation, its feasibility needs to be studied and verified. Therefore, we carried out VLBI observations using the space passive hydrogen maser as the frequency reference. In the experiment, the active hydrogen atomic clock and space passive hydrogen maser were used as the frequency standard, and the alternate VLBI observations of China’s Mars probe TW1 (Tianwen 1) were carried out using the 25 m radio telescope at Sheshan, Shanghai, and other VLBI stations. The results of data processing and analysis show that the standard deviation of VLBI residual group delay based on both active hydrogen atomic clock and space passive hydrogen maser are within 0.5 ns, which indicates that the space passive hydrogen maser can meet the accuracy requirements of VLBI measurement for deep space exploration, and verify its feasibility as the frequency standard of lunar orbital VLBI stations.     

1-Gbps VLBI, The First Detection of Fringes

We report on the detection of VLBI fringes from quasars by a new VLBI system operating at 1 Gbps (1024 Mbits-per-second). Newly developed 1024 Msps (mega sample-per-second) AD samplers and 1024 Mbps recorders were used for the observations. A correlator with external buffers was used for the 1024 Mbps correlation processing of the tapes data.Our new VLBI system enabled 1024 Mbps VLBI, and this allowed the sampling of a 512 MHz bandwidth from a radio telescope receiver. This is the highest sampling speed ever used for VLBI, and the widest bandwidth used for VLBI observation. Initial sensitivity as evaluated by SNR comparison with earlier VLBI systems produced results to matched the expanded bandwidth. In our first observations, simultaneous optical fibre linked real-time VLBI observations were made to check the validity of data and precisely detemine the clock offsets among the radio telescopes.     

Results of the first two years of VLBI observations at the Svetloe Observatory within the framework of international geodynamical programs

We present the results of processing the VLBI observations performed at the Svetloe Observatory of the Institute of Applied Astronomy (IAA), Russian Academy of Sciences, in the period 2003–2005 within the framework of geodynamical programs of the International VLBI Service (IVS) for geodesy and astrometry. We analyzed the observations at the Svetloe Observatory, together with the observations at other stations of the global IVS network, at the IAA using a modified OCCAM package. The package uses new reduction models that decrease the systematic errors of the results. The motion of the stations, primarily of the Svetloe Observatory, is investigated to study the global geotectonic processes. Highly accurate estimates of the coordinate and baseline length variations have been obtained for the first time in Russia from observations at a Russian VLBI station. We determined the coordinates and velocity of the Svetloe VLBI station with errors of ～2 mm and 3 mm yr^?1, respectively, and the baseline lengths between the stations with a sufficiently long observational history with an accuracy of 1–3 mm. The results are shown to be in good agreement with currently available models for the motion of tectonic plates.     

类星体3C 286的一个早期VLBI观测结果

利用基于自校正和ＣＬＥＡＮ算法的现代ＶＬＢＩ混合成图技术,对Ｐｅａｒｓｏｎ等人于１９７６年观测得到的类星体 ３Ｃ ２８６在 １８ ｃｍ波段上的 ＶＬＢＩ数据重新作了处理．由此得到的图像要大大好于最初的处理结果,也获得了更多的有用信息．用几个Ｇａｕｓｓ模型去拟合校正后的ＵＶ数据,发现中央亮区可以被分解为两个致密结构,这与后来的高分辨率观测相一致．与不同历元的ＶＬＢＩ观测结果相比较,发现这两个致密结构间的距离不但在同一时间随观测频率变化,而且很有可能在同一观测频率上随时间变化．     

The radio structure of 1803+784

We report on the progress of world array VLBI observations of the BL Lac object 1803+784 performed at λ=18 cm. A 16-station VLBI array, plus the phased VLA, and the 7-station MERLIN array simultaneously observed the source for 11 hours. These observations provide images of the transition region between the misaligned milliarcsecond- and arcsecond-jet. We show the MERLIN map and a combined full-array map at medium resolution. In addition we describe the present status of short-cm VLBI observations of this source.     

Global very long-baseline interferometry observations of compact radio sources in M82

Observations of the starburst galaxy, M82, have been made with a 20-station global very long-baseline interferometry (VLBI) array at λ 18 cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of ∼1 arcmin with 3-milliarcsecond resolution are discussed. A limit has been placed on the power-law deceleration of the young supernova remnant (SNR) 43.31+592 with an index greater than 0.73±0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR.
The latest global observations have also provided high-resolution images of the most compact radio source in M82, 41.95+575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s⁻¹. However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being a result of the confinement of a supernova by a high-density circumstellar medium.
It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.     

VLBI and VLA observations of intraday polarization variability in 0917+624 and 0954+658

Total intensity and polarization λ =6 cm Very Large Array (VLA) and global very long baseline interferometry (VLBI) images of the quasar 0917+624 and the BL Lacertae object 0954+658 (both at epoch 1991.43) are analysed. Integrated measurements using the VLA during the VLBI observations indicated that, although there were no substantial total intensity variations, there were significant polarization variations for both sources during the 24-h VLBI experiment. The VLBI data were divided into 2–3 h segments in order to try to identify corresponding rapid variability in the VLBI structure. This analysis revealed intraday variability (IDV) in the VLBI core of 0917+624: both the polarized flux and the polarization position angle varied substantially on time-scales of ∼5–10 h. There is evidence that the VLA polarization variations for 0954+658 occurred in an inner VLBI jet component, where the polarized flux varied by ∼30–40 per cent on time-scales of ∼2 h. 0917+624 and 0954+658 were observed together with 0716+714, an object that also displayed IDV in the polarized flux density measured during our experiment (analysed in a separate paper). These three sources were targeted for the VLBI observations since they had been previously identified as intraday variables, but we had no way of knowing whether they would vary during our observations. The fact that all three exhibited IDV in polarization (but not in total intensity) during our experiment suggests that polarization IDV occurs frequently in at least some IDV sources.     

Determination of the red shifts of selected IVS program objects. I

The spectroscopic red shifts of seven optical objects whose coordinates coincide with those of radio sources in the IVS (International VLBI Service for Geodesy and Astrometry) program list are determined from observations with the 6-m BTA telescope at the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences. A comparison of these spectra and red shifts with data in the radio frequency range shows that four of the objects discussed here are correctly identified, while the other three require further study. The distances to the radio sources derived from our measurements yield more accurate estimates of the cosmological model parameters than those based on the proper motions of these objects derived from geodesic VLBI observations.     

视超光速候选源0208-512和2243-123

本文叙述南天区５ＧＨｚＶＬＢＩ普查的第二轮观测中所发现的视超光速候选源,０２０８－５１２和２２４３－１２３,这些结果尚待进一步的观测加以证实．     

Global VLBI polarisation observations of BL Lac objects

In May 1995, we obtained 5-GHz VLBI polarisation observations of 20 sources in the complete sample of BL Lac objects defined by H. Kühr and G. Schmidt. Ten antennas participated in the global VLBI observations. Total intensity and linear polarisation images of five sources have been made and analysed thus far. The images for 0003–066, 1538+149, and 2254+074 are presented, together with an analysis of the evolution of their polarisation structure and estimates of apparent velocities for components in their VLBI jets.     

Tropospheric Delay from VLBI and GNSS Measurements

Using an updated version of the QUASAR software package developed at the Institute of Applied Astronomy of the Russian Academy of Sciences, we have processed the VLBI observations within the international CONT14 program (May 6–20, 2014), in which a global network of 17 stations was involved (a total of ~250 000 observations). The package update concerned the optimization of data structure and the refinement of stochastic models for the random variations in wet tropospheric delay and atomic clock difference. The main goal of this paper is to compare the VLBI determinations of the tropospheric delay with its independent determinations using global navigation satellite systems (GNSS). We show that both these determinations agree well between themselves only in the case of a global analysis of the VLBI observations, where the VLBI station coordinates are also refined, along with the tropospheric delay and the clock synchronization and Earth orientation parameters. If, alternatively, the station coordinates are insufficiently accurate and are not refined from VLBI observations, then it is appropriate not to determine the tropospheric delay from these observations, but to take it from the publicly accessible independent GNSS data. However, this requires that the VLBI and GNSS techniques operate simultaneously at a common observing site. We have established the shortcomings of the universally accepted method of stabilizing the global solution associated with the absence of a criterion for choosing reference stations and radio sources. Two ways of their elimination are proposed: (i) introducing a coordinated list of weight factors for the errors in the coordinates of such stations and sources into the stabilization algorithm and (ii) adopting a coordinated list of stations and sources the refinement of whose coordinates is not required at all for a certain time.     

The radio remnant of SN 1993J: an instrumental explanation for the evolving complex structure

We present simulated images of Supernova 1993J at 8.4 GHz using Very Long Baseline Interferometry (VLBI) techniques. A spherically symmetric source model is convolved with realistic u v -plane distributions, together with standard imaging procedures, to assess the extent of instrumental effects on the recovered brightness distribution. In order to facilitate direct comparisons between the simulations and published VLBI images of SN 1993J, the observed u v -coverage is determined from actual VLBI observations made in the years following its discovery.
The underlying source model only exhibits radial variation in its density profile, with no azimuthal dependence and, even though this model is morphologically simple, the simulated VLBI observations qualitatively reproduce many of the azimuthal features of the reported VLBI observations, such as appearance and evolution of complex azimuthal structure and apparent rotation of the shell. We demonstrate that such features are inexorably coupled to the u v -plane sampling.
The brightness contrast between the peaks and the surrounding shell material are not as prominent in the simulations (which of course assume no antenna- or baseline-based amplitude or phase errors, meaning no self-calibration procedures will have incorporated any such features in models). It is conclusive that incomplete u v -plane sampling has a drastic effect on the final images for observations of this nature. Difference imaging reveals residual emission up to the 8σ level. Extreme care should be taken when using interferometric observations to directly infer the structure of objects such as supernovae.     

地球卫星VLBI观测研究进展

随着地球卫星在资源、导航定位、授时、跟踪以及数据中继等经济和军事领域的作用不断增强,传统的测距测速技术已无法满足对卫星定位精度的实际需求。由于VLBI技术具有高精度角分辨率的特点,人们尝试将其应用于人造地球卫星的观测。针对卫星VLBI观测方面,首先介绍了卫星VLBI观测技术的基本原理及其与传统河外射电源VLBI观测的差异,然后介绍了中外学者应用VLBI技术进行地球卫星观测的研究进展及相关成果,最后对以往近地卫星VLBI观测研究存在的问题进行总结,并讨论了目前以及将来可能的解决方案。     

脉冲星VLBI观测软件相关处理进展

VLBI是进行高分辨率脉冲星观测研究的重要手段.脉冲星信号是非常微弱的脉冲序列,其VLBI观测面临多种挑战.在数据相关过程中采用有效技术提取脉冲星信号可提高观测成功率和精度.DiFX(Distributed FX-style Software Correlator)是目前国际上流行的开源软件相关处理机,它采用非相干消色散技术和“脉冲星数据分箱技术”(Pulsar Binning),在脉冲星VLBI观测数据的相关处理方面具有优异的性能.介绍了DiFX的构架,安装、调试方法,并对利用DiFX处理脉冲星VLBI观测数据的进展情况进行论述.利用单机环境下的DiFX,在普通模式和Pulsar Binning模式下对中国VLBI网(CVN)第一次脉冲星观测数据进行相关处理;利用德国马普射电天文研究所集群计算环境下的Bonn-DiFX,在Pulsar Binning模式下对流量仅有2.6 mJy的毫秒脉冲星PSRJ1022+1001的欧洲VLBI网(EVN)观测数据成功进行相关处理.最后,对使用DiFX处理脉冲星观测数据情况进行总结,并展望了今后CVN开展脉冲星观测研究的前景.