442 research outputs found
Path-decomposition expansion and edge effects in a confined magnetized free-electron gas
Path-integral methods can be used to derive a `path-decomposition expansion'
for the temperature Green function of a magnetized free-electron gas confined
by a hard wall. With the help of this expansion the asymptotic behaviour of the
profiles for the excess particle density and the electric current density far
from the edge is determined for arbitrary values of the magnetic field
strength. The asymptotics are found to depend sensitively on the degree of
degeneracy. For a non-degenerate electron gas the asymptotic profiles are
essentially Gaussian (albeit modulated by a Bessel function), on a length scale
that is a function of the magnetic field strength and the temperature. For a
completely degenerate electron gas the asymptotic behaviour is again
proportional to a Gaussian, with a scale that is the magnetic length in this
case. The prefactors are polynomial and logarithmic functions of the distance
from the wall, that depend on the number of filled Landau levels . As a
consequence, the Gaussian asymptotic decay sets in at distances that are large
compared to the magnetic length multiplied by .Comment: 16 pages, 2 figures, submitted to J. Phys. A: Math. Gen; corrected
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Minimal re-computation for exploratory data analysis in astronomy
We present a technique to automatically minimise the re-computation when a
data processing program is iteratively changed, or added to, as is often the
case in exploratory data analysis in radio astronomy. A typical example is
flagging and calibration of demanding or unusual observations where visual
inspection suggests improvement to the processing strategy. The technique is
based on memoization and referentially transparent tasks. We describe a
prototype implementation for the CASA data reduction package. This technique
improves the efficiency of data analysis while reducing the possibility for
user error and improving the reproducibility of the final result
Correlations in a confined magnetized free-electron gas
Equilibrium quantum statistical methods are used to study the pair
correlation function for a magnetized free-electron gas in the presence of a
hard wall that is parallel to the field. With the help of a path-integral
technique and a Green function representation the modifications in the
correlation function caused by the wall are determined both for a
non-degenerate and for a completely degenerate gas. In the latter case the
asymptotic behaviour of the correlation function for large position differences
in the direction parallel to the wall and perpendicular to the field, is found
to change from Gaussian in the bulk to algebraic near the wall.Comment: 24 pages, 10 figures, submitted to J. Phys. A: Math. Ge
rPICARD: A CASA-based Calibration Pipeline for VLBI Data
Currently, HOPS and AIPS are the primary choices for the time-consuming
process of (millimeter) Very Long Baseline Interferometry (VLBI) data
calibration. However, for a full end-to-end pipeline, they either lack the
ability to perform easily scriptable incremental calibration or do not provide
full control over the workflow with the ability to manipulate and edit
calibration solutions directly. The Common Astronomy Software Application
(CASA) offers all these abilities, together with a secure development future
and an intuitive Python interface, which is very attractive for young radio
astronomers. Inspired by the recent addition of a global fringe-fitter, the
capability to convert FITS-IDI files to measurement sets, and amplitude
calibration routines based on ANTAB metadata, we have developed the the
CASA-based Radboud PIpeline for the Calibration of high Angular Resolution Data
(rPICARD). The pipeline will be able to handle data from multiple arrays: EHT,
GMVA, VLBA and the EVN in the first release. Polarization and phase-referencing
calibration are supported and a spectral line mode will be added in the future.
The large bandwidths of future radio observatories ask for a scalable reduction
software. Within CASA, a message passing interface (MPI) implementation is used
for parallelization, reducing the total time needed for processing. The most
significant gain is obtained for the time-consuming fringe-fitting task where
each scan be processed in parallel.Comment: 6 pages, 1 figure, EVN 2018 symposium proceeding
Psychological functioning in adolescents referred to specialist gender identity clinics across Europe : a clinical comparison study between four clinics
Adolescents seeking professional help with their gender identity development often present with psychological difficulties. Existing literature on psychological functioning of gender diverse young people is limited and mostly bound to national chart reviews. This study examined the prevalence of psychological functioning and peer relationship problems in adolescents across four European specialist gender services (The Netherlands, Belgium, the UK, and Switzerland), using the Child Behavioural Checklist (CBCL) and the Youth Self-Report (YSR). Differences in psychological functioning and peer relationships were found in gender diverse adolescents across Europe. Overall, emotional and behavioural problems and peer relationship problems were most prevalent in adolescents from the UK, followed by Switzerland and Belgium. The least behavioural and emotional problems and peer relationship problems were reported by adolescents from The Netherlands. Across the four clinics, a similar pattern of gender differences was found. Birth-assigned girls showed more behavioural problems and externalising problems in the clinical range, as reported by their parents. According to self-report, internalising problems in the clinical range were more prevalent in adolescent birth-assigned boys. More research is needed to gain a better understanding of the difference in clinical presentations in gender diverse adolescents and to investigate what contextual factors that may contribute to this
High-z radio starbursts host obscured X-ray AGN
We use Virtual Observatory methods to investigate the association between
radio and X-ray emission at high redshifts. Fifty-five of the 92 HDF(N) sources
resolved by combining
MERLIN+VLA data were detected by Chandra, of which 18 are hard enough and
bright enough to be obscured AGN. The high-z population of microJy radio
sources is dominated by starbursts an order of magnitude more active and more
extended than any found at z<1 and at least a quarter of these simultaneously
host highly X-ray-luminous obscured AGN.Comment: 4 pages, 2 figures, To appear in the proceedings of 'At the Edge of
the Universe' (9-13 October 2006, Sintra, Portugal
The SFXC software correlator for Very Long Baseline Interferometry: Algorithms and Implementation
In this paper a description is given of the SFXC software correlator,
developed and maintained at the Joint Institute for VLBI in Europe (JIVE). The
software is designed to run on generic Linux-based computing clusters. The
correlation algorithm is explained in detail, as are some of the novel modes
that software correlation has enabled, such as wide-field VLBI imaging through
the use of multiple phase centres and pulsar gating and binning. This is
followed by an overview of the software architecture. Finally, the performance
of the correlator as a function of number of CPU cores, telescopes and spectral
channels is shown.Comment: Accepted by Experimental Astronom
Time correlations in a confined magnetized free-electron gas
The time-dependent pair correlation functions for a degenerate ideal quantum
gas of charged particles in a uniform magnetic field are studied on the basis
of equilibrium statistics. In particular, the influence of a flat hard wall on
the correlations is investigated, both for a perpendicular and a parallel
orientation of the wall with respect to the field. The coherent and incoherent
parts of the time-dependent structure function in position space are determined
from an expansion in terms of the eigenfunctions of the one-particle
Hamiltonian. For the bulk of the system, the intermediate scattering function
and the dynamical structure factor are derived by taking successive Fourier
transforms. In the vicinity of the wall the time-dependent coherent structure
function is found to decay faster than in the bulk. For coinciding positions
near the wall the form of the structure function turns out to be independent of
the orientation of the wall. Numerical results are shown to corroborate these
findings.Comment: 25 pages, 14 figures, to be published in Journal of Physics
Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. I. Jet Energetics and the Pristine Parsec-Scale Environment of a Supermassive Black Hole
We present continued radio observations of the tidal disruption event
SwiftJ164449.3+573451 extending to \sim216 days after discovery. The data are
part of a long-term program to monitor the expansion and energy scale of the
relativistic outflow, and to trace the parsec-scale environment around a
previously-dormant supermassive black hole (SMBH). The new observations reveal
a significant change in the radio evolution starting at \sim1 month, with a
brightening at all frequencies that requires an increase in the energy by about
an order of magnitude, and an overall density profile around the SMBH of rho
\propto r^{-3/2} (0.1-1.2 pc) with a significant flattening at r\sim0.4-0.6 pc.
The increase in energy cannot be explained with continuous injection from an L
\propto t^{-5/3} tail, which is observed in the X-rays. Instead, we conclude
that the relativistic jet was launched with a wide range of Lorentz factors,
obeying E(>Gamma) \propto Gamma^{-2.5}. The similar ratio of duration to
dynamical timescale for Sw1644+57 and GRBs suggests that this result may be
applicable to GRBs as well. The radial density profile may be indicative of
Bondi accretion, with the inferred flattening at r\sim0.5 pc in good agreement
with the Bondi radius for a \sim10^6 M_sun black hole. The density at \sim0.5
pc is about a factor of 30 times lower than inferred for the Milky Way galactic
center, potentially due to a smaller number of mass-shedding massive stars.
From our latest observations (\sim216 d) we find that the jet energy is
E_{iso}\sim5x10^{53} erg (E_j\sim2.4x10^{51} erg for theta_j=0.1), the radius
is r\sim1.2 pc, the Lorentz factor is Gamma\sim2.2, the ambient density is
n\sim0.2 cm^{-3}, and the projected size is r_{proj}\sim25 microarcsec.
Assuming no future changes in the observed evolution we predict that the radio
emission from Sw1644+57 should be detectable with the EVLA for several decades,
and will be resolvable with VLBI in a few years.Comment: Submitted to ApJ; 22 pages, 2 tables, 9 figure
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