南天区20个河外射电源的VLBI研究

本文报道南天区5GHzVLBI普查式观测结果．首次观测是在1992年11月期间进行的，共观测了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，末发现有致密观源．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．此外还首次发现有3个源中可能有超光速运动现象．     

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.     

Studies of the quasar OH 471

Based on the 1964–1996 observations in the frequency range 0.325–90 GHz, we study peculiarities of the variability of the quasar OH 471 (z=3.4). The double-humped spectrum had peaks at frequencies near 1 and 20 GHz. The flux density of the low-frequency component first decreased and then began to increase. The high-frequency component rose to 2.5 Jy in the late 1970s. The peak frequencies were virtually constant. VLBI observations at 1.6 GHz revealed a core-jet structure with the jet extended eastward to 5 mas. The object is a powerful quasar.     

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.     

Variability of GPS Radio Sources at 5 GHz

We carry out flux monitoring on a sample of 169 Gigahertz Peaked Spectrum (GPS) radio sources at 5 GHz and find that about one-third of them show considerable Inter-Month Variability (IMV), and these IMV phenomena are likely to be caused by interstellar scintillation (ISS). Furthermore, we find that those showing IMV tend to be point-like or core-jet structures at VLBI scale.     

VLBA multi-frequency monitoring of SS433

We present results of VLBA observations of SS433 at two epochs at 5, 8.4, and 15 GHz. The AU-scale region of the source is resolved and shows the familiar quasi-symmetric core-jet structures. But at another epoch, the maps show bright knots ejected a few days before the observation, while the core-complex disappeared. The bright jet components are completely depolarised with fractional polarization less than 0.5%. We also present the first VLBI image of SS433 at 22 GHz.     

S5 2007+777的射电性质研究

S5 2007+777是一个典型的低峰值频率的蝎虎天体,该天体具有kpc尺度的X射线喷流,文献中利用模型估算的方法,得出X射线波段的多普勒因子达到13.0,从而喷流尺度可以达到Mpc量级.在此,搜集了有关S5 2007+777的欧洲甚长基线射电干涉网(European VLBI Network, EVN)高分辨率档案数据、美国甚长基线射电干涉网(Very Long Baseline Array, VLBA) 15 GHz观测数据等,研究了喷流的射电结构、亮温度、自行等方面的性质,发现该源的甚长基线干涉测量(Very Long Baseline Interferometry,VLBI)不同波段的喷流方向一致,但与文献中给出的kpc尺度的X射线喷流和甚大阵(Very Large Array, VLA)射电喷流方向存在一定的差异,说明该源的喷流辐射存在多普勒增亮效应.由VLBI观测得到的亮温度,估算了该源的射电多普勒因子的平均值及中值均为5.0,此值小于文献中X射线波段的多普勒因子,但与文献中利用其他方法得到的射电波段多普勒因子是一致的;另外,对多历元观测数据的拟合发现此源相同波段的各个成分在长历元上没有明显的自行,短历元上的自行甚至是视超光速运动.这可能是由低表面亮度成分中心位置的转移造成的.这同时也验证了之前估算的射电多普勒因子不是很大,小于X射线波段多普勒因子的结论.利用所得到的射电多普勒因子,发现该源具有较大尺度的本征射电喷流,可达到0.5 Mpc,由于这里使用的是均值,因此说明该源也有可能具有接近巨射电星系尺度的喷流.     

在6厘米波长对类星体3C454.3的九站VLBI观测

3C454.3是近几年引起广泛注意的一个强活动射电类星体,它有毫角秒尺度的核——喷流结构,其核在1981年的一次流量密度大爆发之后,显示出“超光速增亮”现象,在6厘米波长,用包括欧洲、美国及南非的9个射电望远镜,于1983年10月对3C454.3进行了甚长基线干涉网观测。观测结果除了证实其核——喷流结构,显示了从毫角秒到角秒尺度上该类星体结构的系统弯曲外,还清楚地揭示了“超光速增亮”所预期的,在1981年8月到1983年1月期间,核的超光速膨胀现象。     

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.     

Bending of Jets in the QSO NRAO 530

We present radio images of NRAO 530 on scales ranging from pc to kpc. The observations include the EVN at 5 GHz, the VLBA at 1.6, 8.6 and 15 GHz, the MERLIN at 1.6 and 5 GHz, and the VLA at 5, 8.4, 15, 22, and 43 GHz. The VLBI images show a core-jet structure with an oscillating trajectory on a scale of about 30 mas north of the strongest compact component (core). Superluminal motions are detected in five of the jet compo-nents with apparent velocities in the range of 13.6 to 25.2c. A new component is detected at 15 GHz with the VLBA observations, which appears to be associated with the outburst in 2002. Significant polarized emission is detected around the core with the VLBA observationsat 15 GHz. Rapid variations of the polarization intensity and angle are found between the epochs in 2002 and 2004. On the kpc-scale, a distant component (labelled as WL) located 11 arcsec west (PA=-86°) of the core is detected beyond the core-jet structure which ex-tended to several hundreds of mas in the north-west direction (-50°). A significant emission between the core-jet structure and the WL is revealed. A clump of diffuse emission (labelled EL, 12 arcsec long) at PA 70° to the core, is also detected in the VLA observations, suggest-ing the presence of double lobes in the source. The core component shows a flat spectrum,while the distant components WL and EL have steep spectra. The steep spectra of the distantcomponents and the detection of the arched emission suggest that the distant components are lobes or hot-spots powered by the core of NRAO 530. The morphologies from pc-to kpc- scales and the bending of jets are investigated. The observed radio morphology from pc to kcp appears to favor the model in which precession or wobbling of the nuclear disk drives the helical motion of the radio plasma and produces the S-shaped structure on kpc scale.     

Search for Binary Black Hole Candidates from the VLBI Images of AGNs

We have searched the core-jet pairs in the VLBI scales (<1 kpc), from several VLBI catalogues, and found out 5 possible Binary Black Hole (BBH) candidates. We present here the search results and analyse the candidates preliminarily. We plan to study with multi-band VLBI observation. We also plan to carry out optical line investigation in future.     

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.     

Results of VLBI monitoring of 3C273 at 22 GHz and 43 GHz

The results of VLBI observations of the quasar 3C273, obtained during a multi-band campaign, are presented. The changes in separation from the core of the components along the first 2 mas of the jet were measured. The jet starts wiggling well inside the first parsec from the core. We derived the period of the oscillating jet path around the principal jet-axis in the range 2–8 mas from the core. The components' angular sizes, their peak fluxes, and turnover frequencies were also derived from the VLBI observations. We have derived lower limits to the Doppler factor by comparing the observed X-ray fluxes with those predicted by the synchrotron self-Compton model. Independent estimates were obtained through the assumption of energy equipartition between particles and magnetic field.     

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.     

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.