1,366 research outputs found
A 6-12 GHz Analogue Lag-Correlator for Radio Interferometry
Aims: We describe a 6-12 GHz analogue correlator that has been developed for
use in radio interferometers. Methods: We use a lag-correlator technique to
synthesis eight complex spectral channels. Two schemes were considered for
sampling the cross-correlation function, using either real or complex
correlations, and we developed prototypes for both of them. We opted for the
``add and square'' detection scheme using Schottky diodes over the more
commonly used active multipliers because the stability of the device is less
critical. Results: We encountered an unexpected problem, in that there were
errors in the lag spacings of up to ten percent of the unit spacing. To
overcome this, we developed a calibration method using astronomical sources
which corrects the effects of the non-uniform sampling as well as gain error
and dispersion in the correlator.Comment: 14 pages, 21 figures, accepted for publication in A&
A 6-12 GHz Analogue Lag-Correlator for Radio Interferometry
Aims: We describe a 6-12 GHz analogue correlator that has been developed for
use in radio interferometers. Methods: We use a lag-correlator technique to
synthesis eight complex spectral channels. Two schemes were considered for
sampling the cross-correlation function, using either real or complex
correlations, and we developed prototypes for both of them. We opted for the
``add and square'' detection scheme using Schottky diodes over the more
commonly used active multipliers because the stability of the device is less
critical. Results: We encountered an unexpected problem, in that there were
errors in the lag spacings of up to ten percent of the unit spacing. To
overcome this, we developed a calibration method using astronomical sources
which corrects the effects of the non-uniform sampling as well as gain error
and dispersion in the correlator.Comment: 14 pages, 21 figures, accepted for publication in A&
Correlated X-ray and Optical Variability in Mkn 509
We present results of a 3 year monitoring campaign of the Seyfert 1 galaxy
Markarian 509, using X-ray data from the Rossi X-ray Timing Explorer (RXTE) and
optical data taken by the SMARTS consortium. Both light curves show significant
variations, and are strongly correlated with the optical flux leading the X-ray
flux by 15 days. The X-ray power spectrum shows a steep high-frequency slope of
-2.0, breaking to a slope of -1.0 at at timescale of 34 days. The lag from
optical to X-ray emission is most likely caused by variations in the accretion
disk propagating inward.Comment: 13 pages, 3 figures. Accepted for publication in the Astrophysical
Journa
Is attending a mental process?
The nature of attention has been the topic of a lively research programme in psychology for over a century. But there is widespread agreement that none of the theories on offer manage to fully capture the nature of attention. Recently, philosophers have become interested in the debate again after a prolonged period of neglect. This paper contributes to the project of explaining the nature of attention. It starts off by critically examining Christopher Moleâs prominent âadverbialâ account of attention, which traces the failure of extant psychological theories to their assumption that attending is a kind of process. It then defends an alternative, process-based view of the metaphysics of attention, on which attention is understood as an activity and not, as psychologists seem to implicitly assume, an accomplishment. The entrenched distinction between accomplishments and activities is shown to shed new light on the metaphysics of attention. It also provides a novel diagnosis of the empirical state of play
Personal and sub-personal: a defence of Dennett's early distinction
Since 1969, when Dennett introduced a distinction between personal and subâpersonal levels of explanation, many philosophers have used âsubâpersonalâ very loosely, and Dennett himself has abandoned a view of the personal level as genuinely autonomous. I recommend a position in which Dennett's original distinction is crucial, by arguing that the phenomenon called mental causation is on view only at the properly personal level. If one retains the commitââ ments incurred by Dennett's early distinction, then one has a satisfactory antiâphysicalistic, antiâdualist philosophy of mind. It neither interferes with the projects of subâpersonal psychology, nor encourages ; instrumentalism at the personal level.
People lose sight of Dennettâs personal/sub-personal distinction because they free it from its philosophical moorings. A distinction that serves a philosophical purpose is typically rooted in doctrine; it cannot be lifted out of context and continue to do its work. So I shall start from Dennettâs distinction as I read it in its original context. And when I speak of âthe distinctionâ, I mean to point not only towards the terms that Dennett first used to define it but also towards the philosophical setting within which its work was cut out
Radio Galaxy NGC 1265 unveils the Accretion Shock onto the Perseus Galaxy Cluster
We present a consistent 3D model for the head-tail radio galaxy NGC 1265 that
explains the complex radio morphology and spectrum by a past passage of the
galaxy and radio bubble through a shock wave. Using analytical solutions to the
full Riemann problem and hydrodynamical simulations, we study how this passage
transformed the plasma bubble into a toroidal vortex ring. Adiabatic
compression of the aged electron population causes it to be energized and to
emit low-surface brightness and steep-spectrum radio emission. The large infall
velocity of NGC 1265 and the low Faraday rotation measure values and variance
of the jet strongly argue that this transformation was due to the accretion
shock onto Perseus situated roughly at R_200. Estimating the volume change of
the radio bubble enables inferring a shock Mach number of M =
4.2_{-1.2}^{+0.8}, a density jump of 3.4_{-0.4}^{+0.2}, a temperature jump of
6.3_{-2.7}^{+2.5}, and a pressure jump of 21.5 +/- 10.5 while allowing for
uncertainties in the equation of state of the radio plasma and volume of the
torus. Extrapolating X-ray profiles, we obtain upper limits on the gas
temperature and density in the infalling warm-hot intergalactic medium of kT <
0.4 keV and n < 5e-5 / cm^3. The orientation of the ellipsoidally shaped radio
torus in combination with the direction of the galaxy's head and tail in the
plane of the sky is impossible to reconcile with projection effects. Instead,
this argues for post-shock shear flows that have been caused by curvature in
the shock surface with a characteristic radius of 850 kpc. The energy density
of the shear flow corresponds to a turbulent-to-thermal energy density of 14%.
The shock-injected vorticity might be important in generating and amplifying
magnetic fields in galaxy clusters. Future LOFAR observations of head-tail
galaxies can be complementary probes of accretion shocks onto galaxy clusters.Comment: 14 pages, 4 figures, ApJ, in print; v3: typos corrected to match the
published version; v2: improved presentation, added 2D numerical simulations
and exact solution to the 1D Riemann problem of a shock overrunning a
spherical bubble that gets transformed into a vortex rin
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Managing digital coordination of design: emerging hybrid practices in an institutionalized project setting
What happens when digital coordination practices are introduced into the institutionalized setting of an engineering project? This question is addressed through an interpretive study that examines how a shared digital model becomes used in the late design stages of a major station refurbishment project. The paper contributes by mobilizing the idea of âhybrid practicesâ to understand the diverse patterns of activity that emerge to manage digital coordination of design. It articulates how engineering and architecture professions develop different relationships with the shared model; the design team negotiates paper-based practices across organizational boundaries; and diverse practitioners probe the potential and limitations of the digital infrastructure. While different software packages and tools have become linked together into an integrated digital infrastructure, these emerging hybrid practices contrast with the interactions anticipated in practice and policy guidance and presenting new opportunities and challenges for managing project delivery. The study has implications for researchers working in the growing field of empirical work on engineering project organizations as it shows the importance of considering, and suggests new ways to theorise, the introduction of digital coordination practices into these institutionalized settings
Scattering in the vicinity of relativistic jets: a method for constraining jet parameters
Relativistic jets of radio loud active galactic nuclei (AGN) produce highly
directed, intense beams of radiation. A fraction of this beamed radiation
scatters on the thermal plasma generally surrounding an AGN. The morphology of
the scattered emission can thus provide constraints on the physical properties
of the jet. We present a model to study the feasibility of constraining the
parameters of a jet, especially its inclination angle and bulk Lorentz factor
in this way. We apply our model to the well studied jet of M87 and the
surrounding diffuse gas and find that the observational limits of the surface
brightness measured in the region of the putative counterjet provide the
tightest constraints on the jet parameters consistent with constraints derived
by other methods. We briefly discuss the applicability of our model to other
sources exhibiting relativistic motionsComment: 17 pages, 15 figures, to appear in A&A, 420, 33 (2004
The 74MHz System on the Very Large Array
The Naval Research Laboratory and the National Radio Astronomy Observatory
completed implementation of a low frequency capability on the VLA at 73.8 MHz
in 1998. This frequency band offers unprecedented sensitivity (~25 mJy/beam)
and resolution (~25 arcsec) for low-frequency observations. We review the
hardware, the calibration and imaging strategies, comparing them to those at
higher frequencies, including aspects of interference excision and wide-field
imaging. Ionospheric phase fluctuations pose the major difficulty in
calibrating the array. Over restricted fields of view or at times of extremely
quiescent ionospheric ``weather'', an angle-invariant calibration strategy can
be used. In this approach a single phase correction is devised for each
antenna, typically via self-calibration. Over larger fields of view or at times
of more normal ionospheric ``weather'' when the ionospheric isoplanatic patch
size is smaller than the field of view, we adopt a field-based strategy in
which the phase correction depends upon location within the field of view. This
second calibration strategy was implemented by modeling the ionosphere above
the array using Zernike polynomials. Images of 3C sources of moderate strength
are provided as examples of routine, angle-invariant calibration and imaging.
Flux density measurements indicate that the 74 MHz flux scale at the VLA is
stable to a few percent, and tied to the Baars et al. value of Cygnus A at the
5 percent level. We also present an example of a wide-field image, devoid of
bright objects and containing hundreds of weaker sources, constructed from the
field-based calibration. We close with a summary of lessons the 74 MHz system
offers as a model for new and developing low-frequency telescopes. (Abridged)Comment: 73 pages, 46 jpeg figures, to appear in ApJ
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