10,652 research outputs found
Engineering calculations for the Delta S method of solving the orbital allotment problem
The method of calculating single-entry separation requirements for pairs of satellites is extended to include the interference on the top link as well as on the down link. Several heuristic models for analyzing the effects of shaped-beam antenna designs on required satellite separations are introduced and demonstrated with gain contour plots. The calculation of aggregate interference is extended to include the effects of up-link interference. The relationship between the single-entry C/I requirements, used in determining satellite separation constraints for various optimization procedures, and the aggregate C/I values of the resulting solutions is discussed
Simulated three-component granular segregation in a rotating drum
Discrete particle simulations are used to model segregation in granular
mixtures of three different particle species in a horizontal rotating drum.
Axial band formation is observed, with medium-size particles tending to be
located between alternating bands of big and small particles. Partial radial
segregation also appears; it precedes the axial segregation and is
characterized by an inner core region richer in small particles. Axial bands
are seen to merge during the long simulation runs, leading to a coarsening of
the band pattern; the relocation of particles involved in one such merging
event is examined. Overall, the behavior is similar to experiment and
represents a generalization of what occurs in the simpler two-component
mixture.Comment: 7 pages, 11 figures (low resolution color figures only; originals at
author's website http://www.ph.biu.ac.il/~rapaport/research/granular.html)
[revised version contains extra figures
PCA of PCA: Principal Component Analysis of Partial Covering Absorption in NGC 1365
We analyse 400 ks of XMM-Newton data on the active galactic nucleus NGC 1365
using principal component analysis (PCA) to identify model independent spectral
components. We find two significant components and demonstrate that they are
qualitatively different from those found in MCG?6-30-15 using the same method.
As the variability in NGC 1365 is known to be due to changes in the parameters
of a partial covering neutral absorber, this shows that the same mechanism
cannot be the driver of variability in MCG-6-30-15. By examining intervals
where the spectrum shows relatively low absorption we separate the effects of
intrinsic source variability, including signatures of relativistic reflection,
from variations in the intervening absorption. We simulate the principal
components produced by different physical variations, and show that PCA
provides a clear distinction between absorption and reflection as the drivers
of variability in AGN spectra. The simulations are shown to reproduce the PCA
spectra of both NGC 1365 and MCG-6-30-15, and further demonstrate that the
dominant cause of spectral variability in these two sources requires a
qualitatively different mechanism.Comment: 8 pages, 10 figures. Accepted for publication in MNRA
Detection of a variable ultra-fast outflow in the Narrow Line Seyfert 1 galaxy PG 1448+273
Relativistically blueshifted absorption features of highly ionised ions, the
so-called ultra-fast outflows (UFOs), have been detected in the X-ray spectra
of a number of accreting supermassive black holes. If these features truly
originate from accretion disc winds accelerated to more than 10 per cent of the
speed of light, their energy budget is very significant and they can contribute
to or even drive galaxy-scale feedback from active galactic nuclei (AGN).
However, the UFO spectral features are often weak due to high ionisation of the
outflowing material, and the inference of the wind physical properties can be
complicated by other spectral features in AGN such as relativistic reflection.
Here we study a highly accreting Narrow Line Seyfert 1 galaxy PG 1448+273. We
apply an automated, systematic routine for detecting outflows in accreting
systems and achieve an unambiguous detection of a UFO in this AGN. The UFO
absorption is observed in both soft and hard X-ray bands with the XMM-Newton
observatory. The velocity of the outflow is (26900 +- 600) km/s (~0.09c), with
an ionisation parameter of log ({\xi} / erg cm s^-1)=4.03_{-0.08}^{+0.10} and a
column density above 10^23 cm^-2. At the same time, we detect weak warm
absorption features in the spectrum of the object. Our systematic outflow
search suggests the presence of further multi-phase wind structure, but we
cannot claim a significant detection considering the present data quality. The
UFO is not detected in a second, shorter observation with XMM-Newton,
indicating variability in time, observed also in other similar AGN.Comment: Accepted for publication in MNRAS. 14 pages, 11 figures, 1 tabl
Generating-function method for fusion rules
This is the second of two articles devoted to an exposition of the
generating-function method for computing fusion rules in affine Lie algebras.
The present paper focuses on fusion rules, using the machinery developed for
tensor products in the companion article. Although the Kac-Walton algorithm
provides a method for constructing a fusion generating function from the
corresponding tensor-product generating function, we describe a more powerful
approach which starts by first defining the set of fusion elementary couplings
from a natural extension of the set of tensor-product elementary couplings. A
set of inequalities involving the level are derived from this set using Farkas'
lemma. These inequalities, taken in conjunction with the inequalities defining
the tensor products, define what we call the fusion basis. Given this basis,
the machinery of our previous paper may be applied to construct the fusion
generating function. New generating functions for sp(4) and su(4), together
with a closed form expression for their threshold levels are presented.Comment: Harvmac (b mode : 47 p) and Pictex; to appear in J. Math. Phy
Modelling the Extreme X-ray Spectrum of IRAS 13224-3809
The extreme NLS1 galaxy IRAS 13224-3809 shows significant variability,
frequency depended time lags, and strong Fe K line and Fe L features in the
long 2011 XMM-Newton observation. In this work we study the spectral properties
of IRAS 13224-3809 in detail, and carry out a series of analyses to probe the
nature of the source, focusing in particular on the spectral variability
exhibited. The RGS spectrum shows no obvious signatures of absorption by
partially ionised material (warm absorbers). We fit the 0.3-10.0 keV spectra
with a model that includes relativistic reflection from the inner accretion
disc, a standard powerlaw AGN continuum, and a low-temperature (~0.1 keV)
blackbody, which may originate in the accretion disc, either as direct or
reprocessed thermal emission. We find that the reflection model explains the
time-averaged spectrum well, and we also undertake flux-resolved and
time-resolved spectral analyses, which provide evidence of gravitational
light-bending effects. Additionally, the temperature and flux of the blackbody
component are found to follow the relation expected for simple
thermal blackbody emission from a constant emitting area, indicating a physical
origin for this component.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Generating Polarization-Entangled Photon Pairs with Arbitrary Joint Spectrum
We present a scheme for generating polarization-entangled photons pairs with
arbitrary joint spectrum. Specifically, we describe a technique for spontaneous
parametric down-conversion in which both the center frequencies and the
bandwidths of the down-converted photons may be controlled by appropriate
manipulation of the pump pulse. The spectral control offered by this technique
permits one to choose the operating wavelengths for each photon of a pair based
on optimizations of other system parameters (loss in optical fiber, photon
counter performance, etc.). The combination of spectral control, polarization
control, and lack of group-velocity matching conditions makes this technique
particularly well-suited for a distributed quantum information processing
architecture in which integrated optical circuits are connected by spans of
optical fiber.Comment: 6 pages, 3 figure
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