1,730 research outputs found
Kinetic approaches to particle acceleration at cosmic ray modified shocks
Kinetic approaches provide an effective description of the process of
particle acceleration at shock fronts and allow to take into account the
dynamical reaction of the accelerated particles as well as the amplification of
the turbulent magnetic field as due to streaming instability. The latter does
in turn affect the maximum achievable momentum and thereby the acceleration
process itself, in a chain of causality which is typical of non-linear systems.
Here we provide a technical description of two of these kinetic approaches and
show that they basically lead to the same conclusions. In particular we discuss
the effects of shock modification on the spectral shape of the accelerated
particles, on the maximum momentum, on the thermodynamic properties of the
background fluid and on the escaping and advected fluxes of accelerated
particles.Comment: 22 pages, 7 figures, accepted for publication in MNRA
The X-ray Emissions from the M87 Jet: Diagnostics and Physical Interpretation
We reanalyze the deep Chandra observations of the M87 jet, first examined by
Wilson & Yang (2002). By employing an analysis chain that includes image
deconvolution, knots HST-1 and I are fully separated from adjacent emission. We
find slight but significant variations in the spectral shape, with values of
ranging from . We use VLA radio observations, as well
as HST imaging and polarimetry data, to examine the jet's broad-band spectrum
and inquire as to the nature of particle acceleration in the jet. As shown in
previous papers, a simple continuous injection model for synchrotron-emitting
knots, in which both the filling factor, , of regions within which
particles are accelerated and the energy spectrum of the injected particles are
constant, cannot account for the X-ray flux or spectrum. Instead, we propose
that is a function of position and energy and find that in the inner
jet, , and
in knots A and B, , where is the emitted photon energy and and is the
emitting electron energy. In this model, the index of the injected electron
energy spectrum () is at all locations in
the jet, as predicted by models of cosmic ray acceleration by ultrarelativistic
shocks. There is a strong correlation between the peaks of X-ray emission and
minima of optical percentage polarization, i.e., regions where the jet magnetic
field is not ordered. We suggest that the X-ray peaks coincide with shock waves
which accelerate the X-ray emitting electrons and cause changes in the
direction of the magnetic field; the polarization is thus small because of beam
averaging.Comment: Accepted for publication in ApJ; 21 pages, 9 figures, 2 tables;
abstract shortened for astro-ph; Figures 1, 7 and 8 at reduced resolutio
Factors associated with attendee adherence to COVID-19 guidance during the 2021 DCMS Events Research Programme Phase 1
As part of the DCMS Events Research Programme 2021, we surveyed and interviewed attendees of the FA Cup Semi-Final (18th April), Carabao Cup Final (25th April), the FA Cup Final (15th May), the Snooker World Championship (17th April â 3rd May) and Sefton Park music event (2nd May) to examine attendee experiences of the events, perceptions of the COVID-19 guidance, and factors most associated with self-reported adherence
Inequalities and identity processes in crises: recommendations for facilitating safe response to the COVID-19 pandemic
Structural inequalities and identity processes are pivotal to understanding public response to COVIDâ19. We discuss how identity processes can be used to promote communityâlevel support, safe normative behaviour, and increase compliance with guidance. However, we caution how government failure to account for structural inequalities can alienate vulnerable groups, inhibit groups from being able to follow guidance, and lead to the creation of new groups in response to illegitimate treatment. Moreover, we look ahead to the longitudinal impacts of inequalities during pandemics and advise government bodies should address identityâbased inequalities to mitigate negative relations with the public and subsequent collective protest
A field study of team working in a new human supervisory control system
This paper presents a case study of an investigation into team behaviour in an energy distribution company. The main aim was to investigate the impact of major changes in the company on system performance, comprising human and technical elements. A socio-technical systems approach was adopted. There were main differences between the teams investigated in the study: the time of year each control room was studied (i.e. summer or winter),the stage of development each team was in (i.e. 10 months), and the team structure (i.e. hierarchical or heterarchical). In all other respects the control rooms were the same: employing the same technology and within the same organization. The main findings were: the teams studied in the winter months were engaged in more `planningâ and `awarenessâ type of activities than those studies in the summer months. Newer teams seem to be engaged in more sharing of information than older teams, which maybe indicative of the development process. One of the hierarchical teams was engaged in more `system-drivenâ activities than the heterarchical team studied at the same time of year. Finally, in general, the heterarchical team perceived a greater degree of team working culture than its hierarchical counterparts. This applied research project confirms findings from laboratory research and emphasizes the importance of involving ergonomics in the design of team working in human supervisory control
Perceived responder legitimacy and group identification predict cooperation and compliance in a mass decontamination field exercise
Emergency respondersâ failure to communicate effectively during decontamination following a chemical or biological incident has been associated with increased public anxiety and reduced public compliance. In this study we applied the social identity approach to evaluating a field exercise that involved mass decontamination. Questionnaires were collected from 115 volunteers, who participated in the exercise as simulated casualties. Volunteersâ perceptions of effective responder communication predicted increased self-reported compliance with decontamination, mediated by perceived responder legitimacy and identification with other group members. Developing effective communication strategies using a social psychology perspective could improve the way in which incidents are managed
Parsimony versus reductionism: how can crowd psychology be introduced into computer simulation?
Computer simulations are increasingly being used to predict the behaviour of crowds. However, the models used are mainly based on video observations, not an understanding of human decision making. Theories of crowd psychology can elucidate the factors underpinning collective behaviour in human crowds. Yet, in contrast to psychology, computer science must rely upon mathematical formulations in order to implement algorithms and keep models manageable. Here we address the problems and possible solutions encountered when incorporating social psychological theories of collective behaviour in computer modelling. We identify that one primary issue is retaining parsimony in a model whilst avoiding reductionism by excluding necessary aspects of crowd psychology, such as the behaviour of groups. We propose cognitive heuristics as a potential avenue to create a parsimonious model that incorporates core concepts of collective behaviour derived from empirical research in crowd psychology
Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Axis-Symmetric Flows
We introduce a simple and economical but effective method for including
relativistic electron transport in multi-dimensional simulations of radio
galaxies. The method is designed to follow explicitly diffusive acceleration at
shocks, and, in smooth flows 2nd order Fermi acceleration plus adiabatic and
synchrotron cooling. We are able to follow both the spatial and energy
distributions of the electrons, so that direct synchrotron emission properties
can be modeled in time-dependent flows for the first time.
Here we present first results in the form of some axis-symmetric MHD
simulations of Mach 20 light jet flows. These show clearly the importance of
nonsteady terminal shocks that develop in such flows even when the jet inflow
is steady. As a result of this and other consequences of the fundamentally
driven character of jets, we find complex patterns of emissivities and
synchrotron spectra, including steep spectral gradients in hot spots, islands
of distinct spectra electrons within the lobes and spectral gradients coming
from the dynamical histories of a given flow element rather than from
synchrotron aging of the embedded electrons. In addition, spectral aging in the
lobes tends to proceed more slowly than one would estimate from regions of high
emissivity.Comment: 30 pages of Latex generated text plus 7 figures in gif format.
Accepted for publication in the Astrophysical Journal. High resolution
postscript figures available through anonymous ftp at
ftp://ftp.msi.umn.edu/pub/users/twj/RGje
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