14,533 research outputs found
Synthetic Observations of Simulated Radio Galaxies I: Radio and X-ray Analysis
We present an extensive synthetic observational analysis of numerically-
simulated radio galaxies designed to explore the effectiveness of conventional
observational analyses at recovering physical source properties. These are the
first numerical simulations with sufficient physical detail to allow such a
study. The present paper focuses on extraction of magnetic field properties
from nonthermal intensity information. Synchrotron and inverse-Compton
intensities provided meaningful information about distributions and strengths
of magnetic fields, although considerable care was called for. Correlations
between radio and X-ray surface brightness correctly revealed useful dynamical
relationships between particles and fields. Magnetic field strength estimates
derived from the ratio of X-ray to radio intensity were mostly within about a
factor of two of the RMS field strength along a given line of sight. When
emissions along a given line of sight were dominated by regions close to the
minimum energy/equipartition condition, the field strengths derived from the
standard power-law-spectrum minimum energy calculation were also reasonably
close to actual field strengths, except when spectral aging was evident.
Otherwise, biases in the minimum- energy magnetic field estimation mirrored
actual differences from equipartition. The ratio of the inverse-Compton
magnetic field to the minimum-energy magnetic field provided a rough measure of
the actual total energy in particles and fields in most instances, within an
order of magnitude. This may provide a practical limit to the accuracy with
which one may be able to establish the internal energy density or pressure of
optically thin synchrotron sources.Comment: 43 pages, 14 figures; accepted for publication in ApJ, v601 n2
February 1, 200
Slowly driven sandpile formation with granular mixtures
We introduce a one-dimensional sandpile model with different particle types and an infinitesimal driving rate. The parameters for the model are the N^2 critical slopes for one type of particle on top of another. The model is trivial when N=1, but for N=2 we observe four broad classes of sandpile structure in different regions of the parameter space. We describe and explain the behaviour of each of these classes, giving quantitative analysis wherever possible. The behaviour of sandpiles with N>2 essentially consists of combinations of these four classes. We investigate the model's robustness and highlight the key areas that any experiment designed to reproduce these results should focus on
On the interpretation of spin-polarized electron energy loss spectra
We study the origin of the structure in the spin-polarized electron energy
loss spectroscopy (SPEELS) spectra of ferromagnetic crystals. Our study is
based on a 3d tight-binding Fe model, with constant onsite Coulomb repulsion U
between electrons of opposite spin. We find it is not the total density of
Stoner states as a function of energy loss which determines the response of the
system in the Stoner region, as usually thought, but the densities of Stoner
states for only a few interband transitions. Which transitions are important
depends ultimately on how strongly umklapp processes couple the corresponding
bands. This allows us to show, in particular, that the Stoner peak in SPEELS
spectra does not necessarily indicate the value of the exchange splitting
energy. Thus, the common assumption that this peak allows us to estimate the
magnetic moment through its correlation with exchange splitting should be
reconsidered, both in bulk and surface studies. Furthermore, we are able to
show that the above mechanism is one of the main causes for the typical
broadness of experimental spectra. Finally, our model predicts that optical
spin waves should be excited in SPEELS experiments.Comment: 11 pages, 7 eps figures, REVTeX fil
The Line-of-Sight Proximity Effect and the Mass of Quasar Host Halos
We show that the Lyman-alpha optical depth statistics in the proximity
regions of quasar spectra depend on the mass of the dark matter halos hosting
the quasars. This is owing to both the overdensity around the quasars and the
associated infall of gas toward them. For a fiducial quasar host halo mass of
(3.0+/-1.6) h^-1 x 10^12 Msun, as inferred by Croom et al. from clustering in
the 2dF QSO Redshift Survey, we show that estimates of the ionizing background
(Gamma^bkg) from proximity effect measurements could be biased high by a factor
of ~2.5 at z=3 owing to neglecting these effects alone. The clustering of
galaxies and other active galactic nuclei around the proximity effect quasars
enhances the local background, but is not expected to skew measurements by more
than a few percent. Assuming the measurements of Gamma^bkg based on the mean
flux decrement in the Ly-alpha forest to be free of bias, we demonstrate how
the proximity effect analysis can be inverted to measure the mass of the dark
matter halos hosting quasars. In ideal conditions, such a measurement could be
made with a precision comparable to the best clustering constraints to date
from a modest sample of only about 100 spectra. We discuss observational
difficulties, including continuum flux estimation, quasar systematic redshift
determination, and quasar variability, which make accurate proximity effect
measurements challenging in practice. These are also likely to contribute to
the discrepancies between existing proximity effect and flux decrement
measurements of Gamma^bkg.Comment: 25 pages, including 14 figures, accepted by Ap
Observation of magnetization reversal and negative magnetization in a double perovskite compound Sr2YbRuO6
Detailed magnetic properties of the compound Sr2YbRuO6 are presented here.
The compound belongs to the family of double perovskites forming a monoclinic
structure. Magnetization meas-urements reveal clear evidence for two components
of magnetic ordering aligned opposite to each other, leading to a magnetization
reversal, compensation temperature (T* = 34 K) and neg-ative magnetization at
low temperatures and low magnetic fields. Heat capacity measurements
corroborate the presence of two components in the magnetic ordering and a
noticeable third anomaly at low temperatures (~15 K) which cannot be attributed
the Schottky effect. The calcu-lated magnetic entropy is substantially lower
than that expected for the ground states of the or-dered moments of Ru5+ and
Yb3+, indicating the presence of large crystal field effects and/ or
in-complete magnetic ordering and/or magnetic frustrations well above the
magnetic ordering. An attempt is made to explain the magnetization reversal
within the frameworks of available models.Comment: 15 pages text, 6 figures Journal-ref: J.Phys.:Condens.Matter
20(2008)23520
Evaluation of low temperature waste heat as a low carbon heat resource in the UK
The capture and transport of waste heat represents a great opportunity for the decarbonisation of heat supply in buildings. To date, mostly high temperature waste heat has been reused and reported. However, with the recent advent of low and ambient temperature (4th and 5th generation) district energy networks, there is scope for the recovery and utilisation of heat from a range of novel, low temperature sources. The current study represents one of the first attempts to quantify the size of this opportunity, with particular focus in the UK, and complements the few previous attempts at estimating low temperature waste heat by focussing on a range of novel sources. The approach used was to evaluate a number of low temperature waste heat sources to determine: (a) the annual quantity of waste heat generated; and (b) the temperature(s) of the waste heat, for each heat source. In many cases, this was achieved using methodology and assumptions derived from the authors’ earlier investigations. The relative merits and potential of each heat source are also discussed, with respect to location, proximity to end users, need for upgrade using a heat pump, continuity of supply and distribution options for reuse, for example by using district energy networks with different operating temperatures. The total quantity of waste heat energy identified from the heat sources considered in this study, for England, Wales and Northern Ireland, was estimated to be 572 TWh.a−1, which would represent 132% of the total energy consumption for heat in these countries (432 TWh.a−1). Although this study focused on the UK potential for low temperature waste heat, the estimation methods developed and resulting analysis are generic and could also be applied in the context of other countries
The song of the dunes as a self-synchronized instrument
Since Marco Polo (1) it has been known that some sand dunes have the peculiar
ability of emitting a loud sound with a well defined frequency, sometimes for
several minutes. The origin of this sustained sound has remained mysterious,
partly because of its rarity in nature (2). It has been recognized that the
sound is not due to the air flow around the dunes but to the motion of an
avalanche (3), and not to an acoustic excitation of the grains but to their
relative motion (4-7). By comparing several singing dunes and two controlled
experiments, one in the laboratory and one in the field, we here demonstrate
that the frequency of the sound is the frequency of the relative motion of the
sand grains. The sound is produced because some moving grains synchronize their
motions. The existence of a velocity threshold in both experiments further
shows that this synchronization comes from an acoustic resonance within the
flowing layer: if the layer is large enough it creates a resonance cavity in
which grains self-synchronize.Comment: minor changes, essentially more references
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