南天区20个河外射电源的VLBI研究

本报道南天区５ＧＨｚ普查式观测结果，首次观测是在１９９２年１１月期间进行的，共观测２３个河外射电源，获得了２０个河外射电源的ＶＬＢＩ高分辨率的射电图象，有１５个射电源呈现核喷流结构，１个有双向喷流，３个未分解，未发现有致密双源，在１６个核喷流（包括一个双向喷流源）源中有１１个呈现弯曲喷流现象。此外还首次发现了３个源中可能有超光速运动现象。     

高频VLBI观测射电源的结构

高频VLBI观测揭示出河外射电源在pc或者亚pc尺度上大都有核-喷流的内部结构，尤其是对平谱致密射电源中毫角秒尺度的子源，能以相当高的概率观测到它的视超光速现象。多频观测可监测到一些新喷流子源的出现，特别在射电激变源中这些新喷流子源的出现与宽带内（从光学到γ射线）的爆发相关。且得到的一些源的核区谱指数分布，为确认源的VLBI核提供了支持。     

高频VLBI观测射电源的结构

高频VLBI观测揭示出河外射电源在pc或者亚pc尺度上大都有核-喷流的内部结构。尤其是对平谱致密射电源中毫角秒尺度的子源，能以相当高的概率观测到它的视超光速现象。多频观测可监测到一些新喷流子源的出现，特别在射电激变源中这些新喷流子源的出现与宽带内(从光学到γ射线)的爆发相关。且得到的一些源的核区谱指数分布，为确认源的VLBI核提供了支持。     

南天区耀变体的VLBI研究

本文报道南天区５ＧＨｚＶＬＢＩ的第二轮观测．第二轮观测是在１９９３年５月期间进行的，共观测了２５个射电源，检测到２４个源，获得了２３个南天区耀变体（Ｂｌａｚａｒ）的ＶＬＢＩ高分辨率的图象，其中１５个是第一次ＶＬＢＩ成图，７个是高能（＞１００ＭｅＶ）γ射电源，１２个源呈现核－喷流结构，其余１１个源是单一的致密核，未发现致密双源．有两个源可能有视超光速现象．     

活动星系致密核视角大小的统计研究

本文对ＶＬＢＩ观测得到的河外射电源致密核ｃｏｒｅ的视角大小进行了统计研究，发现对单个源平均上视角大小近似反比于观测频率，与目前的模型相一致。在５ＧＨｚ上，在红移范围０．０４＜ｚ＜２．５内，平均的ｃｏｒｅ视角大小近似为常数，在非均匀喷流模型框架下，一种可能的解释是在河外射电源致密结构中磁场存在着演化。     

南天区耀变体的VLBI研究

本报道南天区５ＧＨｚＶＬＢＩ的第二轮观测，第二轮观测是在１９９３年５月期间进行的，共观测２５个射电源，检测到２４个源，获得了２３个南天区耀变体的ＶＬＢＩ高分辨率的图象，其中１５个是第一次ＶＬＢＩ成图，７个高能γ射电源，１２个泊呈现核－喷流结构，其余１１个源是单一的致密核，未发现致密双源。有两个源可能有视超光速现象。     

活动星系致密核视角大小的统计研究

本对ＶＬＢＩ观测得到的河外射电源致密核ｃｏｒｅ的视角大小进行了统计研究，发现对单个源平均上视角大小近似反比于观测频率，与目前的模型相一致。在５ＧＨｚ上，在红移范围０．０４＜ｚ＜２．５内，平均的ｃｏｒｅ视角大小近似为常数，在非均匀喷流模型框架下，一种可能的解释是在河外射电源致密结构中磁场存在着演化。     

活动星系和类星体中的喷流（Ⅰ）

自从人们获得河外射电源的第一个结构图像以来，３０多年时间已经过去了。在这段时间的后半期，人们对许多源中的喷流状结构作了大量研究。目前，我们正在分析所获得的有关喷流的第一代结果。这包括高分辨率观测，数值模拟和理论研究等诸方面的成就。在本文 详细地讨论河外射电源中的喷流。     

河外射电源PKS2345—267视超光速运动的新证据

给出了利用上海、乌鲁木齐、意大利NOTO、南非HartRAO射电望远镜对河外射电源PKS2345－167的VLBI观测结果,并讨论其喷流中的子源的视超光速运动。     

河外射电源变化的观测研究

在本文中评述河外射电源变化的观测进展。首先对几个射电变源的一些专门术语和定义作介绍。然后叙述在整个频谱上各种变化测量的结果，包括流量、频谱和偏振的变化。讨论了ＶＬＢＩ对变化研究的重要影响。简要地触及短时标变化引发的逆康普顿灾变的理论问题，最后，强调指出了多波段同时观测河外射电源在深入了解活动星系核方面的重要作用。     

A VLBI study of 20 southern extragalactic radio sources

This paper describes a 5 GHz VLBI survey in the southern hemisphere. The first observation of 23 extragalactic radio sources was conducted in November 1992. Twenty VLBI images were obtained, of which 15 sources show core-jet structure, one has a two-sided jet, and three sources are unresolved. No compact double source was found. Eleven of the 16 sources with core-jet (including a two-aided jet) show evidence of jet bending. In addition, four sources appear to be superluminal, of which three sources are new.     

Combined-array images of quasar 3C 286 at 18 cm wavelength

MERLIN, VLBI and MERLIN+VLBI images of QSO 3C286 at wave-length 18cm are presented. Low-brightness features in the MERLIN image are just resolved in the MERLIN+VLBI image. But some vestiges still remain. The extension of the central part of the MERLIN+VLBI image is longer than that of the VLBI image. Even though the dynamic range of the MERLIN+VLBI image is better than 1000:1, the counter jet is not found. Several models are used to interpret the observed structures.     

Analysis of λ = 6 cm VLBI polarization observations of a complete sample of northern BL Lacertae objects

The results of very long baseline interferometry (VLBI) total intensity ( I ) and linear polarization ( P ) observations at     are presented for 10 radio bright BL Lacertae objects. These images complete first-epoch polarization observations for the 1-Jy sample of northern BL Lacertae objects defined by Kühr & Schmidt. Estimates of superluminal speeds are presented for several sources, bringing the total number of sources in the sample for which such estimates are available to 16. Second-epoch observations currently being reduced should yield speed estimates for VLBI features in essentially all the sources in the sample. The jet magnetic fields of these BL Lacertae objects are usually transverse to the local jet direction, but a sizeable minority (about 30 per cent) have VLBI jet components with longitudinal magnetic fields. This may suggest that the conditions in the VLBI jets of BL Lacertae objects are favourable for the formation of relativistic shocks; alternatively, it may be that the toroidal component of the intrinsic jet magnetic field is characteristically dominant in these sources.     

The λ=6 cm VLBI polarization structure of nine BL Lacertae objects

VLBI total intensity ( I ) and linear polarization ( P ) images at λ =6 cm have been obtained for nine radio-bright BL Lacertae objects. These are the first VLBI P images for these sources, and, in a number of cases, the first I images as well. They confirm the previously noted tendency for the jet magnetic fields of BL Lacertae objects to be transverse to the local jet direction, but also provide new evidence that a sizeable minority of BL Lacertae objects have VLBI jet components with longitudinal magnetic fields. In addition, two sources have VLBI jet components in which the direction of the electric vector χ bears no obvious relation to the apparent local jet direction; the origin of these arbitrary χ offsets is unclear. A new tentative superluminal speed of β =6.3  h ⁻¹ has been determined for 0828+493; tentative speeds for two knots in 1418+546 are β =4.3 and 2.5  h ⁻¹. This work is part of an ongoing programme to determine the VLBI I and P structure of all 34 sources in the 1-Jy sample of northern BL Lacertae objects defined by Kühr & Schmidt.     

类星体3C286在λ18厘米的联合阵图像

本文给出类星体3C286在λ18厘米的MERLIN图像、VLBI图像和MERLIN＋VLBI图像．呈现在MERLIN图像上的低亮度的结构在MERLIN VLBI联合阵的图像上基本被分解，仅留下一些支离破碎的痕迹．MERLIN＋VLBI图像的中央部分比VLBI图像有更长的延伸．MERLIN＋VLBI图像的动态范围好于1000：1，但没有搜索到反向喷流．我们提出了几种可能的模式对所得到的结构进行解释．     

Multifrequency VLBI observations of faint gigahertz peaked spectrum sources

We present the data and analysis of VLBI observations at 1.6, 5 and 15 GHz of a sample of faint gigahertz peaked spectrum (GPS) sources selected from the Westerbork Northern Sky Survey. The 5-GHz observations involved a global array of 16 stations and yielded data on the total sample of 47 sources. A subsample of 26 GPS sources with peak frequencies     and/or peak flux densities     was observed with the VLBA at 15 GHz. A second subsample of 29 sources, with     was observed at 1.6 GHz using a 14-station global VLBI array. In this way, 44 of the 47 sources (94 per cent) in the sample were observed above and at or below their spectral peak. Spectral decomposition allowed us to identify three, 11, seven and two objects as compact symmetric objects, compact doubles, core–jet and complex sources, respectively. However, many of the sources classified as compact double or core–jet sources show only two components making their classification rather tentative. This may explain why the strong morphological dichotomy of GPS quasars and galaxies found for radio-bright GPS sources is not as clear in this faint sample.     

Relation of the jet in M 87 to nearby galaxies in the virgo cluster

The massive and active galaxies in the core of the Virgo Cluster were shown in 1968 to be aligned on either side of M 87, along the direction of the jet and counter-jet. Recent observations confirm the significance of this alignment by showing that the brightest X-ray sources, including additional large galaxies, define the same line. Detailed X-ray maps of M 86 show that this galaxy, which is part of the alignment, is probably blown by a wind from M 87 due to its close alignment with the jet. But the large radio E galaxy, M 84, which is 1.4 degrees away from M 87, andexactly along the line of the jet, is shown by the compression of its X-ray isophotes to be actually moving out along the line of the jet. This evidence furnishes rather spectacular support for the earlier conclusion that M 84 had originated as a proto-galaxy within M 87 and had been ejected out along the line of the jet. Alexander von Humboldt Senior Scientist awardee at the Max-Planck-Institut für Astrophysik, Garching, FRG.     

Extragalactic sources with very asymmetric radio structure: VLA and MERLIN observations

As part of our study to understand the nature of extragalactic radio sources which are very asymmetric in the surface brightness of the two lobes, often with radio emission on only one side of the nucleus, we have observed a large number of them with high angular resolution and good surface brightness sensitivity at radio frequencies. In this paper we present VLA and MERLIN observations of 15 such sources. We discuss their observed structures and spectra, and possible explanations for their morphologies. We report evidence of a possible correlation between the hot-spot brightness ratio and the degree of core prominence, used as a Statistical measure of source orientation, suggesting that relativistic beaming of the hot-spot emission does play a significant role in the observed brightness asymmetry. To explain the apparently one-sided sources within the relativistic beaming framework, the velocities required are in the range of 0.2 to 0.8c. We discuss the possibility that the lobe which is seen to the south of the jet in 3C273 is the counter-lobe seen in projection. We also draw-attention to a number of one-sided sources with very weak cores, and discuss their possible nature.     

EVN observations of low-luminosity flat-spectrum active galactic nuclei

We present and discuss the results of very-long baseline interferometry (VLBI, EVN) observations of three low-luminosity     broad emission line active galactic nuclei (AGNs) carefully selected from a sample of flat-spectrum radio sources (CLASS). Based on the total and the extended radio power at 5 and at 1.4 GHz respectively, these objects should be technically classified as radio-quiet AGN and thus the origin of their radio emission is not clearly understood. The VLBI observations presented in this paper have revealed compact radio cores which imply a lower limit on the brightness temperature of about     . This result rules out a thermal origin for the radio emission and strongly suggests an emission mechanism similar to that observed in more powerful radio-loud AGNs. Since, by definition, the three objects show a flat (or inverted) radio spectrum between 1.4 and 8.4 GHz, the observed radio emission could be relativistically beamed. Multi-epoch VLBI observations can confirm this possibility in two years' time.     

Bulk motion of ultrarelativistic conical blazar jets

Allowing for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales, we show that their bulk Lorentz factors and viewing angles can be much larger than the values usually inferred by combining their flux-variability and proper-motion measurements. This is in accord with our earlier finding that such ultrarelativistic (Lorentz factor,  Γ > 30  ) conical jets can reconcile the relatively slow apparent motions of Very Long Baseline Interferometry (VLBI) knots in TeV blazars with the extremely fast flows implied by their rapid γ-ray variability. This jet geometry also implies that de-projected jet opening angles will typically be significantly underestimated from VLBI measurements. In addition, de-projected jet lengths will be considerably overestimated if high Lorentz factors and significant opening angles are not taken into account.