7,981 research outputs found
WSRT Faraday tomography of the Galactic ISM at \lambda \sim 0.86 m
We investigate the distribution and properties of Faraday rotating and
synchrotron emitting regions in the Galactic ISM in the direction of the
Galactic anti-centre. We apply Faraday tomography to a radio polarization
dataset that we obtained with the WSRT. We developed a new method to calculate
a linear fit to periodic data, which we use to determine rotation measures from
our polarization angle data. From simulations of a Faraday screen + noise we
could determine how compatible the data are with Faraday screens. An
unexpectedly large fraction of 14% of the lines-of-sight in our dataset show an
unresolved main component in the Faraday depth spectrum. For lines-of-sight
with a single unresolved component we demonstrate that a Faraday screen in
front of a synchrotron emitting region that contains a turbulent magnetic field
component can explain the data.Comment: 5 pages, 5 figures. Accepted for publication as a Letter to the
Editor in A&
Motion of a sphere through an aging system
We have investigated the drag on a sphere falling through a clay suspension
that has a yield stress and exhibits rheological aging. The drag force
increases with both speed and the rest time between preparation of the system
and the start of the experiment, but there exists a nonzero minimum speed below
which steady motion is not possible. We find that only a very thin layer of
material around the sphere is fluidized when it moves, while the rest of
suspension is deformed elastically. This is in marked contrast to what is found
for yield-stress fluids that do not age.Comment: latex, 4 figure
On hyperovals of polar spaces
We derive lower and upper bounds for the size of a hyperoval of a finite polar space of rank 3. We give a computer-free proof for the uniqueness, up to isomorphism, of the hyperoval of size 126 of H(5, 4) and prove that the near hexagon E-3 has up to isomorphism a unique full embedding into the dual polar space DH(5, 4)
Response of thin-film SQUIDs to applied fields and vortex fields: Linear SQUIDs
In this paper we analyze the properties of a dc SQUID when the London
penetration depth \lambda is larger than the superconducting film thickness d.
We present equations that govern the static behavior for arbitrary values of
\Lambda = \lambda^2/d relative to the linear dimensions of the SQUID. The
SQUID's critical current I_c depends upon the effective flux \Phi, the magnetic
flux through a contour surrounding the central hole plus a term proportional to
the line integral of the current density around this contour. While it is well
known that the SQUID inductance depends upon \Lambda, we show here that the
focusing of magnetic flux from applied fields and vortex-generated fields into
the central hole of the SQUID also depends upon \Lambda. We apply this
formalism to the simplest case of a linear SQUID of width 2w, consisting of a
coplanar pair of long superconducting strips of separation 2a, connected by two
small Josephson junctions to a superconducting current-input lead at one end
and by a superconducting lead at the other end. The central region of this
SQUID shares many properties with a superconducting coplanar stripline. We
calculate magnetic-field and current-density profiles, the inductance
(including both geometric and kinetic inductances), magnetic moments, and the
effective area as a function of \Lambda/w and a/w.Comment: 18 pages, 20 figures, revised for Phys. Rev. B, the main revisions
being to denote the effective flux by \Phi rather than
LOFAR: A new radio telescope for low frequency radio observations: Science and project status
LOFAR, the Low Frequency Array, is a large radio telescope consisting about
100 soccer field sized antenna stations spread over a region of 400 km in
diameter. It will operate in the frequency range from ~10 to 240 MHz, with a
resolution at 240 MHz of better than an arcsecond. Its superb sensitivity will
allow for a broad range of astrophysical studies. In this contribution we first
discuss four major areas of astrophysical research in which LOFAR will
undoubtedly make important contributions: reionisation, distant galaxies and
AGNs, transient radio sources and cosmic rays. Subsequently, we will discuss
the technical concept of the instrument and the status of the LOFAR projectComment: 8 pages, 2 figures, to appear in the proceedings of the XXI Texas
Symposium on Relativistic Astrophysics held on December 9--13 2002, in
Florence, Ital
Mircorheology and jamming in a yield-stress fluid
Abstract We study the onset of a yield stress in a polymer microgel dispersion using a combination of particle-tracking microrheology and shear rheometry. On the bulk scale, the dispersion changes from a predominantly viscous fluid to a stiff elastic gel as the concentration of the microgel particles increases. On the microscopic scale, the tracer particles see two distinct microrheological environments over a range of concentrations-one being primarily viscous, the other primarily elastic. The fraction of the material that is elastic on the microscale increases from zero to one as the concentration increases. Our results indicate that the yield stress appears as the result of jamming of the microgel particles, and we infer a model for the small-scale structure and interactions within the dispersion and their relationship to the bulk viscoelastic properties
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