379 research outputs found
Acceleration of energetic particles by large-scale compressible magnetohydrodynamic turbulence
Fast particles diffusing along magnetic field lines in a turbulent plasma can
diffuse through and then return to the same eddy many times before the eddy is
randomized in the turbulent flow. This leads to an enhancement of particle
acceleration by large-scale compressible turbulence relative to previous
estimates in which isotropic particle diffusion is assumed.Comment: 13 pages, 3 figures, accepted for publication in Ap
Extending emission line Doppler tomography ; mapping modulated line flux
Emission line Doppler tomography is a powerful tool that resolves the
accretion flow in binaries on micro-arcsecond scales using time-resolved
spectroscopy. I present an extension to Doppler tomography that relaxes one of
its fundamental axioms and permits the mapping of time-dependent emission
sources. Significant variability on the orbital period is a common
characteristic of the emission sources that are observed in the accretion flows
of cataclysmic variables and X-ray binaries. Modulation Doppler tomography maps
sources varying harmonically as a function of the orbital period through the
simultaneous reconstruction of three Doppler tomograms. One image describes the
average flux distribution like in standard tomography, while the two additional
images describe the variable component in terms of its sine and cosine
amplitudes. I describe the implementation of such an extension in the form of
the maximum entropy based fitting code MODMAP. Test reconstructions of
synthetic data illustrate that the technique is robust and well constrained.
Artifact free reconstructions of complex emission distributions can be achieved
under a wide range of signal to noise levels. An application of the technique
is illustrated by mapping the orbital modulations of the asymmetric accretion
disc emission in the dwarf nova IP Pegasi.Comment: 8 pages, 4 figures; accepted for publication in MNRA
Feedback Heating by Cosmic Rays in Clusters of Galaxies
Recent observations show that the cooling flows in the central regions of
galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles
are a leading candidate for suppressing cooling, usually via some form of
mechanical heating. At the same time, observed X-ray cavities and synchrotron
emission point toward a significant non-thermal particle population. Previous
studies have focused on the dynamical effects of cosmic-ray pressure support,
but none have built successful models in which cosmic-ray heating is
significant. Here we investigate a new model of AGN heating, in which the
intracluster medium is efficiently heated by cosmic-rays, which are injected
into the ICM through diffusion or the shredding of the bubbles by
Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal
conduction as well. Using numerical simulations, we show that the cooling
catastrophe is efficiently suppressed. The cluster quickly relaxes to a
quasi-equilibrium state with a highly reduced accretion rate and temperature
and density profiles which match observations. Unlike the conduction-only case,
no fine-tuning of the Spitzer conduction suppression factor f is needed. The
cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within
observational bounds. Cosmic ray heating is a very attractive alternative to
mechanical heating, and may become particularly compelling if GLAST detects the
gamma-ray signature of cosmic-rays in clusters.Comment: Revised version accepted for publication in MNRAS. Significantly
expanded discussion and new simulations exploring parameter space/model
robustness; conclusions unchange
Maximum entropy and the problem of moments: A stable algorithm
We present a technique for entropy optimization to calculate a distribution
from its moments. The technique is based upon maximizing a discretized form of
the Shannon entropy functional by mapping the problem onto a dual space where
an optimal solution can be constructed iteratively. We demonstrate the
performance and stability of our algorithm with several tests on numerically
difficult functions. We then consider an electronic structure application, the
electronic density of states of amorphous silica and study the convergence of
Fermi level with increasing number of moments.Comment: 4 pages including 3 figure
Fine Structures of Shock of SN 1006 with the Chandra Observation
The north east shell of SN 1006 is the most probable acceleration site of
high energy electrons (up to ~ 100 TeV) with the Fermi acceleration mechanism
at the shock front. We resolved non-thermal filaments from thermal emission in
the shell with the excellent spatial resolution of Chandra. The thermal
component is extended widely over about ~ 100 arcsec (about 1 pc at 1.8 kpc
distance) in width, consistent with the shock width derived from the Sedov
solution. The spectrum is fitted with a thin thermal plasma of kT = 0.24 keV in
non-equilibrium ionization (NEI), typical for a young SNR. The non-thermal
filaments are likely thin sheets with the scale widths of ~ 4 arcsec (0.04 pc)
and ~ 20 arcsec (0.2 pc) at upstream and downstream, respectively. The spectra
of the filaments are fitted with a power-law function of index 2.1--2.3, with
no significant variation from position to position. In a standard diffusive
shock acceleration (DSA) model, the extremely small scale length in upstream
requires the magnetic field nearly perpendicular to the shock normal. The
injection efficiency (eta) from thermal to non-thermal electrons around the
shock front is estimated to be ~ 1e-3 under the assumption that the magnetic
field in upstream is 10 micro G. In the filaments, the energy densities of the
magnetic field and non-thermal electrons are similar to each other, and both
are slightly smaller than that of thermal electrons. in the same order for each
other. These results suggest that the acceleration occur in more compact region
with larger efficiency than previous studies.Comment: 24 pages, 11 figures, Accepted for publication in ApJ, the paper with
full resolution images in
http://www-cr.scphys.kyoto-u.ac.jp/member/bamba/Paper/SN1006.pd
Bayesian coherent analysis of in-spiral gravitational wave signals with a detector network
The present operation of the ground-based network of gravitational-wave laser
interferometers in "enhanced" configuration brings the search for gravitational
waves into a regime where detection is highly plausible. The development of
techniques that allow us to discriminate a signal of astrophysical origin from
instrumental artefacts in the interferometer data and to extract the full range
of information are some of the primary goals of the current work. Here we
report the details of a Bayesian approach to the problem of inference for
gravitational wave observations using a network of instruments, for the
computation of the Bayes factor between two hypotheses and the evaluation of
the marginalised posterior density functions of the unknown model parameters.
The numerical algorithm to tackle the notoriously difficult problem of the
evaluation of large multi-dimensional integrals is based on a technique known
as Nested Sampling, which provides an attractive alternative to more
traditional Markov-chain Monte Carlo (MCMC) methods. We discuss the details of
the implementation of this algorithm and its performance against a Gaussian
model of the background noise, considering the specific case of the signal
produced by the in-spiral of binary systems of black holes and/or neutron
stars, although the method is completely general and can be applied to other
classes of sources. We also demonstrate the utility of this approach by
introducing a new coherence test to distinguish between the presence of a
coherent signal of astrophysical origin in the data of multiple instruments and
the presence of incoherent accidental artefacts, and the effects on the
estimation of the source parameters as a function of the number of instruments
in the network.Comment: 22 page
Developing a self-healing supramolecular nucleoside hydrogel
Low molecular weight gelator hydrogels provide a viable alternative to traditional polymer based drug delivery platforms, owing to their tunable stability and in most cases inherent biocompatibility. Here we report the first self-healing nucleoside hydrogel using N4-octanoyl-2′-deoxycytidine (0.5% w/v) for drug delivery. The hydrogel's cross-linked nanofibrillar structure, was characterised using oscillatory rheology and confirmed using SEM and TEM imaging. The potential of this gel for drug delivery was explored in vitro using fluorescently labelled tracers. Cell viability assays were conducted using pancreatic cell lines which tolerated the gels well; whilst no adverse effects on the viability or proliferation of cells were observed for fibroblast cell lines
Extending political participation in China: new opportunities for citizens in the policy process
Authoritarian political systems are portrayed as offering few opportunities for citizens to participate in politics – particularly in the policy process. This paper’s contribution is to set out new mechanisms that enable Authoritarian political systems are portrayed as offering few opportunities for citizens to participate in politics – particularly in the policy process. This paper’s contribution is to set out new mechanisms that enable Chinese citizens to evaluate government performance, contribute to decision-making, shape policy agendas and feed back on implementation. Based on fieldwork in the city of Hangzhou, we argue that the local party-state orchestrates citizen participation in the policy process, but members of the public nevertheless do have influence. Political participation is widening in China, but it is still controlled. It is not yet clearly part of a process of democratization, but it does establish the principle of citizen rights to oversee the government
Maximum Entropy for Gravitational Wave Data Analysis: Inferring the Physical Parameters of Core-Collapse Supernovae
The gravitational wave signal arising from the collapsing iron core of a Type
II supernova progenitor star carries with it the imprint of the progenitor's
mass, rotation rate, degree of differential rotation, and the bounce depth.
Here, we show how to infer the gravitational radiation waveform of a core
collapse event from noisy observations in a network of two or more LIGO-like
gravitational wave detectors and, from the recovered signal, constrain these
source properties. Using these techniques, predictions from recent core
collapse modeling efforts, and the LIGO performance during its S4 science run,
we also show that gravitational wave observations by LIGO might have been
sufficient to provide reasonable estimates of the progenitor mass, angular
momentum and differential angular momentum, and depth of the core at bounce,
for a rotating core collapse event at a distance of a few kpc.Comment: 44 pages, 12 figures; accepted version scheduled to appear in Ap J 1
April 200
Constraints On the Diffusive Shock Acceleration From the Nonthermal X-ray Thin Shells In SN1006 NE Rim
Characteristic scale lengths of nonthermal X-rays from the SN1006 NE rim,
which are observed by Chandra, are interpreted in the context of the diffusive
shock acceleration on the assumption that the observed spatial profile of
nonthermal X-rays corresponds to that of accelerated electrons with energies of
a few tens of TeV. To explain the observed scale lengths, we construct two
simple models with a test particle approximation, where the maximum energy of
accelerated electrons is determined by the age of SN1006 (age-limited model) or
the energy loss (energy loss-limited model), and constrain the magnetic field
configuration and the diffusion coefficients of accelerated electrons. When the
magnetic field is nearly parallel to the shock normal, the magnetic field
should be in the range of 20-85 micro Gauss and highly turbulent both in
upstream and downstream, which means that the mean free path of accelerated
electrons is on the order of their gyro-radius (Bohm limit). This situation can
be realized both in the age-limited and energy loss-limited model. On the other
hand, when the magnetic field is nearly perpendicular to the shock normal,
which can exist only in the age-limited case, the magnetic field is several
micro Gauss in the upstream and 14-20 micro Gauss in the downstream, and the
upstream magnetic field is less turbulent than the downstream.Comment: 9 pages, 4 figures, accepted for publication in A&
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