1,821 research outputs found
Extremely Anisotropic Scintillations
A small number of quasars exhibit interstellar scintillation on time-scales
less than an hour; their scintillation patterns are all known to be
anisotropic. Here we consider a totally anisotropic model in which the
scintillation pattern is effectively one-dimensional. For the persistent rapid
scintillators J1819+3845 and PKS1257-326 we show that this model offers a good
description of the two-station time-delay measurements and the annual cycle in
the scintillation time-scale. Generalising the model to finite anisotropy
yields a better match to the data but the improvement is not significant and
the two additional parameters which are required to describe this model are not
justified by the existing data. The extreme anisotropy we infer for the
scintillation patterns must be attributed to the scattering medium rather than
a highly elongated source. For J1819+3845 the totally anisotropic model
predicts that the particular radio flux variations seen between mid July and
late August should repeat between late August and mid November, and then again
between mid November and late December as the Earth twice changes its direction
of motion across the scintillation pattern. If this effect can be observed then
the minor-axis velocity component of the screen and the orientation of that
axis can both be precisely determined. In reality the axis ratio is finite,
albeit large, and spatial decorrelation of the flux pattern along the major
axis may be observable via differences in the pairwise fluxes within this
overlap region; in this case we can also constrain both the major-axis velocity
component of the screen and the magnitude of the anisotropy.Comment: 5 pages, 4 figures, MNRAS submitte
Machine Learning and Irresponsible Inference: Morally Assessing the Training Data for Image Recognition Systems
Just as humans can draw conclusions responsibly or irresponsibly, so too can computers. Machine learning systems that have been trained on data sets that include irresponsible judgments are likely to yield irresponsible predictions as outputs. In this paper I focus on a particular kind of inference a computer system might make: identification of the intentions with which a person acted on the basis of photographic evidence. Such inferences are liable to be morally objectionable, because of a way in which they are presumptuous. After elaborating this moral concern, I explore the possibility that carefully procuring the training data for image recognition systems could ensure that the systems avoid the problem. The lesson of this paper extends beyond just the particular case of image recognition systems and the challenge of responsibly identifying a personâs intentions. Reflection on this particular case demonstrates the importance (as well as the difficulty) of evaluating machine learning systems and their training data from the standpoint of moral considerations that are not encompassed by ordinary assessments of predictive accuracy
New Forms of Data for Understanding Urban Activity in Developing Countries
Emerging forms of data offer new opportunities for developing a deeper understanding of poorly understood social and spatial processes. This is no more important than in developing countries, where large-scale data collection and processing has been relatively limited. In this paper, we explore how two new datasets can be used to enhance our understanding of human activity and communication interactions in Dakar, Senegal. Starting from a premise of little contextual knowledge about the setting in which we are working, we explore how much these data, combined with novel quantitative methods, are able to inform us about the urban environment in question. Fine-grained infrastructural data are combined with k-means clustering to produce an 11-class land use classification, distinguishing dense and sparse, single and mixed use regions. Using these classifications and over 1.5 billion mobile phone call records, patterns of activity and interaction within and between land use types are analysed. These analyses reveal strong activity associated with high density commercial, governmental and administrative regions. These regions are also identified as relatively strong âattractorsâ of communication, and wider patterns show higher interactions between areas with similar land use characteristics. Analyses of dynamic activity and interaction patterns highlight the movement of people from workplace zones to residential areas. Importantly, these analyses reflect the expected patterns of urban activity, providing some validation of the data, methods, and the empirical approach. The paper concludes in addressing the strengths and potential of these approaches, while recognising current limitations and areas for further work
On rapid interstellar scintillation of quasars: PKS 1257-326 revisited
The line of sight towards the compact, radio loud quasar PKS 1257-326 passesthrough a patch of scattering plasma in the local Galactic ISM that causes large and rapid,intra-hour variations in the received flux density at centimetre wavelengths. This rapid interstellarscintillation (SS) has been occurring for at least 15 years, implying that the scatteringâscreenâ is at least 100 AU in physical extent. Through observations of the ISS we have measuredmicroarcsecond-scale âcore shiftsâ in PKS 1257-326, corresponding to changing opacityduring an intrinsic outburst. Recent analysis of VLA data of a sample of 128 quasars found 6sources scintillating with a characteristic time-scale of < 2 hours, suggesting that nearby scatteringscreens in the ISM may have a covering fraction of a few percent. That is an importantconsideration for proposed surveys of the transient and variable radio sk
Seeing motion and apparent motion
In apparent motion experiments, participants are presented with what is in fact a succession of two brief stationary stimuli at two different locations, but they report an impression of movement. Philosophers have recently debated whether apparent motion provides evidence in favour of a particular account of the nature of temporal experience. I argue that the existing discussion in this area is premised on a mistaken view of the phenomenology of apparent motion and, as a result, the space of possible philosophical positions has not yet been fully explored. In particular, I argue that the existence of apparent motion is compatible with an account of the nature of temporal experience that involves a version of direct realism. In doing so, I also argue against two other claims often made about apparent motion, viz. that apparent motion is the psychological phenomenon that underlies motion experience in the cinema, and that apparent motion is subjectively indistinguishable from real motion
Detection of Six Rapidly Scintillating AGNs and the Diminished Variability of J1819+3845
The extreme, intra-hour and > 10% rms flux density scintillation observed in
AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has
been attributed to scattering in highly turbulent, nearby regions in the
interstellar medium. Such behavior has been found to be rare. We searched for
rapid scintillators among 128 flat spectrum AGNs and analyzed their properties
to determine the origin of such rapid and large amplitude radio scintillation.
The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two
hour intervals over 11 days. We detected six rapid scintillators with
characteristic time-scales of
10%. We found strong lines of evidence linking rapid scintillation to the
presence of nearby scattering regions, estimated to be < 12 pc away for ~ 200
muas sources and < 250 pc away for ~ 10 muas sources. We attribute the scarcity
of rapid and large amplitude scintillators to the requirement of additional
constraints, including large source compact fractions. J1819+3845 was found to
display ~ 2% rms variations at ~ 6 hour time-scales superposed on longer > 11
day variations, suggesting that the highly turbulent cloud responsible for its
extreme scintillation has moved away, with its scintillation now caused by a
more distant screen ~ 50 to 150 pc away.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
First Results from MASIV: The Micro-Arcsecond Scintillation-Induced Variability Survey
We are undertaking a large-scale, Micro-Arcsecond Scintillation-Induced
Variability (MASIV) survey of the northern sky, Dec > 0 deg, at 4.9 GHz with
the VLA. Our objective is to construct a sample of 100 to 150 scintillating
extragalactic sources with which to examine both the microarcsecond structure
and the parent populations of these sources, and to probe the turbulent
interstellar medium responsible for the scintillation. We report on our first
epoch of observations which revealed variability on timescales ranging from
hours to days in 85 of 710 compact flat-spectrum sources. The number of highly
variable sources, those with RMS flux density variations greater than 4% of the
mean, increases with decreasing source flux density but rapid, large amplitude
variables such as J1819+3845 are very rare. When compared with a model for the
scintillation due to irregularities in a 500 pc thick electron layer, our
preliminary results indicate maximum brightness temperatures ~10E+12 K, similar
to those obtained from VLBI surveys even though interstellar scintillation is
not subject to the same angular resolution limit.Comment: 18 pages, 5 figures. To appear in the Astronomical Journa
Untangling the Conceptual Isssues Raised in Reydon and Scholzâs Critique of Organizational Ecology and Darwinian Populations
Reydon and Scholz raise doubts about the Darwinian status of organizational ecology by arguing that Darwinian principles are not applicable to organizational populations. Although their critique of organizational ecologyâs typological essentialism is correct, they go on to reject the Darwinian status of organizational populations. This paper claims that the distinction between replicators and interactors, raised in modern philosophy of biology but not discussed by Reydon and Scholz, points the way forward for organizational ecologists. It is possible to conceptualise evolving Darwinian populations providing the inheritance mechanism is appropriately specified. By this approach, adaptation and selection are no longer dichotomised, and the evolutionary significance of knowledge transmission is highlightedPeer reviewe
Conceptual design study for heat exhaust management in the ARC fusion pilot plant
The ARC pilot plant conceptual design study has been extended beyond its
initial scope [B. N. Sorbom et al., FED 100 (2015) 378] to explore options for
managing ~525 MW of fusion power generated in a compact, high field (B_0 = 9.2
T) tokamak that is approximately the size of JET (R_0 = 3.3 m). Taking
advantage of ARC's novel design - demountable high temperature superconductor
toroidal field (TF) magnets, poloidal magnetic field coils located inside the
TF, and vacuum vessel (VV) immersed in molten salt FLiBe blanket - this
follow-on study has identified innovative and potentially robust power exhaust
management solutions.Comment: Accepted by Fusion Engineering and Desig
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