5,679 research outputs found
The Jet Angular Profile and the Afterglow Light Curves
We investigate how the angular structure of GRB jets effects the afterglow
light curves at different viewing angles, , from the jet symmetry
axis. A numerical hydrodynamical modeling for the evolution of a relativistic
collimated outflow, as it interacts with the surrounding medium, is carried
out, and compared to two simple models that make opposite and extreme
assumptions for the degree of lateral energy transfer. The Lorentz factor,
, and kinetic energy per unit solid angle, , are initially
taken to be power laws of the angle from the jet axis. We find that
the lateral velocity in the comoving frame, , is typically much
smaller than the sound speed, , as long as , and the dynamics
of relativistic structured jets may be reasonably described by a simple
analytic model where is independent of time, as long as
\Gamma(\theta=0)\ga a few. We perform a qualitative comparison between the
resulting light curves and afterglow observations. This constrains the jet
structure, and poses problems for a `universal' jet model, where all GRB jets
are assumed to be intrinsically identical, and differ only by our viewing
angle, .Comment: 4 pages, 2 figures, contributed talk at the meeting "Gamma Ray Bursts
in the Afterglow Era - Third Workshop", September 2002, Rome, Ital
Extended Equal Service and Differentiated Service Models for Peer-to-Peer File Sharing
Peer-to-Peer (P2P) systems have proved to be the most effective and popular
file sharing applications in recent years. Previous studies mainly focus on the
equal service and the differentiated service strategies when peers have no
initial data before their download. In an upload-constrained P2P file sharing
system, we model both the equal service process and the differentiated service
process when peers' initial data distribution satisfies some special
conditions, and also show how to minimize the time to get the file to any
number of peers. The proposed models can reveal the intrinsic relations among
the initial data amount, the size of peer set and the minimum last finish time.
By using the models, we can also provide arbitrary degree of differentiated
service to a certain number of peers. We believe that our analysis process and
achieved theoretical results could provide fundamental insights into studies on
bandwidth allocation and data scheduling, and can give helpful reference both
for improving system performance and building effective incentive mechanism in
P2P file sharing systems
Identifying relationship between skid resistance and road crashes using probability-based approach
Road accidents are of great concerns for road and transport departments around world, which cause tremendous loss and dangers for public. Reducing accident rates and crash severity are imperative goals that governments, road and transport authorities, and researchers are aimed to achieve. In Australia, road crash trauma costs the nation A1.7 million. Serious injury cases can cost the taxpayer many times the cost of a fatality. Crashes are in general uncontrolled events and are dependent on a number of interrelated factors such as driver behaviour, traffic conditions, travel speed, road geometry and condition, and vehicle characteristics (e.g. tyre type pressure and condition, and suspension type and condition). Skid resistance is considered one of the most important surface characteristics as it has a direct impact on traffic safety. Attempts have been made worldwide to study the relationship between skid resistance and road crashes. Most of these studies used the statistical regression and correlation methods in analysing the relationships between skid resistance and road crashes. The outcomes from these studies provided mix results and not conclusive. The objective of this paper is to present a probability-based method of an ongoing study in identifying the relationship between skid resistance and road crashes. Historical skid resistance and crash data of a road network located in the tropical east coast of Queensland were analysed using the probability-based method. Analysis methodology and results of the relationships between skid resistance, road characteristics and crashes are presented
Isospin-Violating Dark Matter Benchmarks for Snowmass 2013
Isospin-violating dark matter (IVDM) generalizes the standard
spin-independent scattering parameter space by introducing one additional
parameter, the neutron-to-proton coupling ratio f_n/f_p. In IVDM the
implications of direct detection experiments can be altered significantly. We
review the motivations for considering IVDM and present benchmark models that
illustrate some of the qualitatively different possibilities. IVDM strongly
motivates the use of a variety of target nuclei in direct detection
experiments.Comment: LaTeX, 5 pages, 4 figures. v2: minor figure revision
Isospin-Violating Dark Matter
Searches for dark matter scattering off nuclei are typically compared
assuming that the dark matter's spin-independent couplings are identical for
protons and neutrons. This assumption is neither innocuous nor well motivated.
We consider isospin-violating dark matter (IVDM) with one extra parameter, the
ratio of neutron to proton couplings, and include the isotope distribution for
each detector. For a single choice of the coupling ratio, the DAMA and CoGeNT
signals are consistent with each other and with current XENON constraints, and
they unambiguously predict near future signals at XENON and CRESST. We provide
a quark-level realization of IVDM as WIMPless dark matter that is consistent
with all collider and low-energy bounds.Comment: 5 pages; v2: added references and fixed figures for Mac users; v3:
published version; v4: fixed erroneous Ar values in table
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