2,061 research outputs found
Evaluating the Applicability of the Fokker-Planck Equation in Polymer Translocation: A Brownian Dynamics Study
Brownian dynamics (BD) simulations are used to study the translocation
dynamics of a coarse-grained polymer through a cylindrical nanopore. We
consider the case of short polymers, with a polymer length, N, in the range
N=21-61. The rate of translocation is controlled by a tunable friction
coefficient, gamma_{0p}, for monomers inside the nanopore. In the case of
unforced translocation, the mean translocation time scales with polymer length
N as ~ (N-N_p)^alpha, where N_p is the average number of monomers in the
nanopore. The exponent approaches the value alpha=2 when the pore friction is
sufficiently high, in accord with the prediction for the case of the
quasi-static regime where pore friction dominates. In the case of forced
translocation, the polymer chain is stretched and compressed on the cis and
trans sides, respectively, for low gamma_{0p}. However, the chain approaches
conformational quasi-equilibrium for sufficiently large gamma_{0p}. In this
limit the observed scaling of with driving force and chain length
supports the FP prediction that is proportional to N/f_d for sufficiently
strong driving force. Monte Carlo simulations are used to calculate
translocation free energy functions for the system. The free energies are used
with the Fokker-Planck equation to calculate translocation time distributions.
At sufficiently high gamma_{0p}, the predicted distributions are in excellent
agreement with those calculated from the BD simulations. Thus, the FP equation
provides a valid description of translocation dynamics for sufficiently high
pore friction for the range of polymer lengths considered here. Increasing N
will require a corresponding increase in pore friction to maintain the validity
of the FP approach. Outside the regime of low N and high pore friction, the
polymer is out of equilibrium, and the FP approach is not valid.Comment: 13 pages, 11 figure
AGN-Induced Cavities in NGC 1399 and NGC 4649
We present an analysis of archival Chandra and VLA observations of the E0
galaxy NGC 1399 and the E2 galaxy NGC 4649 in which we investigate cavities in
the surrounding X-ray emitting medium caused by the central AGN. We calculate
the jet power required for the AGN to evacuate these cavities and find values
of ~8x10^{41} erg/s and ~14x10^{41} erg/s for the lobes of NGC 1399 and
~7x10^{41} erg/s and ~6x10^{41} erg/s for those of NGC 4649. We also calculate
the k/f values for each cavity, where k is the ratio of the total particle
energy to that of electrons radiating in the range of 10 MHz to 10 GHz, and f
is the volume filling factor of the plasma in the cavity. We find that the
values of k/f for the lobes of NGC 1399 are ~93 and ~190, and those of the
lobes of NGC 4649 are ~15000 and ~12000. We conclude that the assumed spectrum
describes the electron distribution in the lobes of NGC 1399 reasonably well,
and that there are few entrained particles. For NGC 4649, either there are many
entrained particles or the model spectrum does not accurately describe the
population of electrons.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Jobseekers Regime and Flexible New Deal, the Six Month Offer and Support for the Newly Unemployed evaluations: An early process study
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On viscosity, conduction and sound waves in the intracluster medium
Recent X-ray and optical observations of the Perseus cluster indicate that
the viscous and conductive dissipation of sound waves is the mechanism
responsible for heating the intracluster medium and thus balancing radiative
cooling of cluster cores. We discuss this mechanism more generally and show how
the specific heating and cooling rates vary with temperature and radius. It
appears that the heating mechanism is most effective above 10^7K, which allows
for radiative cooling to proceed within normal galaxy formation but will stifle
the growth of very massive galaxies. The scaling of the wavelength of sound
waves with cluster temperature and feedback in the system are investigated.Comment: 5 pages, 4 figures, MNRAS accepte
Fossil AGN jets as ultra high energy particle accelerators
Remnants of AGN jets and their surrounding cocoons leave colossal
magnetohydrodynamic (MHD) fossil structures storing total energies ~10^{60}
erg. The original active galacic nucleus (AGN) may be dead but the fossil will
retain its stable configuration resembling the reversed-field pinch (RFP)
encountered in laboratory MHD experiments.
In an RFP the longitudinal magnetic field changes direction at a critical
distance from the axis, leading to magnetic re-connection there, and to slow
decay of the large-scale RFP field. We show that this field decay induces
large-scale electric fields which can accelerate cosmic rays with an E^{-2}
power-law up to ultra-high energies with a cut-off depending on the fossil
parameters. The cut-off is expected to be rigidity dependent, implying the
observed composition would change from light to heavy close to the cut-off if
one or two nearby AGN fossils dominate. Given that several percent of the
universe's volume may house such slowly decaying structures, these fossils may
even re-energize ultra-high energy cosmic rays from distant/old sources,
offsetting the ``GZK-losses'' due to interactions with photons of the cosmic
microwave background radiation and giving evidence of otherwise undetectable
fossils. In this case the composition would remain light to the highest
energies if distant sources or fossils dominated, but otherwise would be mixed.
It is hoped the new generation of cosmic ray experiments such as the Pierre
Auger Observatory and ultra-high energy neutrino telescopes such as ANITA and
lunar Cherenkov experiments will clarify this.Comment: 11 pages, 6 figures, additional references and explanations. Accepted
for publication in MNRA
The prevalence and causes of vision loss in Indigenous Australians: the National Indigenous Eye Health Survey
Aim: To determine the prevalence and causes of vision loss in Indigenous Australians. Design, setting and participants: A national, stratified, random cluster sample was drawn from 30 communities across Australia that each included about 300 Indigenous people of all ages. A sample of non-Indigenous adults aged 40 years was also tested at several remote sites for comparison. Participants were examined using a standardised protocol that included a questionnaire (self-administered or completed with the help of field staff), visual acuity (VA) testing on presentation and after correction, visual field testing, trachoma grading, and fundus and lens photography. The data were collected in 2008. Main outcome measures: VA; prevalence of low vision and blindness; causes of vision loss; rates of vision loss in Indigenous compared with non-Indigenous adults.
Results: 1694Indigenouschildrenand1189Indigenousadultswereexamined, representing recruitment rates of 84% for children aged 5–15 years and 72% for adults aged 40 years. Rates of low vision (VA \u3c 6/12 to 6/60) were 1.5% (95% CI, 0.9%–2.1%) in children and 9.4% (95% CI, 7.8%–11.1%) in adults. Rates of blindness (VA \u3c 6/60) were 0.2% (95% CI, 0.04%–0.5%) in children and 1.9% (95% CI, 1.1%–2.6%) in adults. The principal cause of low vision in both adults and children was refractive error. The principal causes of blindness in adults were cataract, refractive error and optic atrophy. Relative risks (RRs) of vision loss and blindness in Indigenous adults compared with adults in the mainstream Australian population were 2.8 and 6.2, respectively. By contrast, RRs of vision loss and blindness in Indigenous children compared with mainstream children were 0.2 and 0.6, respectively. Conclusion: Many causes of vision loss in our sample were readily avoidable. Better allocation of services and resources is required to give all Australians equal access to eye health services
Radio Bubbles in Clusters
We extend our earlier work on cluster cores with distinct radio bubbles,
adding more active bubbles, i.e. those with Ghz radio emission, to our sample,
and also investigating ``ghost bubbles,'' i.e. those without GHz radio
emission. We have determined k, which is the ratio of the total particle energy
to that of the electrons radiating between 10 MHz and 10 GHz. Constraints on
the ages of the active bubbles confirm that the ratio of the energy factor, k,
to the volume filling factor, f lies within the range 1 < k/f < 1000. In the
assumption that there is pressure equilibrium between the radio-emitting plasma
and the surrounding thermal X-ray gas, none of the radio lobes has
equipartition between the relativistic particles and the magnetic field. A
Monte-Carlo simulation of the data led to the conclusion that there are not
enough bubbles present in the current sample to be able to determine the shape
of the population. An analysis of the ghost bubbles in our sample showed that
on the whole they have higher upper limits on k/f than the active bubbles,
especially when compared to those in the same cluster. A study of the Brightest
55 cluster sample shows that 17, possibly 20, clusters required some form of
heating as they have a short central cooling time, t_cool < 3 Gyr, and a large
central temperature drop, T_centre/T_outer< 1/2. Of these between 12 (70 per
cent) and 15 (75 per cent), contain bubbles. This indicates that the duty cycle
of bubbles is large in such clusters and that they can play a major role in the
heating process.Comment: 12 pages, Accepted for publication in MNRA
Particle Energies and Filling Fractions of Radio Bubbles in Cluster Cores
Using Chandra images of cluster cores with clear radio bubbles, we have
determined k, which is the ratio of the total particle energy to that of the
electrons radiating between 10 MHz and 10 GHz. Radiative and dynamical
constraints on the bubbles indicate that the ratio of the energy factor, k, to
the volume filling factor, f, lies within the range 1 < k/f < 1000. Assuming
pressure equilibrium between the radio-emitting plasma and the surrounding
X-ray gas, none of the lobes have equipartition between relativistic particles
and magnetic field. There is no evidence for any dependence of the upper limit
of the k/f ratio on any physical parameter of the cluster or the radio source.
The distribution of the upper limit on k/f appears to be bimodal, the value for
some clusters being ~3 and for the others ~300. We show that this is may due to
the composition of the jet which forms the bubbles, the variation in the volume
filling fraction or variation in the amount of re-acceleration occurring in the
bubble.Comment: 12 pages, 9 figures; accepted for publication in MNRA
The radio properties of a complete, X-ray selected sample of nearby, massive elliptical galaxies
We investigate the radio properties of a complete sample of nearby, massive,
X-ray bright elliptical and S0 galaxies. Our sample contains 18 galaxies with
ROSAT All-Sky Survey X-ray fluxes Fx_(0.1-2.4 keV) > 3 x 10^(-12) erg/s/cm^2,
within a distance of 100 Mpc. For these galaxies, we have complete (18/18) VLA
radio and Chandra X-ray coverage. Nuclear radio emission is detected from 17/18
of the galaxies. Ten of the galaxies exhibit extended radio emission; of these
ten, all but one also exhibit clear evidence of interaction of the radio source
with the surrounding, X-ray emitting gas. Among the seven galaxies with
unresolved radio sources, one has clear, and one has small, cavity-like
features in the Chandra X-ray images; a third has a disturbed X-ray morphology.
Using a radio luminosity limit equivalent to L_(1.4 Ghz) > 10^(23) W/Hz to
calculate the radio-loud fraction, we find that this misses the majority of the
radio detected galaxies in the sample. We determine integrated radio-to-X-ray
flux ratios for the galaxies, GRx, which are shown to span a large range
(factor of 100). We calculate the mass-weighted cooling times within 1 kpc, and
find hints for an anticorrelation with the radio luminosity. We also calculate
limits on k/f, where k is the ratio of the total particle energy to that of
relativistic electrons radiating in the range 10 MHz-10 GHz and f is the volume
filling factor of the plasma in the cavity. The k/f distribution is also broad,
reflecting previous results for larger galaxy clusters. Lowering the X-ray flux
limit, at the expense of less complete VLA and Chandra coverage, increases the
size of our sample to 42 galaxies. Nuclear radio activity is detected in at
least 34/42 of this extended sample.Comment: Accepted for publication in MNRAS, 19 pages, 11 Figures and 7 Table
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