R. M. Campbell

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

A multi-epoch VLBI survey of the kinematics of CFJ sources. II. Analysis of the kinematics

Context. This is the second of a series of papers presenting VLBI observations of the 293 Caltech-Jodrell Bank Flat-Spectrum (hereafter CJF) sources and their analysis. Aims. To obtain a consistent motion dataset large enough to allow the systematic properties of the population to be studied. Methods. We present the detailed kinematic analysis of the complete fl ux-density limited CJF survey. We computed 2-D kinematic models based on the optimal model-fitting parameters of mult i-epoch VLBA observations. This allows us to calculate not only radial, but also orthogonal motions, and thus to study curvature and acceleration. Statistical tests of the motions measured an d their reliability have been performed. A correlation analysis between the derived apparent motions, luminosities, spectral indices, an d core dominance and the resulting consequences is described. Results. With at least one velocity in each of 237 sources, this sample is much larger than any available before and allows a meaningful statistical investigation of apparent motions and any poss ible correlations with other parameters in AGN jets. The main results to emerge are as follows: - In general motions are not consistent with a single uniform velocity applicable to all components along a jet. - We find a slight trend towards a positive outward accele ration and also adduce some evidence for greater acceleration in the inner-most regions. - We find a lack of fast components at phys ical distances less than a few pc from the reference feature. - Only ∼4% of the components from galaxies and<2% of those from quasars undergo large bends i.e. within 15 ◦ of±90 ◦ . - The distribution of radial velocities shows a broad distribution of velociti es (apparent velocities up to 30 c). 15% of the best sampled jet components exhibit ”low” velocities which may need to be explained in a different manner to the ”fast” motions. - Some negative or ”backwards” superluminal motions are seen and in 15 cases (6%) these are definitely significant. - We find a strong correlation between t he 5 GHz luminosity and the apparent velocity. - The CJF galaxies, on average, show slower apparent jet component velocities than the quasars. - The mean velocity in the VLBA 2cm survey (Kellermann et al. 2004) is substantially higher than in the CJF survey ‐ the rati o could be roughly a factor of 1.5‐2. This supports the observed trend of increasing apparent velocity with increasing observing frequency. Conclusions. This AGN survey provides the basis for any statistical analysis of jet and jet-component properties.

The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales

The millisecond-duration radio flashes known as fast radio bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (∼100 mas precision) of FRB 121102 using the Very Large Array has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here, wereport very-long-baseline radio interferometric observations using the European VLBI Network and the 305 m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of ≲40 pc (≲12 mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (∼2–4 mas at 1.7 GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be ≲ 0.7 pc (≲0.2 mas angular extent at 5.0 GHz) and a lower limit for the brightness temperature of Tb ≳ 5× 107 K. Together, these observations provide strong evidence for a direct physical link between FRB 121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB 121102 that best match the observed data.

The SFXC software correlator for very long baseline interferometry: algorithms and implementation

In this paper a description is given of the SFXC software correlator, developed and maintained at the Joint Institute for VLBI in Europe (JIVE). The software is designed to run on generic Linux-based computing clusters. The correlation algorithm is explained in detail, as are some of the novel modes that software correlation has enabled, such as wide-field VLBI imaging through the use of multiple phase centres and pulsar gating and binning. This is followed by an overview of the software architecture. Finally, the performance of the correlator as a function of number of CPU cores, telescopes and spectral channels is shown.

The kinematics in the pc-scale jets of AGN - The case of S5 1803+784

We present a kinematic analysis of jet component motion in the VLBI jet of the BL Lac object S5 1803+784, which does not reveal long-term outward motion for most of the components. Understanding the complex kinematic phenomena can possibly provide insights into the differences between quasars and BL Lac objects. The blazar S5 1803+784 has been studied with VLBI at

The soft X-ray properties of AGN from the CJF sample: A correlation analysis between soft X-ray and VLBI properties

\nu

A multi-epoch VLBI survey of the kinematics of CJF sources I. Model-fit parameters and maps

=1.6, 2.3, 5, 8.4, and 15 GHz between 1993.88 and 2005.68 in 26 observing runs. We (re)analyzed the data and present Gaussian model-fits. We collected the already published kinematic information for this source from the literature and re-identified the components according to the new scenario presented in this paper. Altogether, 94 epochs of observations have been investigated. A careful study of the long-term kinematics reveals a new picture for component motion in S5 1803+784. In contrast to previously discussed motion scenarios, we find that the jet structure within 12 mas of the core can most easily be described by the coexistence of several bright jet features that remain on the long-term at roughly constant core separations (in addition to the already known {\it stationary} jet component

The EVN-MarkIV VLBI Data Processor

\sim

Transient relativistic ejections and stationary core in XTE J1752−223

1.4 mas) and one faint component moving with an apparent superluminal speed (

Very Long Baseline Interferometry Measured Proper Motion and Parallax of the {

\sim