4,804 research outputs found
Contact line stability of ridges and drops
Within the framework of a semi-microscopic interface displacement model we
analyze the linear stability of sessile ridges and drops of a non-volatile
liquid on a homogeneous, partially wet substrate, for both signs and arbitrary
amplitudes of the three-phase contact line tension. Focusing on perturbations
which correspond to deformations of the three-phase contact line, we find that
drops are generally stable while ridges are subject only to the long-wavelength
Rayleigh-Plateau instability leading to a breakup into droplets, in contrast to
the predictions of capillary models which take line tension into account. We
argue that the short-wavelength instabilities predicted within the framework of
the latter macroscopic capillary theory occur outside its range of validity and
thus are spurious.Comment: 6 pages, 1 figur
Generalized Symmetries of Impulsive Gravitational Waves
We generalize previous \cite{AiBa2} work on the classification of
() symmetries of plane-fronted waves with an impulsive profile. Due
to the specific form of the profile it is possible to extend the group of
normal-form-preserving diffeomorphisms to include non-smooth transformations.
This extension entails a richer structure of the symmetry algebra generated by
the (non-smooth) Killing vectors.Comment: 18 pages, latex2e, no figure
Radio-loud Active Galaxies in the Northern ROSAT All-Sky Survey III: New Spectroscopic Identifications from the RGB BL Lac Survey
We present new spectroscopic identifications for 169 objects in the
RASS-Green Bank (RGB) catalog of radio- and X-ray-emitting AGN. These data
significantly increase the fraction of bright RGB objects with classifications.
Specifically, we report and discuss the classification of 66 radio-loud
quasars, 53 BL Lacs, 33 Broad Line Radio Galaxies, 5 Narrow Line Radio
Galaxies, 1 Seyfert I galaxy and 11 galaxies or galaxies in clusters. Over 78%
of the identifications we present here are new. The observations we report were
undertaken as part of our targeted search program to identify a new, large
unbiased sample of BL Lac Objects and we therefore discuss the BL Lac sample
extensively. Unlike many previous surveys, we impose no selection criteria
based on optical morphology, color or broadband spectral energy distribution.
Our classifications are based solely on a carefully defined set of
self-consistent spectroscopic classification criteria. We show the 53 RGB
presented here exhibit transitional properties between normal galaxies and BL
Lacs discovered previously. We show there is no clear separation in CaII break
strength between RGB BL Lacs and galaxies, with the distribution of break
strengths varying smoothly between 0% and 50%. We also show that the newly
discovered RGB BL Lacs reside in a "zone of avoidance" in the log(S_x/S_r) vs.
log(S_o/S_r) diagram. This has important implications for BL Lac search
strategies since it shows that RASS BL Lac samples will be severely incomplete
if candidates are chosen only from among those objects with the highest S_x/S_r
flux ratios.Comment: 21 pages text, 189 Figures, 4 tables, LaTeX2E, 4.2MB tar file
(compressed); special style file paper.sty provide
The Optical Emission from Gamma-ray Quasars
We present photometric observations of six radio-loud quasars that were
detected by the COMPTEL gamma-ray telescope. The data encompasses seven
wavebands in the optical and near-infrared. After correction for Galactic
extinction, we find a wide range in optical slopes. Two sources are as blue as
optically-selected quasars, and are likely to be dominated by the accretion
disc emission, while three others show colours consistent with a red
synchrotron component. We discuss the properties of the COMPTEL sample of
quasars, as well as the implications our observations have for multi-wavelength
modelling of gamma-ray quasars.Comment: 12 pages, 3 figures, accepted for publication in P.A.S.A; minor typos
correcte
Synthetic X-ray light curves of BL Lacs from relativistic hydrodynamic simulations
We present the results of relativistic hydrodynamic simulations of the
collision of two dense shells in a uniform external medium, as envisaged in the
internal shock model for BL Lac jets. The non-thermal radiation produced by
highly energetic electrons injected at the relativistic shocks is computed
following their temporal and spatial evolution. The acceleration of electrons
at the relativistic shocks is parametrized using two different models and the
corresponding X-ray light curves are computed. We find that the interaction
time scale of the two shells is influenced by an interaction with the external
medium. For the chosen parameter sets, the efficiency of the collision in
converting dissipated kinetic energy into the observed X-ray radiation is of
the order of one percent.Comment: 22 pages, 6 figures, accepted to A&
Investigation of the Spin-Peierls transition in CuGeO_3 by Raman scattering
Raman experiments on the spin-Peierls compound CuGeO and the substituted
(Cu,Zn)GeO and Cu(Ge,Ga)O compounds were
performed in order to investigate the response of specific magnetic excitations
of the one-dimensional spin-1/2 chain to spin anisotropies and
substitution-induced disorder. In pure CuGeO, in addition to normal phonon
scattering which is not affected at all by the spin-Peierls transition, four
types of magnetic scattering features were observed. Below T=14 K a
singlet-triplet excitation at 30 cm, two-magnon scattering from 30 to
227 cm and folded phonon modes at 369 and 819 cm were identified.
They were assigned by their temperature dependence and lineshape. For
temperatures between the spin-Peierls transition T and approximately 100
K a broad intensity maximum centered at 300 cm is observed.Comment: 7 pages, LaTex2e, including 3 figures (eps) to be published in
Physica B (1996
Beam Based Alignment of Interaction Region Magnets
In conventional beam based alignment (BBA) procedures, the relative alignment
of a quadrupole to a nearby beam position monitor is determined by finding a
beam position in the quadrupole at which the closed orbit does not change when
the quadrupole field is varied. The final focus magnets of the interaction
regions (IR) of circular colliders often have some specialized properties that
make it difficult to perform conventional beam based alignment procedures. At
the HERA interaction points, for example, these properties are: (a) The
quadrupoles are quite strong and long. Therefore a thin lens approximation is
quite imprecise. (b) The effects of angular magnet offsets become significant.
(c) The possibilities to steer the beam are limited as long as the alignment is
not within specifications. (d) The beam orbit has design offsets and design
angles with respect to the axis of the low-beta quadrupoles. (e) Often
quadrupoles do not have a beam position monitor in their vicinity. Here we
present a beam based alignment procedure that determines the relative offset of
the closed orbit from a quadrupole center without requiring large orbit changes
or monitors next to the quadrupole. Taking into account the alignment angle
allows us to reduce the sensitivity to optical errors by one to two orders of
magnitude. We also show how the BBA measurements of all IR quadrupoles can be
used to determine the global position of the magnets. The sensitivity to errors
of this method is evaluated and its applicability to HERA is shown
Topological Designs
We give an exponential upper and a quadratic lower bound on the number of
pairwise non-isotopic simple closed curves can be placed on a closed surface of
genus g such that any two of the curves intersects at most once. Although the
gap is large, both bounds are the best known for large genus. In genus one and
two, we solve the problem exactly.
Our methods generalize to variants in which the allowed number of pairwise
intersections is odd, even, or bounded, and to surfaces with boundary
components.Comment: 14 p., 4 Figures. To appear in Geometriae Dedicat
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