175,616 research outputs found
Properties of tug-of-war model for cargo transport by molecular motors
Molecular motors are essential components for the biophysical functions of
the cell. Our current quantitative understanding of how multiple motors move
along a single track is not complete; even though models and theories for
single motor chemomechanics abound. Recently, M.J.I. Mller {\em
et al.} have developed a tug-of-war model to describe the bidirectional
movement of the cargo (PNAS(2008) 105(12) P4609-4614). Through Monte Carlo
simulations, they discovered that the tug-of-war model exhibits several
qualitative different motility regimes, which depend on the precise value of
single motor parameters, and they suggested the sensitivity can be used by a
cell to regulate its cargo traffic. In the present paper, we carry out a
thorough analysis of the tug-of-war model. All the stable, i.e., biophysically
observable, steady states are obtained. Depending on several parameters, the
system exhibits either uni-, bi- or tristability. Based on the separating
boundary of the different stable states and the initial numbers of the
different motor species that are bound to the track, the steady state of the
cargo movement can be predicted, and consequently the steady state velocity can
be obtained. It is found that, the velocity, even the direction, of the cargo
movement change with the initial numbers of the motors which are bound to the
track and several other parameters
Off-Axis Afterglow Light Curves from High-Resolution Hydrodynamical Jet Simulations
Numerical jet simulations serve a valuable role in calculating gamma-ray
burst afterglow emission beyond analytical approximations. Here we present the
results of high resolution 2D simulations of decelerating relativistic jets
performed using the RAM adaptive mesh refinement relativistic hydrodynamics
code. We have applied a separate synchrotron radiation code to the simulation
results in order to calculate light curves at frequencies varying from radio to
X-ray for observers at various angles from the jet axis. We provide a
confirmation from radio light curves from simulations rather than from a
simplified jet model for earlier results in the literature finding that only a
very small number of local Ibc supernovae can possibly harbor an orphan
afterglow.
Also, recent studies have noted an unexpected lack of observed jet breaks in
the Swift sample. Using a jet simulation with physical parameters
representative for an average Swift sample burst, such as a jet half opening
angle of 0.1 rad and a source redshift of z = 2.23, we have created synthetic
light curves at 1.5 keV with artificial errors while accounting for Swift
instrument biases as well. A large set of these light curves have been
generated and analyzed using a Monte Carlo approach. Single and broken power
law fits are compared. We find that for increasing observer angle, the jet
break quickly becomes hard to detect. This holds true even when the observer
remains well within the jet opening angle. We find that the odds that a Swift
light curve from a randomly oriented 0.1 radians jet at z = 2.23 will exhibit a
jet break at the 3 sigma level are only 12 percent. The observer angle
therefore provides a natural explanation for the lack of perceived jet breaks
in the Swift sample.Comment: 4 pages, 3 figures. First of two contributions to proceedings GRB2010
Maryland conference. Editors: McEnery, Racusin and Gehrels. The data from
this paper is publicly available from http://cosmo.nyu.edu/afterglowlibrary
An on-line library of afterglow light curves
Numerical studies of gamma-ray burst afterglow jets reveal significant
qualitative differences with simplified analytical models. We present an
on-line library of synthetic afterglow light curves and broadband spectra for
use in interpreting observational data. Light curves have been calculated for
various physics settings such as explosion energy and circumburst structure, as
well as differing jet parameters and observer angle and redshift. Calculations
gave been done for observer frequencies ranging from low radio to X-ray and for
observer times from hours to decades after the burst. The light curves have
been calculated from high-resolution 2D hydrodynamical simulations performed
with the RAM adaptive-mesh refinement code and a detailed synchrotron radiation
code.
The library will contain both generic afterglow simulations as well as
specific case studies and will be freely accessible at
http://cosmo.nyu.edu/afterglowlibrary . The synthetic light curves can be used
as a check on the accuracy of physical parameters derived from analytical model
fits to afterglow data, to quantitatively explore the consequences of varying
parameters such as observer angle and for accurate predictions of future
telescope data.Comment: 4 pages, 2 figures. Second of two contributions to proceedings
GRB2010 Maryland conference. Editors: McEnery, Racusin and Gehrels. The data
from this paper is publicly available from
http://cosmo.nyu.edu/afterglowlibrary
Sensitivity dependent model of protein-protein interaction networks
The scale free structure p(k)~k^{-gamma} of protein-protein interaction
networks can be reproduced by a static physical model in simulation. We inspect
the model theoretically, and find the key reason for the model to generate
apparent scale free degree distributions. This explanation provides a generic
mechanism of "scale free" networks. Moreover, we predict the dependence of
gamma on experimental protein concentrations or other sensitivity factors in
detecting interactions, and find experimental evidence to support the
prediction.Comment: organization improved, and experimental evidence of predicted
dependence on sensitivity is addresse
Study of X-ray Radiation Damage in Silicon Sensors
The European X-ray Free Electron Laser (XFEL) will deliver 30,000 fully
coherent, high brilliance X-ray pulses per second each with a duration below
100 fs. This will allow the recording of diffraction patterns of single complex
molecules and the study of ultra-fast processes. Silicon pixel sensors will be
used to record the diffraction images. In 3 years of operation the sensors will
be exposed to doses of up to 1 GGy of 12 keV X-rays. At this X-ray energy no
bulk damage in silicon is expected. However fixed oxide charges in the
insulating layer covering the silicon and interface traps at the Si-SiO2
interface will be introduced by the irradiation and build up over time.
We have investigated the microscopic defects in test structures and the
macroscopic electrical properties of segmented detectors as a function of the
X-ray dose. From the test structures we determine the oxide charge density and
the densities of interface traps as a function of dose. We find that both
saturate (and even decrease) for doses between 10 and 100 MGy. For segmented
sensors the defects introduced by the X-rays increase the full depletion
voltage, the surface leakage current and the inter-pixel capacitance. We
observe that an electron accumulation layer forms at the Si-SiO2 interface. Its
width increases with dose and decreases with applied bias voltage. Using TCAD
simulations with the dose dependent parameters obtained from the test
structures, we are able to reproduce the observed results. This allows us to
optimize the sensor design for the XFEL requirements
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