Jet opening angles and gamma-ray brightness of AGN

We have investigated the differences in apparent opening angles between the parsec-scale jets of the active galactic nuclei (AGN) detected by the Fermi Large Area Telescope (LAT) during its first three months of operations and those of non-LAT-detected AGN. We used 15.4 GHz VLBA observations of sources from the 2 cm VLBA MOJAVE program, a subset of which comprise the statistically complete flux density limited MOJAVE sample. We determined the apparent opening angles by analyzing transverse jet profiles from the data in the image plane and by applying a model fitting technique to the data in the (u,v) plane. Both methods provided comparable opening angle estimates. The apparent opening angles of gamma-ray bright blazars are preferentially larger than those of gamma-ray weak sources. At the same time, we have found the two groups to have similar intrinsic opening angle distributions, based on a smaller subset of sources. This suggests that the jets in gamma-ray bright AGN are oriented at preferentially smaller angles to the line of sight resulting in a stronger relativistic beaming. The intrinsic jet opening angle and bulk flow Lorentz factor are found to be inversely proportional, as predicted by standard models of compact relativistic jets. If a gas dynamical jet acceleration model is assumed, the ratio of the initial pressure of the plasma in the core region P_0 to the external pressure P_ext lies within the range 1.1 to 34.6, with a best fit estimate of P_0/P_ext=2.Comment: 4 pages, 3 figures; accepted for publication in the A&A Letters; table in electronic form can be extracted from the preprint sourc

MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. VII. Blazar Jet Acceleration

We discuss acceleration measurements for a large sample of extragalactic radio jets from the MOJAVE program which studies the parsec-scale jet structure and kinematics of a complete, flux-density-limited sample of Active Galactic Nuclei (AGN). Accelerations are measured from the apparent motion of individual jet features or "components" which may represent patterns in the jet flow. We find that significant accelerations are common both parallel and perpendicular to the observed component velocities. Parallel accelerations, representing changes in apparent speed, are generally larger than perpendicular acceleration that represent changes in apparent direction. The trend for larger parallel accelerations indicates that a significant fraction of these changes in apparent speed are due to changes in intrinsic speed of the component rather than changes in direction to the line of sight. We find an overall tendency for components with increasing apparent speed to be closer to the base of their jets than components with decreasing apparent speed. This suggests a link between the observed pattern motions and the underlying flow which, in some cases, may increase in speed close to the base and decrease in speed further out; however, common hydro-dynamical processes for propagating shocks may also play a role. About half of the components show "non-radial" motion, or a misalignment between the component's structural position angle and its velocity direction, and these misalignments generally better align the component motion with the downstream emission. Perpendicular accelerations are closely linked with non-radial motion. When observed together, perpendicular accelerations are usually in the correct direction to have caused the observed misalignment.Comment: 17 pages, 11 figures, 1 table, accepted by the Astrophysical Journa

The Relation between Radio Polarization and Gamma-ray Emission in AGN Jets

We have compared the parsec-scale jet linear polarization properties of the Fermi LAT-detected and non-detected sources in the complete flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE sources, 30 were detected by the LAT during its first three months of operation. We find that during the era since the launch of Fermi, the unresolved core components of the LAT-detected jets have significantly higher median fractional polarization at 15 GHz. This complements our previous findings that these LAT sources have higher apparent jet speeds, brightness temperatures and Doppler factors, and are preferentially found in higher activity states.Comment: 6 pages, 3 figures, to appear in the proceedings of "High Energy Phenomena In Relativistic Outflows II" (Buenos Aires, Argentina, October 26-30, 2009) International Journal of Modern Physics

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VI. Kinematics Analysis of a Complete Sample of Blazar Jets

We discuss the jet kinematics of a complete flux-density-limited sample of 135 radio-loud active galactic nuclei (AGN) resulting from a 13 year program to investigate the structure and evolution of parsec-scale jet phenomena. Our analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained between 2002 and 2007, but includes our previously published observations made at the same wavelength, and is supplemented by VLBA archive data. In all, we have used 2424 images spanning the years 1994-2007 to study and determine the motions of 526 separate jet features in 127 jets. The data quality and temporal coverage (a median of 15 epochs per source) of this complete AGN jet sample represents a significant advance over previous kinematics surveys. In all but five AGNs, the jets appear one-sided, most likely the result of differential Doppler boosting. In general the observed motions are directed along the jet ridge line, outward from the optically thick core feature. We directly observe changes in speed and/or direction in one third of the well-sampled jet components in our survey. While there is some spread in the apparent speeds of separate features within an individual jet, the dispersion is about three times smaller than the overall dispersion of speeds among all jets. This supports the idea that there is a characteristic flow that describes each jet, which we have characterized by the fastest observed component speed. The observed maximum speed distribution is peaked at ~10c, with a tail that extends out to ~50c. This requires a distribution of intrinsic Lorentz factors in the parent population that range up to ~50. We also note the presence of some rare low-pattern speeds or even stationary features in otherwise rapidly flowing jets... (abridged)Comment: 19 pages, 10 figures, 2 tables, accepted by the Astronomical Journal; online only material is available from http://www.cv.nrao.edu/2cmVLBA/pub/MOJAVE_VI_suppl.zi

What’s in a label? An exploration of how people acquire the label ‘autistic’ in adulthood and the consequences of doing so

Background. Since the 1960s the estimated prevalence of autism has increased. This has been accompanied with greater public awareness of the condition and a growing demand for diagnosis, particularly in adulthood. For sociologists, diagnoses such as autism play a fundamental role in modern social life. As well as organising the clinical picture for patients – determining their prognosis and treatment options – diagnoses also have the capacity to change how a person thinks about themselves and other people. Previous research has shown that obtaining an autism diagnosis in adulthood comes with significant benefits (greater self-awareness and access to support services) as well as some undesirable drawbacks (shame and a sense of helplessness). Yet a medical diagnosis is not the only way of acquiring the label. An individual can also label themselves – that is, self-identify – as autistic, and they can be labelled as such by other autistic people. To date, little has been done to investigate these other ways of acquiring the label, and more broadly the implications of being labelled autistic, by any means, have yet to be clearly theorised by sociologists. I aim to address these gaps in this study. Methods. I conducted a qualitative study in order to answer the question: “How do people come to be labelled, or to label themselves, as autistic in adulthood, and what are the consequences of doing so?” Using snowball and theoretical sampling, I recruited twenty-one autistic adults, eleven with a medical diagnosis and ten who self- identified as such, to take part in two loosely structured qualitative interviews (forty-two interviews in total). These accounts were analysed using a method called situational analysis, a form of constructivist grounded theory. Findings. I present three theoretical concepts that illustrate how people go about acquiring the label autistic and what it means to live with it. The first is the concept of the ‘sticky-slippy’ label, which is a figurative expression used to illustrate some of the properties of the label autistic. Once acquired, the label has an inherent ‘stickiness’ to it (a sense of permanence) whilst at the same time exhibiting more ‘slippery’ qualities (a fluid and shifting prominence in a person’s identity). The second concept relates specifically to people self-identifying as autistic and their reasons for doing so, which are conceptualised as four different ways: (1) somebody who self-identifies as autistic as a precursor to seeking a medical diagnosis, (2) somebody who self-identifies as autistic despite a negative diagnosis, (3) somebody who self-identifies as autistic as an alternative to a diagnosis, and (4) somebody who self-identifies as only having autistic traits. The third concept relates to the practice of autistic lay people labelling other lay people as autistic (which I call a ‘lay diagnosis’). Within this, I distinguish between ‘passively spotting’ and ‘actively seeking’ autism in others. Discussion. The ambition of this study is to provide a conceptual vocabulary for thinking about the nature of the label autistic, the different ways in which people can acquire it, and the implications of doing so. The concepts presented here may be applicable to other physical and psychiatric categories, of which autism serves as an illustrative example. The concept of the sticky-slippy label offers a means of understanding and reporting the consequences of being labelled autistic, something that is markedly absent in the current literature. The four ways of self-identification represent a sustained engagement with the growing phenomenon of people labelling themselves as autistic, which may be of interest to those researching the self-identification or self-diagnosis of other psychiatric conditions. Finally, I open the door on a potentially interesting research agenda: the act of lay people diagnosing other lay people with physical and psychological afflictions – lay diagnosis – which could sit alongside current research endeavours within the sociology of diagnosis

Plural Policing and the Challenge of Democratic Accountability

This chapter explores the accountability of the mixed economy of public and private policing in the England and Wales. Its starting point is the recognition that a variety of public, private and hybrid actors are engaged in the authorization and provision of policing. As a consequence, the article does not restrict its analytical gaze to how state power, as deployed by police forces, is made accountable. Rather, it considers how, under market conditions, networks of plural policing can be governed according to, and accommodated within, a set of democratic principles. In so doing, it is argued that ‘local security networks’ comprising state, civil society and market actors, and whose governance and accountability mechanisms frequently stand outside of extant political structures, raise specific challenges if they are to be governed not only effectively but also democratically. The article proceeds to consider options for bringing democratically accountable governance to plural policing networks

On the Frequency of Words Used in Answers to Explain in Plain English Questions by Novice Programmers

© 2019 Association for Computing Machinery. Most previous research studies using Explain in Plain English questions have focussed on categorising the answers of novice programmers according to the SOLO taxonomy, and/or the relationship between explaining code and writing code. In this paper, we study the words used in the explanations of novice programmers. Our data is from twelve Explain in plain English questions presented to over three hundred students in an exam at the end of the students' first semester of programming. For each question, we compare the frequency of certain words used in correct answers, between students who scored a perfect twelve on all the Explain in plain English questions and students with lower scores. We report a number of statistically significant differences in word frequency between the students who answered all questions correctly and students who did not. The students who answered all twelve questions correctly tended to be more precise, more comprehensive, and more likely to choose words not explicitly in the code, but instead words that are an abstraction beyond the code