Proper motion and apparent contraction in J0650+6001

We present a multi-epoch and multi-frequency VLBI study of the compact radio source J0650+6001. In VLBI images the source is resolved into three components. The central component shows a flat spectrum, suggesting the presence of the core, while the two outer regions, with a steeper spectral index, display a highly asymmetric flux density. The time baseline of the observations considered to derive the source expansion covers about 15 years. During this time interval, the distance between the two outer components has increased by 0.28+/-0.13 mas, that corresponds to an apparent separation velocity of 0.39c+/-0.18c and a kinematic age of 360+/-170 years. On the other hand, a multi-epoch monitoring of the separation between the central and the southern components points out an apparent contraction of about 0.29+/-0.02 mas, corresponding to an apparent contraction velocity of 0.37c+/-0.02c. Assuming that the radio structure is intrinsically symmetric, the high flux density ratio between the outer components can be explained in terms of Doppler beaming effects where the mildly relativistic jets are separating with an intrinsic velocity of 0.43c+/-0.04c at an angle between 12 and 28 degrees to the line of sight. In this context, the apparent contraction may be interpreted as a knot in the jet that is moving towards the southern component with an intrinsic velocity of 0.66c+/-0.03c, and its flux density is boosted by a Doppler factor of 2.0.Comment: 7 pages, 5 pages. Accepted for publication in MNRA

High-resolution images of five radio quasars at early cosmological epochs

Context: Until now, there have only been seven quasars at z>4.5 whose the high-resolution radio structure had been studied in detail with Very Long Baseline Interferometry (VLBI) imaging. Aims: We almost double the number of VLBI-imaged quasars at these high redshifts with the aim of studying their redshift-dependent structural and physical properties in a larger sample. Methods: We observed five radio quasars (J0813+3508, J1146+4037, J1242+5422, J1611+0844, and J1659+2101) at 4.5<z<5 with the European VLBI Network (EVN) at 1.6 GHz on 29 October 2008 and at 5 GHz on 22 October 2008. The angular resolution achieved ranges from 1.5 to 25 milli-arcseconds (mas), depending on the observing frequency, the position angle in the sky, and the source's celestial position. Results: The sources are all somewhat extended on mas scales, but compact enough to be detected at both frequencies. With one exception of a flat-spectrum source (J1611+0844), their compact emission is characterised by a steep radio spectrum. We found no evidence of Doppler-boosted radio emission in the quasars in our sample. The radio structure of one of them (J0813+3508) is extended to ~7", which corresponds to 43 kpc projected linear size. Many of the highest redshift compact radio sources are likely to be young, evolving objects, far-away cousins of the powerful gigahertz peaked-spectrum (GPS) and compact steep-spectrum (CSS) sources that populate the Universe at lower redshifts.Comment: 7 pages, 2 figures, accepted for Astronomy & Astrophysic

A View through Faraday's Fog 2: Parsec Scale Rotation Measures in 40 AGN

Results from a survey of the parsec scale Faraday rotation measure properties for 40 quasars, radio galaxies and BL Lac objects are presented. Core rotation measures for quasars vary from approximately 500 to several thousand radians per meter squared. Quasar jets have rotation measures which are typically 500 radians per meter squared or less. The cores and jets of the BL Lac objects have rotation measures similar to those found in quasar jets. The jets of radio galaxies exhibit a range of rotation measures from a few hundred radians per meter squared to almost 10,000 radians per meter squared for the jet of M87. Radio galaxy cores are generally depolarized, and only one of four radio galaxies (3C-120) has a detectable rotation measure in the core. Several potential identities for the foreground Faraday screen are considered and we believe the most promising candidate for all the AGN types considered is a screen in close proximity to the jet. This constrains the path length to approximately 10 parsecs, and magnetic field strengths of approximately 1 microGauss can account for the observed rotation measures. For 27 out of 34 quasars and BL Lacs their optically thick cores have good agreement to a lambda squared law. This requires the different tau = 1 surfaces to have the same intrinsic polarization angle independent of frequency and distance from the black hole.Comment: Accepted to the Astrophysical Journal: 71 pages, 40 figure

Searching (the) FIRST radio arcs near ACO clusters

Gravitational lensing (GL) of distant radio sources by galaxy clusters should produce radio arc(let)s. We extracted radio sources from the FIRST survey near Abell cluster cores and found their radio position angles to be uniformly distributed with respect to the cluster centres. This result holds even when we restrict the sample to the richest or most centrally condensed clusters, and to sources with high S/N and large axial ratio. Our failure to detect GL with statistical methods may be due to poor cluster centre positions. We did not find convincing candidates for arcs either. Our result agrees with theoretical estimates predicting that surveys much deeper than FIRST are required to detect the effect. This is in apparent conflict with the detection of such an effect claimed by Bagchi & Kapahi (1995).Comment: 6 pages; 8 figures and 1 style file are included; to appear in Proc. "Observational Cosmology with the New Radio Surveys", eds. M. Bremer, N. Jackson & I. Perez-Fournon, Kluwer Acad. Pres

Chandra Discovery of a 300 kpc X-ray Jet in the GPS Quasar PKS1127-145

We have discovered an X-ray jet with Chandra imaging of the z=1.187 radio-loud quasar PKS1127-145. In this paper we present the Chandra X-ray data, follow-up VLA observations, and optical imaging using the HST WFPC2. The X-ray jet contains 273+/-5 net counts in 27ksec and extends ~30 arcsec, from the quasar core, corresponding to a minimum projected linear size of ~330/h_50 kpc. The evaluation of the X-ray emission processes is complicated by the observed offsets between X-ray and radio brightness peaks. We discuss the problems posed by these observations to jet models. In addition, PKS1127-145 is a Giga-Hertz Peaked Spectrum radio source, a member of the class of radio sources suspected to be young or ``frustrated'' versions of FRI radio galaxies. However the discovery of an X-ray and radio jet extending well outside the host galaxy of PKS1127-145 suggests that activity in this and other GPS sources may be long-lived and complex.Comment: 22 pages, 11 ps figures, 1 figure in a JPG file, 3 tables. AASTEX. Accepted by The Astrophysical Journa

Radio Emission from GRO J1655-40 during the 1994 Jet Ejection Episodes

We report multifrequency radio observations of GRO J1655-40 obtained with the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope and the Hartebeesthoek Radio Astronomy Observatory at the time of the major hard X-ray and radio outbursts in 1994 August-September. The radio emission reached levels of the order of a few Jy and was found to be linearly polarized by up to 10%, indicating a synchrotron origin. The light curves are in good agreement with those measured with the VLA, but our closer time sampling has revealed two new short-lived events and significant deviations from a simple exponential decay. The polarization data show that the magnetic field is well ordered and aligned at right angles to the radio jets for most of the monitoring period. The time evolution of the polarization cannot be explained solely in terms of a simple synchrotron bubble model, and we invoke a hybrid `core-lobe' model with a core which contributes both synchrotron and free-free emission and `lobes' which are classical synchrotron emitters.Comment: 36 pages, 5 tables, 9 figures; accepted for publication in Ap

Parsec-scale morphology and spectral index distribution in faint high frequency peakers

We investigate the parsec-scale structure of 17 high frequency peaking radio sources from the faint HFP sample. VLBA observations were carried out at two adjacent frequencies, 8.4 and 15.3 GHz, both in the optically-thin part of the spectrum, to obtain the spectral index information. We found that 64% of the sources are resolved into subcomponents, while 36% are unresolved even at the highest frequency. Among the resolved sources, 7 have a morphology and a spectral index distribution typical of young radio sources, while in other 4 sources, all optically associated with quasars, the radio properties resemble those of the blazar population. The equipartition magnetic field of the single components are a few tens milliGauss, similar to the values found in the hotspots of young sources with larger sizes. Such high magnetic fields cause severe radiative losses, precluding the formation of extended lobe structures emitting at centimeter wavelengths. The magnetic fields derived in the various components of individual source are usually very different, indicating a non self-similar source evolution, at least during the very first stages of the source growth.Comment: 14 pages, 5 figures, accepted for publication in MNRA

HI absorption towards nearby compact radio sources

We present the results of HI absorption measurements towards a sample of nearby Compact Steep-Spectrum (CSS) and Giga-Hertz Peaked Spectrum (GPS) radio sources, the CORALZ sample, using the Giant Metrewave Radio Telescope (GMRT). We observed a sample of 18 sources and find 7 new detections. These sources are of lower luminosity than earlier studies of CSS and GPS objects and we investigate any dependence of HI absorption features on radio luminosity. Within the uncertainties, the detection rates and column densities are similar to the more luminous objects, with the GPS objects exhibiting a higher detection rate than for the CSS objects. The relative velocity of the blueshifted absorption features, which may be due to jet-cloud interactions, are within $\sim$$-$250 km s$^{-1}$ and do not appear to extend to values over 1000 km s$^{-1}$ seen for the more luminous objects. This could be due to the weaker jets in these objects, but requires confirmation from observations of a larger sample of sources. There appears to be no evidence of any dependence of HI column density on either luminosity or redshift, but these new detections are consistent with the inverse relation between HI column density and projected linear size.Comment: 10 pages, 6 figures, accepted for publication in MNRA

Chandra Observations of 3C Radio Sources with z<0.3: Nuclei, Diffuse Emission, Jets and Hotspots

We report on our Chandra Cycle 9 program to observe half of the 60 (unobserved by Chandra) 3C radio sources at z<0.3 for 8 ksec each. Here we give the basic data: the X-ray intensity of the nuclei and any features associated with radio structures such as hot spots and knots in jets. We have measured fluxes in soft, medium and hard bands and are thus able to isolate sources with significant intrinsic column density. For the stronger nuclei, we have applied the standard spectral analysis which provides the best fit values of X-ray spectral index and column density. We find evidence for intrinsic absorption exceeding a column density of 10^{22} cm^{-2} for one third of our sources.Comment: 12 pages, 37 figures (the complete version of the paper with all figures is available on line, see appendix for details), ApJ accepte

An optical-IR jet in 3C133

We report the discovery of a new optical-IR synchrotron jet in the radio galaxy 3C133 from our HST/NICMOS snapshot survey. The jet and eastern hotspot are well resolved, and visible at both optical and IR wavelengths. The IR jet follows the morphology of the inner part of the radio jet, with three distinct knots identified with features in the radio. The radio-IR SED's of the knots are examined, along with those of two more distant hotspots at the eastern extreme of the radio feature. The detected emission appears to be synchrotron, with peaks in the NIR for all except one case, which exhibits a power-law spectrum throughout.Comment: ApJ accepted. 14 pages, 6 figure