12,374 research outputs found
A Probabilistic Analysis of Kademlia Networks
Kademlia is currently the most widely used searching algorithm in P2P
(peer-to-peer) networks. This work studies an essential question about Kademlia
from a mathematical perspective: how long does it take to locate a node in the
network? To answer it, we introduce a random graph K and study how many steps
are needed to locate a given vertex in K using Kademlia's algorithm, which we
call the routing time. Two slightly different versions of K are studied. In the
first one, vertices of K are labelled with fixed IDs. In the second one,
vertices are assumed to have randomly selected IDs. In both cases, we show that
the routing time is about c*log(n), where n is the number of nodes in the
network and c is an explicitly described constant.Comment: ISAAC 201
Pure hydrogen low-temperature plasma exposure of HOPG and graphene: Graphane formation?
Single- and multilayer graphene and highly ordered pyrolytic graphite (HOPG) were exposed to a pure hydrogen low-temperature plasma (LTP). Characterizations include various experimental techniques such as photoelectron spectroscopy, Raman spectroscopy and scanning probe microscopy. Our photoemission measurement shows that hydrogen LTP exposed HOPG has a diamond-like valence-band structure, which suggests double-sided hydrogenation. With the scanning tunneling microscopy technique, various atomic-scale charge-density patterns were observed, which may be associated with different C-H conformers. Hydrogen-LTP-exposed graphene on SiO₂ has a Raman spectrum in which the D peak to G peak ratio is over 4, associated with hydrogenation on both sides. A very low defect density was observed in the scanning probe microscopy measurements, which enables a reverse transformation to graphene. Hydrogen-LTP-exposed HOPG possesses a high thermal stability, and therefore, this transformation requires annealing at over 1000 °C
The Closest Look at 1H0707-495: X-ray Reverberation Lags with 1.3 Ms of Data
Reverberation lags in AGN were first discovered in the NLS1 galaxy,
1H0707-495. We present a follow-up analysis using 1.3 Ms of data, which allows
for the closest ever look at the reverberation signature of this remarkable
source. We confirm previous findings of a hard lag of ~100 seconds at
frequencies v ~ [0.5 - 4] e-4 Hz, and a soft lag of ~30 seconds at higher
frequencies, v ~ [0.6 - 3] e-3 Hz. These two frequency domains clearly show
different energy dependences in their lag spectra. We also find evidence for a
signature from the broad Fe K line in the high frequency lag spectrum. We use
Monte Carlo simulations to show how the lag and coherence measurements respond
to the addition of Poisson noise and to dilution by other components. With our
better understanding of these effects on the lag, we show that the lag-energy
spectra can be modelled with a scenario in which low frequency hard lags are
produced by a compact corona responding to accretion rate fluctuations
propagating through an optically thick accretion disc, and the high frequency
soft lags are produced by short light-travel delay associated with reflection
of coronal power-law photons off the disc.Comment: 11 pages, 10 figures. Accepted for publication in MNRA
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FARS2 mutations presenting with pure spastic paraplegia and lesions of the dentate nuclei.
Mutations in FARS2, the gene encoding the mitochondrial phenylalanine-tRNA synthetase (mtPheRS), have been linked to a range of phenotypes including epileptic encephalopathy, developmental delay, and motor dysfunction. We report a 9-year-old boy with novel compound heterozygous variants of FARS2, presenting with a pure spastic paraplegia syndrome associated with bilateral signal abnormalities in the dentate nuclei. Exome sequencing identified a paternal nonsense variant (Q216X) lacking the catalytic core and anticodon-binding regions, and a maternal missense variant (P136H) possessing partial enzymatic activity. This case confirms and expands the phenotype related to FARS2 mutations with regards to clinical presentation and neuroimaging findings
Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs)
in low-mass X-ray binary systems can be used to constrain NS masses and radii.
Most previous work of this type has set these constraints using Planck function
fits as a proxy: both the models and the data are fit with diluted blackbody
functions to yield normalizations and temperatures which are then compared
against each other. Here, for the first time, we fit atmosphere models of X-ray
bursting NSs directly to the observed spectra. We present a hierarchical
Bayesian fitting framework that uses state-of-the-art X-ray bursting NS
atmosphere models with realistic opacities and relativistic exact Compton
scattering kernels as a model for the surface emission. We test our approach
against synthetic data, and find that for data that are well-described by our
model we can obtain robust radius, mass, distance, and composition
measurements. We then apply our technique to Rossi X-ray Timing Explorer
observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to
all five bursts shows that the theoretical atmosphere models describe the data
well but there are still some unmodeled features in the spectrum corresponding
to a relative error of 1-5% of the energy flux. After marginalizing over this
intrinsic scatter, we find that at 68% credibility the circumferential radius
of the NS in 4U 1702-429 is R = 12.4+-0.4 km, the gravitational mass is
M=1.9+-0.3 Msun, the distance is 5.1 < D/kpc < 6.2, and the hydrogen mass
fraction is X < 0.09.Comment: 15 pages, 11 figures, submitted to A&
Device-spectroscopy of magnetic field effects in a polyfluorene organic light-emitting diode
We perform charge-induced absorption and electroluminescence spectroscopy in
a polyfluorene organic magnetoresistive device. Our experiments allow us to
measure the singlet exciton, triplet exciton and polaron densities in a live
device under an applied magnetic field, and to distinguish between three
different models that were proposed to explain organic magnetoresistance. These
models are based on different spin-dependent interactions, namely exciton
formation, triplet exciton-polaron quenching and bipolaron formation. We show
that the singlet exciton, triplet exciton and polaron densities and
conductivity all increase with increasing magnetic field. Our data are
inconsistent with the exciton formation and triplet-exciton polaron quenching
models.Comment: 4 pages, two figure
A Parallax Distance to the Microquasar GRS 1915+105 and a Revised Estimate of its Black Hole Mass
Using the Very Long Baseline Array, we have measured a trigonometric parallax
for the micro quasar GRS 1915+105, which contains a black hole and a K-giant
companion. This yields a direct distance estimate of 8.6 (+2.0,-1.6) kpc and a
revised estimate for the mass of the black hole of 12.4 (+2.0,-1.8) Msun. GRS
1915+105 is at about the same distance as some HII regions and water masers
associated with high-mass star formation in the Sagittarius spiral arm of the
Galaxy. The absolute proper motion of GRS 1915+105 is -3.19 +/- 0.03 mas/y and
-6.24 +/- 0.05 mas/y toward the east and north, respectively, which corresponds
to a modest peculiar speed of 22 +/-24 km/s at the parallax distance,
suggesting that the binary did not receive a large velocity kick when the black
hole formed. On one observational epoch, GRS 1915+105 displayed superluminal
motion along the direction of its approaching jet. Considering previous
observations of jet motions, the jet in GRS 1915+105 can be modeled with a jet
inclination to the line of sight of 60 +/- 5 deg and a variable flow speed
between 0.65c and 0.81c, which possibly indicates deceleration of the jet at
distances from the black hole >2000 AU. Finally, using our measurements of
distance and estimates of black hole mass and inclination, we provisionally
confirm our earlier result that the black hole is spinning very rapidly.Comment: 20 pages; 2 tables; 6 figure
The Statistical Multifragmentation Model with Skyrme Effective Interactions
The Statistical Multifragmentation Model is modified to incorporate the
Helmholtz free energies calculated in the finite temperature Thomas-Fermi
approximation using Skyrme effective interactions. In this formulation, the
density of the fragments at the freeze-out configuration corresponds to the
equilibrium value obtained in the Thomas-Fermi approximation at the given
temperature. The behavior of the nuclear caloric curve at constant volume is
investigated in the micro-canonical ensemble and a plateau is observed for
excitation energies between 8 and 10 MeV per nucleon. A kink in the caloric
curve is found at the onset of this gas transition, indicating the existence of
a small excitation energy region with negative heat capacity. In contrast to
previous statistical calculations, this situation takes place even in this case
in which the system is constrained to fixed volume. The observed phase
transition takes place at approximately constant entropy. The charge
distribution and other observables also turn out to be sensitive to the
treatment employed in the calculation of the free energies and the fragments'
volumes at finite temperature, specially at high excitation energies. The
isotopic distribution is also affected by this treatment, which suggests that
this prescription may help to obtain information on the nuclear equation of
state
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