15,502 research outputs found
Eight Things you should Know about Open Source Integrated Library Systems.
Open source library management systems are free alternative to costly commercial library systems. It helps to automate library functions and give a tremendous savings on library automation expenses. User ’s participation in all stages of software project ensures the development of the features that the library really wants. Lack of awareness and knowledge in open source technology among library professionals restrict wide adoption of open source library management system. This article gives an insight into the use and maintenance of open source library management systems
Generation of Magnetic Fields in the Relativistic Shock of Gamma-Ray-Burst Sources
We show that the relativistic two-stream instability can naturally generate
strong magnetic fields with 1e-5 - 1e-1 of the equipartition energy density, in
the collisionless shocks of Gamma-Ray-Burst (GRB) sources. The generated fields
are parallel to the shock front and fluctuate on the very short scale of the
plasma skin depth. The synchrotron radiation emitted from the limb-brightened
source image is linearly polarized in the radial direction relative to the
source center. Although the net polarization vanishes under circular symmetry,
GRB sources should exhibit polarization scintillations as their radio afterglow
radiation gets scattered by the Galactic interstellar medium. Detection of
polarization scintillations could therefore test the above mechanism for
magnetic field generation.Comment: 12 pages, 2 figures included. Submitted to Ap
Modeling the line variations from the wind-wind shock emissions of WR 30a
The study of Wolf-Rayet stars plays an important role in evolutionary
theories of massive stars. Among these objects, ~ 20% are known to be in binary
systems and can therefore be used for the mass determination of these stars.
Most of these systems are not spatially resolved and spectral lines can be used
to constrain the orbital parameters. However, part of the emission may
originate in the interaction zone between the stellar winds, modifying the line
profiles and thus challenging us to use different models to interpret them. In
this work, we analyzed the HeII4686\AA + CIV4658\AA blended lines of WR30a
(WO4+O5) assuming that part of the emission originate in the wind-wind
interaction zone. In fact, this line presents a quiescent base profile,
attributed to the WO wind, and a superposed excess, which varies with the
orbital phase along the 4.6 day period. Under these assumptions, we were able
to fit the excess spectral line profile and central velocity for all phases,
except for the longest wavelengths, where a spectral line with constant
velocity seems to be present. The fit parameters provide the eccentricity and
inclination of the binary orbit, from which it is possible to constrain the
stellar masses.Comment: accepted for publication in the MNRA
Description of Atmospheric Conditions at the Pierre Auger Observatory Using Meteorological Measurements and Models
Atmospheric conditions at the site of a cosmic ray observatory must be known
well for reconstructing observed extensive air showers, especially when
measured using the fluorescence technique. For the Pierre Auger Observatory, a
sophisticated network of atmospheric monitoring devices has been conceived.
Part of this monitoring was a weather balloon program to measure atmospheric
state variables above the Observatory. To use the data in reconstructions of
air showers, monthly models have been constructed. Scheduled balloon launches
were abandoned and replaced with launches triggered by high-energetic air
showers as part of a rapid monitoring system. Currently, the balloon launch
program is halted and atmospheric data from numerical weather prediction models
are used. A description of the balloon measurements, the monthly models as well
as the data from the numerical weather prediction are presented
Probing the Magnetic Field Structure in Gamma-Ray Bursts through Dispersive Plasma Effects on the Afterglow Polarization
(Abr) The origin and structure of magnetic fields in Gamma-Ray Burst (GRB)
fireball plasmas are two of the most important open questions in all GRB
models. We show that the structure and strength of the magnetic field may be
constrained by radio and IR observations of the early afterglow, where plasma
effects on the polarization of propagating radiation are significant. We
calculate these propagation effects for cold and relativistic plasmas, and find
that in the presence of a uniform equipartition field the degree of linear
polarization is suppressed, and circular polarization prevails at low
frequencies, nu < 1-3 GHz, (2x10^11 Hz < nu < few x 10^14 Hz) in the forward
(reverse) shock. At higher frequencies linear polarization dominates. At the
frequency of the transition between circular and linear polarization, the net
level of polarization is minimal, ~10-20%. These features are nearly
independent of the circumburst density. The transition frequency is smaller by
a factor of ~10 when the uniform field is much weaker than equipartition. The
dependence of these results on viewing geometry, outflow collimation and
magnetic field orientation is discussed. When the configuration of the field is
entangled over length scales much smaller than the extent of the emitting
plasma, the aforementioned effects should not be observed and a linear
polarization at the few % level is expected. Polarimetric observations during
the early afterglow, and particularly of the reverse shock emission, may
therefore place strong constraints on the structure and strength of the
magnetic field within the fireball plasma.Comment: 12 pages, 6 figures. Accepted for publication in ApJ. Revised version
includes improved discussion of viewing and fireball geometry, with
implications to resulting polarizatio
Cosmic Rays from Cosmic Strings with Condensates
We re-visit the production of cosmic rays by cusps on cosmic strings. If a
scalar field (``Higgs'') has a linear interaction with the string world-sheet,
such as would occur if there is a bosonic condensate on the string, cusps on
string loops emit narrow beams of very high energy Higgses which then decay to
give a flux of ultra high energy cosmic rays. The ultra-high energy flux and
the gamma to proton ratio agree with observations if the string scale is GeV. The diffuse gamma ray and proton fluxes are well below current
bounds. Strings that are {\it lighter} and have linear interactions with
scalars produce an excess of direct and diffuse cosmic rays and are ruled out
by observations, while heavier strings ( GeV) are constrained by
their gravitational signatures. This leaves a narrow window of parameter space
for the existence of cosmic strings with bosonic condensates.Comment: 9 pages, 5 figures; revised reference
Trigonometric Sutherland systems and their Ruijsenaars duals from symplectic reduction
Besides its usual interpretation as a system of indistinguishable
particles moving on the circle, the trigonometric Sutherland system can be
viewed alternatively as a system of distinguishable particles on the circle or
on the line, and these 3 physically distinct systems are in duality with
corresponding variants of the rational Ruijsenaars-Schneider system. We explain
that the 3 duality relations, first obtained by Ruijsenaars in 1995, arise
naturally from the Kazhdan-Kostant-Sternberg symplectic reductions of the
cotangent bundles of the group U(n) and its covering groups
and , respectively. This geometric interpretation
enhances our understanding of the duality relations and simplifies Ruijsenaars'
original direct arguments that led to their discovery.Comment: 34 pages, minor additions and corrections of typos in v
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