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.     

Intraday polarization variability outside the VLBI core of the active galactic nucleus 0716+714

Rapid ('intraday') cm-wavelength variations in both total and polarized flux density have been observed in a number of strong extragalactic radio sources. It is difficult to explain these variations purely as propagation effects, but if they are intrinsic to the sources, implausibly high brightness temperatures are required. We discuss here rapid polarization variability during our λ =6 cm global very long-baseline interferometry (VLBI) observations of the active galactic nucleus 0716+714. Measurements made with the Very Large Array (VLA) during the VLBI observations indicate a ≃50° swing in the position angle χ of the VLA core polarization in 12 hours. Corresponding variations were observed only for short VLBI baselines, so that they could not have occurred in the VLBI core (the only feature detected in our VLBI polarization map). The fact that the variations appear both in the VLA data and in the VLBI data for short baselines makes it difficult to explain them as instrumental effects. This leads us to conclude that the rapid variations occurred outside the area covered by our VLBI map; we estimate that the variability occurred in some compact feature roughly 25 milliarcseconds from the nucleus. It is clear that compact structures on a wide range of scales must be taken into account in studies of intraday variability in AGN.     

High resolution imaging of the Hubble Deep and Flanking Fields

42 hours of A-array VLA data and 18 days MERLIN data at 1.4 GHz have been combined to image a 10 arcminute field centred on the Hubble Deep Field (HDF). This area encloses both the Hubble Deep and Flanking Fields. A complete sample of 87 sources have been detected with flux densities above 40 μJy. All these have been imaged with the MERLIN+VLA combination to produce images with 0.2, 0.3 and 0.5 arcsecond resolution. These are the most sensitive 1.4 GHz images yet made with rms noise levels of 3.3 μJy/beam in the 0.2 arcsecond images. About 70% of the microJy sources are found to be starburst type systems associated with major disk galaxies in the redshift range 0.4–1. Some 20% are found to be low-luminosity AGN systems identified with field ellipticals at redshifts close to 1. The remaining 10% are associated with optically faint systems close to or beyond the HDF limit; many of these may be dust-shrouded starbursts at high redshift. We propose to extend this study to include VLBI data of comparable sensitivity to investigate the compact radio structures found in the microJy source population.     

A new quadruple gravitational lens system: CLASS B0128+437

High-resolution MERLIN observations of a newly discovered four-image gravitational lens system, B0128+437, are presented. The system was found after a careful re-analysis of the entire CLASS data set. The MERLIN observations resolve four components in a characteristic quadruple-image configuration; the maximum image separation is 542 mas and the total flux density is 48 mJy at 5 GHz. A best-fitting lens model with a singular isothermal ellipsoid results in large errors in the image positions. A significantly improved fit is obtained after the addition of a shear component, suggesting that the lensing system is more complex and may consist of multiple deflectors. The integrated radio spectrum of the background source indicates that it is a gigahertz peaked spectrum source. It may therefore be possible to resolve structure within the radio images with deep VLBI observations and thus to constrain the lensing mass distribution better.     

VSOP/global VLBI observations of 2021+614: Detection of hotspot advance

We have analysed VSOP and global VLBI data at 5 GHz and VLBA + Effelsberg data at 15 GHz for 2021+614. By identifying the core as being between two bright hotspots, we show that 2021+614 is a compact symmetric object of size 40 pc. From comparison with earlier observations we deduce an apparent age of 375 yr for 2021+614. This provides additional support for the contention that compact symmetric radio objects associated with galaxies are in fact young radio sources and the possible precursors of the classical FR I or FR II radio galaxies.     

High resolution imaging of parsec-scale jets: the impact of the VSOP observations

Resolution improvements achieved with the ongoing space VLBI mission VSOP provide an excellent opportunity for studying compact, parsec-scale jets in active galactic nuclei. Visibilities on space baselines contain structural information that cannot be recovered by super-resolving ground-array data at the same frequency. We illustrate this by discussing the results of recent VSOP observations of parsec-scale jets in 0836+710 and 3C273.     

VSOP and ground-based VLBI observations of Markarian 421

We have produced 22 VLBI images of the TeV blazar Markarian 421 at 11 epochs, including a Space VLBI observation with the HALCA satellite. We measure the speeds of the three innermost jet components to be 0.64±0.33, 0.48±0.09, and 0.06±0.09c (H₀=65 km s⁻¹ Mpc⁻¹). Interpretation of these subluminal speeds in terms of the high Doppler factor demanded by the TeV observations is discussed.     

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.     

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．     

PKS 1117+146: a new large compact symmetric object

We present 23-GHz MERLIN observations of the high-luminosity radio galaxy PKS 1117+146. The radio image obtained from these data shows, for the first time, a central weak component (designated C) that we identify as the core of the radio galaxy, and two outer components (designated N and S) which are the hotspots of the extended lobes observed at lower frequencies. Extended emission in components C and S is an indication of a possible jet connecting the core to the strongest component. The overall optical and radio properties of PKS 1117+146 are consistent with the source being classified as a compact symmetric object (CSO). We discuss this hypothesis, which would make PKS 1117+146 the largest CSO known so far.     

The environment of Hercules A

We have made VLA radio total intensity and polarisation observations in the A, B and C configurations at 1665, 1435, 1365 and 1295 MHz and in the B, C and D configurations at 8465 and 8415 MHz to study the environment of the powerful radio galaxy Hercules A. We have also made ROSAT PSPC and HRI X-ray observations to study the intracluster gas in the Hercules A cluster. We have mapped the Faraday rotation field with high resolution (1.′′42.5 h⁻¹₁₀₀ kpc for q₀=0), and combined this with the X-ray data on the gas distribution in order to map the magnetic field of the cluster. We have found that Hercules A exhibits a strong Laing-Garrington effect: the western side of the radio emission is more depolarised than the eastern side. The X-ray observations have revealed an extended X-ray emission elongated along the radio galaxy axis and a weak nuclear component. The Hercules A cluster is a cooling flow cluster, which appears isothermal at large radii. Comparing the Faraday dispersion profile with the X-ray estimated density profile, we found that the magnetic field is decreasing with radius and we have estimated a central value of 3B₀ (μG) 9. The estimated core electron density of n₀6.6×10³ m⁻³ reveals a dense environment in which Hercules A is situated.     

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.     

Simulations and interpretation of the 6-cm MERLIN images of the classical nova V723 Cas

We compare the predictions of simple models for the radio emission from classical novae with the MERLIN radio observations of nova V723 Cas (Nova Cas 1995). Spherically symmetric and ellipsoidal radiative transfer models are implemented in order to generate synthetic emission maps. These are then convolved with an accurate representation of the u v coverage of MERLIN. The parameters and geometry of the shell model are based on those returned by fitting models to the observed light curve. This allows direct comparison of the model images with the nine 6-cm MERLIN images of V723 Cas.
It is found that the seemingly complex structure (clumping, apparent rotation) evident in the observations can actually be reproduced with a simple spherical emission model. The simulations show that a 24-h track greatly reduces the instrumental effects and the synthetic radio map is a closer representation of the true (model) sky brightness distribution. It is clear that interferometric arrays with sparse u v coverage (e.g. MERLIN, VLBA) will be more prone to these instrumental effects especially when imaging ring-like objects with time-dependent structure variations. A modelling approach such as that adopted here is essential when interpreting observations.     

1144+35: A giant radio galaxy with superluminal motion

We report on centimeter VLA and VLBI observations of the giant, low power radio galaxy 1144+35. On the parsec scale, we see a complex jet component moving away from the center of activity at 2.7h₅₀⁻¹ c. We detect a faint parsec-scale counter-jet and derive a jet velocity of 0.95c and an angle to the line of sight of 25°, consistent with an intrinsically symmetric ejection. These findings lend credence to the claim that even the jets of low-power radio galaxies start out relativistically.     

OQ208, a non-ideal VLBI calibrator of flux density

OQ208 is used ore and more frequently as a VLBI calibrator of flux density. A VLBI image of OQ208 at 5 GHz is firstly provided in this paper. Its structure consists of a compact core and a component. The angular distance between the core and the component is about 7 milli-arcsecond; the position angle is −125 degrees. In principle, OQ208 is not an ideal VLBI calibrator of flux density. This paper provides the structural information for correcting the structure effect when OQ208 is to be used as a calibrator.     

The youngest lobe-dominated radio sources

We present an analysis of multi-epoch global VLBI observations of the Compact Symmetric Objects: 2352+495 and 0710+439 at 5 GHz. Analysis of data spread over almost two decades shows strong evidence for an increase in separation of the outer components of both sources at a rate of 0.2h⁻¹c (for q=0.5 and H=100h km s⁻¹Mpc⁻¹). Dividing the overall sizes of the sources by their separation rates implies that these Compact Symmetric Objects have a kinematic age 10⁴ years. These results (and those for other CSOs) strongly argue that CSOs are indeed very young sources that probably evolve into much larger classical doubles.     

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.     

Is the jet of AO 0235+164 rotating?

We present the VLBI map of a superluminal radio source AO 0235+164 at 5 GHz. This shows that the object's radio structure is dominated by a strong, nearly unresolved core with two extended asymmetric weak jet components. Based on the variation of its flux density with time at 5 GHz, we suggest that the flux density of AO 0235+164 exhibits two periodic variations: a shorter-period variation of 1.81±0.06 yr and a longer-period one of 3.64±0.08 yr. These two periodic variations could be the result of the joint action of jet outbursts and jet rotation.     

VLA and MERLIN observations of RZ Cassiopeiae

We present radio interferometric observations of the Algol-type binary system RZ Cassiopeiae made with the VLA and MERLIN arrays at 6 cm over an incomplete orbital cycle of the system (1.195 d). We detected RZ Cas with both instruments. The images were unresolved in both cases, with angular extents comparable to the synthesized beams. The peak flux density in the VLA image was 1.14 mJy beam⁻¹ and in the MERLIN image it was 0.93 mJy beam⁻¹. The derived brightness temperatures are  4.02 × 10⁸  and  4.35 × 10⁸ K  and the effective electron energies are 0.347 and 0.346 MeV for the MERLIN and VLA data respectively. The radio light curve shows an interesting modulation centred close to the primary eclipse which seems to correlate with ASCA SIS observations of the system. The results can be interpreted as an emitting region on the outer hemisphere of the cool component aligned along the centroid axis of the binary system.     

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.