2,005 research outputs found
The Littlewood-Gowers problem
We show that if A is a subset of Z/pZ (p a prime) of density bounded away
from 0 and 1 then the A(Z/pZ)-norm (that is the l^1-norm of the Fourier
transform) of the characterstic function of A is bounded below by an absolute
constant times (log p)^{1/2 - \epsilon} as p tends to infinity. This improves
on the exponent 1/3 in recent work of Green and Konyagin.Comment: 31 pp. Corrected typos. Updated references
The X-ray Emissions from the M87 Jet: Diagnostics and Physical Interpretation
We reanalyze the deep Chandra observations of the M87 jet, first examined by
Wilson & Yang (2002). By employing an analysis chain that includes image
deconvolution, knots HST-1 and I are fully separated from adjacent emission. We
find slight but significant variations in the spectral shape, with values of
ranging from . We use VLA radio observations, as well
as HST imaging and polarimetry data, to examine the jet's broad-band spectrum
and inquire as to the nature of particle acceleration in the jet. As shown in
previous papers, a simple continuous injection model for synchrotron-emitting
knots, in which both the filling factor, , of regions within which
particles are accelerated and the energy spectrum of the injected particles are
constant, cannot account for the X-ray flux or spectrum. Instead, we propose
that is a function of position and energy and find that in the inner
jet, , and
in knots A and B, , where is the emitted photon energy and and is the
emitting electron energy. In this model, the index of the injected electron
energy spectrum () is at all locations in
the jet, as predicted by models of cosmic ray acceleration by ultrarelativistic
shocks. There is a strong correlation between the peaks of X-ray emission and
minima of optical percentage polarization, i.e., regions where the jet magnetic
field is not ordered. We suggest that the X-ray peaks coincide with shock waves
which accelerate the X-ray emitting electrons and cause changes in the
direction of the magnetic field; the polarization is thus small because of beam
averaging.Comment: Accepted for publication in ApJ; 21 pages, 9 figures, 2 tables;
abstract shortened for astro-ph; Figures 1, 7 and 8 at reduced resolutio
MAGIC MOORE-PENROSE INVERSES AND PHILATELIC MAGIC SQUARES WITH SPECIAL EMPHASIS ON THE DANIELSâZLOBEC MAGIC SQUARE
We study singular magic matrices in which the numbers in the rows and columns and in the two main diagonals all add up to the same sum. Our interest focuses on such magic matrices for which the Mooreâ
Penrose inverse is also magic. Special attention is given to the âDanielsâZlobec magic squareââ introduced by the British magician and television performer Paul Daniels (b. 1938) and considered by Zlobec (2001);
see also Murray (1989, pp. 30â32). We introduce the concept of a âphilatelic magic squareâ as a square arrangement of images of postage stamps so that the associated nominal values form a magic square. Three philatelic magic squares with stamps especially chosen for Sanjo Zlobec are presented in celebration of his 70th birthday; most helpful in identifying these stamps was an Excel checklist by MĂ€nnikkö (2009)
High Energy Neutrinos: Sources and Fluxes
We discuss briefly the potential sources of high energy astrophysical
neutrinos and show estimates of the neutrino fluxes that they can produce. A
special attention is paid to the connection between the highest energy cosmic
rays and astrophysical neutrinos.Comment: 7 pages, 2 figures, submitted to the Proceedings of TAUP 2005
workshop, corrected left panel of figure
Water Table Management Reduces Tile Nitrate Loss in Continuous Corn and in a Soybean-Corn Rotation
Water table management systems can be designed to alleviate soil water excesses and deficits, as well as reduce nitrate leaching losses in tile discharge. With this in mind, a standard tile drainage (DR) system was compared over 8 years (1991 to 1999) to a controlled tile drainage/subirrigation (CDS) system on a low-slope (0.05 to 0.1%) Brookston clay loam soil (Typic Argiaquoll) in southwestern Ontario, Canada. In the CDS system, tile discharge was controlled to prevent excessive drainage, and water was pumped back up the tile lines (subirrigation) to replenish the crop root zone during water deficit periods. In the first phase of the study (1991 to 1994), continuous corn (Zea mays, L.) was grown with annual nitrogen (N) fertilizer inputs as per local soil test recommendations. In the second phase (1995 to 1999), a soybean (Glycine max L., Merr.)-corn rotation was used with N fertilizer added only during the two corn years. In Phase 1 when continuous corn was grown, CDS reduced total tile discharge by 26% and total nitrate loss in tile discharge by 55%, compared to DR. In addition, the 4-year flow weighted mean (FWM) nitrate concentration in tile discharge exceeded the Canadian drinking water guideline (10 mg N lâ1) under DR (11.4 mg N lâ1), but not under CDS (7.0 mg N lâ1). In Phase 2 during the soybean-corn rotation, CDS reduced total tile discharge by 38% and total nitrate loss in tile discharge by 66%, relative to DR. The 4-year FWM nitrate concentration during Phase 2 in tile discharge was below the drinking water guideline for both DR (7.3 mg N lâ1) and CDS (4.0 mg N lâ1). During both phases of the experiment, the CDS treatment caused only minor increases in nitrate loss in surface runoff relative to DR. Hence CDS decreased FWM nitrate concentrations, total drainage water loss, and total nitrate loss in tile discharge relative to DR. In addition, soybean-corn rotation reduced FWM nitrate concentrations and total nitrate loss in tile discharge relative to continuous corn. CDS and crop rotations with reduced N fertilizer inputs can thus improve the quality of tile discharge water substantially
Rayleigh-Taylor Instabilities in Young Supernova Remnants Undergoing Efficient Particle Acceleration
We employ hydrodynamic simulations to study the effects of high shock
compression ratios, as expected for fast shocks with efficient particle
acceleration, on the convective instability of driven waves in supernova
remnants. We find that the instability itself does not depend significantly on
the compression ratio, but because the width of the interaction region between
the forward and reverse shocks can shrink significantly with increasing shock
compression, we find that convective instabilities can reach all the way to the
forward shock front if compression ratios are high enough.Comment: Submitted to The Astrophysical Journa
Nonthermal Emission from a Supernova Remnant in a Molecular Cloud
In evolved supernova remnants (SNRs) interacting with molecular clouds, such
as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a
forward shock of moderate Mach number, a cooling layer, a dense radiative shell
and an interior region filled with hot tenuous plasma is expected. We present a
kinetic model of nonthermal electron injection, acceleration and propagation in
that environment and find that these SNRs are efficient electron accelerators
and sources of hard X- and gamma-ray emission. The energy spectrum of the
nonthermal electrons is shaped by the joint action of first and second order
Fermi acceleration in a turbulent plasma with substantial Coulomb losses.
Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal
electrons produce multiwavelength photon spectra in quantitative agreement with
the radio and the hard emission observed by ASCA and EGRET from IC 443. We
distinguish interclump shock wave emission from molecular clump shock wave
emission accounting for a complex structure of molecular cloud. Spatially
resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and
3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST
would distinguish the contribution of the energetic lepton component to the
gamma-rays observed by EGRET.Comment: 14 pages, 4 figure, Astrophysical Journal, v.538, 2000 (in press
The Contribution of Different Supernova Populations to the Galactic Gamma-Ray Background
The contribution of Source Cosmic Rays (SCRs), accelerated and still confined
in Supernova Remnants (SNRs), to the diffuse high energy \gr emission above 1
GeV from the Galactic disk is studied. The \grs produced by the SCRs have a
much harder spectrum than those generated by the Galactic Cosmic Rays (GCRs).
Extending a previous paper, a simple SNR population synthesis is considered and
the Inverse Compton emission from the SCR electrons is evaluated in greater
detail. Then the combined spectrum of \gr emission from the Galactic Supernova
Remnant population is calculated and this emission at low Galactic latitudes is
compared with the diffuse \gr emission observed by the EGRET and ground based
instruments. The average contribution of SCRs is comparable to the GCR
contribution already at GeV energies, due to Supernovae of types II and Ib
exploding into the wind bubbles of quite massive progenitor stars, and becomes
dominant at \gr energies above 100 GeV. At TeV energies the dominant
contribution is from SCRs in SNRs that expand into a uniform interstellar
medium. In fact, the sum of hadronic and Inverse Compton \grs would exceed the
limits given by the existing experimental data, unless the confinement time
, i.e. the time until which SNRs confine the main fraction of
accelerated SCRs, is as small as yr and the typical magnetic
field strength in SNRs as large as 30 G. Both situations are however
possible as a result of field amplification through CR backreaction in the
acceleration process. It is pointed out that accurate measurements of the
low-latitude diffuse Galactic \gr spectrum at TeV-energies can serve as a
unique consistency test for CR origin from the Supernova Remnant population as
a whole.Comment: To appear in Astrophysical Journa
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