Radiative transfer of synchrotron radiation through a compressed random magnetic field

The generally low polarization levels present in extragalactic radio sources suggest magnetic fields that are largely turbulent, presumably maintained by turbulence motion of the underlying plasma. The polarization of radio emission is often generated as the plasma is compressed by shock waves, and this type of configuration has been widely applied, from the cores of AGN to the lobes of classical double radio sources. Here we note that, in the absence of Faraday rotation, the synchrotron radiative transfer equations for such a configuration, separate, leading to relatively simple solutions for opaque or partially opaque sources. Expressions for the emission and absorption coefficients are derived, and, although in general, these must be evaluated numerically, the process is much simpler than a full numerical solution of the transfer equations

Magnetic field structure of the extended 3C 380 jet

An earlier study of the complex jet of 3C 380 by Papageorgiou et al. revealed total intensity and polarization structure associated with a bright knot K1 about 0.7 arcsec from the core that was reminiscent of that expected for a conical shock wave. In this new study, 1.42, 1.66 and 4.99 GHz total intensity, polarization and Faraday rotation images are presented and analysed. These images were derived from observations with the Very Long Baseline Array plus one antenna of the Very Large Array, obtained in 2006 March. These new images confirm the overall magnetic field structure of the knot K1 indicated in the earlier observations. In addition, a clear Faraday rotation gradient has been detected across the jet, extending roughly from 10 to 30 mas (70–200 pc) along the jet from the core (a radial distance of approximately two beamwidths). The gradient spans roughly 3.5 beamwidths in the transverse direction, and the difference in the rotation measures on either side of the jet is 4–5σ, demonstrating that the detection of the gradient is firm. We interpret this transverse Faraday rotation gradient as reflecting systematic variation of the line-of-sight component of a helical or toroidal magnetic field (B) associated with the jet of 3C 380. These results provide evidence that the helical field arising due to the joint action of the rotation of the central black hole and its accretion disc and the jet outflow can survive to distances of hundreds of parsecs from the central engine

The parsec-scale radio jet of BL Lac

Polarization-sensitive lambda= 6 cm global VLBI images of BL Lac for data taken at three epochs are presented and analysed, together with an additional epoch for which only total intensity data are available. Previous lambda= 6 cm and lambda= 3.6 cm VLBI polarization observations of BL Lac have shown that the jet components in this source tend to have magnetic field transverse to the jet direction. On the whole, these images confirm this tendency, though there is evidence for possibly oblique or longitudinal magnetic fields in some features at some epochs. The flux densities of individual features moving from the core can initially drop rapidly, but the rate of flux decrease slows with time, so that many features are, in fact, remarkably long-lived. This has made it possible to trace the movements of some components over more than a decade. The trajectories of individual components show oscillations whose amplitude appears to grow with distance from the VLBI core

86 GHz Very Long Baseline Polarimetry of 3C273 and 3C279 with the Coordinated Millimeter VLBI Array

86 GHz Very Long Baseline Polarimetry probes magnetic field structures within the cores of Active Galactic Nuclei at higher angular resolutions and a spectral octave higher than previously achievable. Observations of 3C273 and 3C279 taken in April 2000 with the Coordinated Millimeter VLBI Array have resulted in the first total intensity (Stokes I) and linear polarization VLBI images reported of any source at 86 GHz. These results reveal the 86 GHz electric vector position angles within the jets of 3C273 and 3C279 to be orthogonal to each other, and the core of 3C273 to be unpolarized. If this lack of polarization is due to Faraday depolarization alone, the dispersion in rotation measure is >=90000 rad/m^2 for the core of 3C273.Comment: AASTeX v5.02; 10 pages; 4 figures; accepted for publication in the Astrophysical Journal Letter

Space very long baseline interferometry observations of polarization in the jet of 3C 380

A comparison between low-frequency space very long baseline interferometry (VLBI) and high-frequency ground-based VLBI images can, in principle, be used to detect small variations in rotation measure (RM) on fine angular scales inaccessible to ground arrays alone. This paper reports an attempt to perform such a comparison using the jet in the quasar 3C 380. Observations made with the VSOP antenna HALCA together with a ground array at wavelength 1.6 GHz provide total intensity and polarization images of comparable resolution to those from the ground array alone at 5 GHz. The results provide an image showing derotated magnetic vector position angle of somewhat higher resolution than that available earlier. The results show variations in an RM around component A of the order of 10 rad m(-2) that could not have been detected with the ground array alone. It is concluded that satellite VLBI observations provide a promising means to study the distribution of matter and magnetic fields around parsec-scale jets. The ground observations used here follow the steady outward drift of component A, which has approximately doubled its distance from the core since the first observations in 1982. They also reveal total intensity and polarization structure associated with a bright knot 0.7 arcsec from the core which is reminiscent of that expected for a conical shock wave

Analysing the Transverse Structure of the Relativistic Jets of AGN

This paper describes a method of fitting total intensity and polarization profiles in VLBI images of astrophysical jets to profiles predicted by a theoretical model. As an example, the method is used to fit profiles of the jet in the Active Galactic Nucleus Mrk501 with profiles predicted by a model in which a cylindrical jet of synchrotron plasma is threaded by a magnetic field with helical and disordered components. This fitting yields model Stokes Q profiles that agree with the observed profiles to within the 1-2 \sigma uncertainties; the I model and observed profiles are overall not in such good agreement, with the model I profiles being generally more symmetrical than the observed profiles. Consistent fitting results are obtained for profiles derived from 6cm VLBI images at two distances from the core, and also for profiles obtained for different wavelengths at a single location in the VLBI jet. The most striking success of the model is its ability to reproduce the spine-sheath polarization structure observed across the jet. Using the derived viewing angle in the jet rest frame, \delta' approximately 83 degrees, together with a superluminal speed reported in the literature, \beta apparent = 3.3, yields a solution for the viewing angle and velocity of the jet in the observer's frame \delta degrees and \beta approximately 0.96. Although these results for Mrk501 must be considered tentative, the combined analysis of polarization profiles and apparent component speeds holds promise as a means of further elucidating the magnetic field structures and other parameters of parsec-scale AGN jets

Returning Students in Engineering Education: Making a Case for “Experience Capital”

Students returning to college are not generally studied, where most of the research on non-traditional students is focused on individuals returning to earn their undergraduate degree. There are, however, many students returning to receive graduate degrees as they pursue new directions in life by interest or economic necessity. Undergraduate students with experience have clear educational related goals, practical approaches to problem-solving, and high learning motivation.Returning graduate students are expected to model similar behaviors. These individuals bring a lifetime of personal and professional expertise, which we identify as “experience capital.”A review of the literature reveals that capital has been pondered since early western philosophers considered the concept of social capital in terms of „community governance‟. Others credit Dewey with the first use of the term „social capital‟. Since then, development of other capitals include human, cultural, and symbolic. Human capital is viewed as knowledge, skills, and attributes; cultural capital as an indicator of class position acquired by family and education ; and symbolic as the prestige, recognition, and fame. Today, social capital is viewed as the networks,relationships, and connections of influence and support. Experience capital is the partial union of social, human, cultural, and symbolic capital, which individuals develop from their persona land professional experiences as they progress through life.This is an exploratory study capturing the perceptions of “experience capital” of individuals with several years of professional experience in their discipline returning for a doctoral degree in engineering education. The research question this study addresses is: what “experience capital”do returning students bring to an engineering education doctoral program? The participants will be interviewed; open coding will be used to identify common themes. The results of this qualitative study will position the experiences of the participants at the partial union of social,human, cultural, and symbolic capital, in a space called experience capital

PolyTB: a genomic variation map for Mycobacterium tuberculosis

Tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) is the second major cause of death from an infectious disease worldwide. Recent advances in DNA sequencing are leading to the ability to generate whole genome information in clinical isolates of M. tuberculosis complex (MTBC). The identification of informative genetic variants such as phylogenetic markers and those associated with drug resistance or virulence will help barcode Mtb in the context of epidemiological, diagnostic and clinical studies. Mtb genomic datasets are increasingly available as raw sequences, which are potentially difficult and computer intensive to process, and compare across studies. Here we have processed the raw sequence data (>1500 isolates, eight studies) to compile a catalogue of SNPs (n = 74,039, 63% non-synonymous, 51.1% in more than one isolate, i.e. non-private), small indels (n = 4810) and larger structural variants (n = 800). We have developed the PolyTB web-based tool (http://pathogenseq.lshtm.ac.uk/polytb) to visualise the resulting variation and important meta-data (e.g. in silico inferred strain-types, location) within geographical map and phylogenetic views. This resource will allow researchers to identify polymorphisms within candidate genes of interest, as well as examine the genomic diversity and distribution of strains. PolyTB source code is freely available to researchers wishing to develop similar tools for their pathogen of interest

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

A B S T R A C T The results of very long baseline interferometry (VLBI) total intensity (I) and linear polarization (P) observations at l 6 cm 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 Ku Èhr & 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