4,849 research outputs found
Active management of multi-service networks.
Future multiservice networks will be extremely large and complex. Novel management solutions will be required to keep the management costs reasonable. Active networking enables management to be delegated to network users as a large set of independent small scale management systems. A novel architecture for an active network based management solution for multiservice networking is presented
CONFLLVM: A Compiler for Enforcing Data Confidentiality in Low-Level Code
We present an instrumenting compiler for enforcing data confidentiality in
low-level applications (e.g. those written in C) in the presence of an active
adversary. In our approach, the programmer marks secret data by writing
lightweight annotations on top-level definitions in the source code. The
compiler then uses a static flow analysis coupled with efficient runtime
instrumentation, a custom memory layout, and custom control-flow integrity
checks to prevent data leaks even in the presence of low-level attacks. We have
implemented our scheme as part of the LLVM compiler. We evaluate it on the SPEC
micro-benchmarks for performance, and on larger, real-world applications
(including OpenLDAP, which is around 300KLoC) for programmer overhead required
to restructure the application when protecting the sensitive data such as
passwords. We find that performance overheads introduced by our instrumentation
are moderate (average 12% on SPEC), and the programmer effort to port OpenLDAP
is only about 160 LoC.Comment: Technical report for CONFLLVM: A Compiler for Enforcing Data
Confidentiality in Low-Level Code, appearing at EuroSys 201
Measurement of electron screening in muonic lead
Energies of the transitions between high-lying (n≥6) states of muonic lead were accurately determined. The results are interpreted as a ∼2% test of the electron screening. The agreement between experiment and theory is good if it is assumed that the refilling of the electron K shell is fast. The present results furthermore severely restrict possible ionization of the electron L shell
Cosmic Ray Acceleration at the Forward Shock in Tycho's Supernova Remnant: Evidence from Chandra X-ray Observations
We present evidence for cosmic ray acceleration at the forward shock in
Tycho's supernova remnant (SNR) from three X-ray observables: (1) the proximity
of the contact discontinuity to the forward shock, or blast wave, (2) the
morphology of the emission from the rim of Tycho, and (3) the spectral nature
of the rim emission. We determine the locations of the blast wave (BW), contact
discontinuity (CD), and reverse shock (RS) around the rim of Tycho's supernova
remnant using a principal component analysis and other methods applied to new
Chandra data. The azimuthal-angle-averaged radius of the BW is 251". For the CD
and RS we find average radii of 241" and 183", respectively. Taking account of
projection effects, we find ratios of 1:0.93:0.70 (BW:CD:RS). We show these
values to be inconsistent with adiabatic hydrodynamical models of SNR
evolution. The CD:BW ratio can be explained if cosmic ray acceleration of ions
is occurring at the forward shock. The RS:BW ratio, as well as the strong Fe Ka
emission from the Tycho ejecta, imply that the RS is not accelerating cosmic
rays. We also extract radial profiles from ~34% of the rim of Tycho and compare
them to models of surface brightness profiles behind the BW for a purely
thermal plasma with an adiabatic shock. The observed morphology of the rim is
much more strongly peaked than predicted by the model, indicating that such
thermal emission is implausible here. Spectral analysis also implies that the
rim emission is non-thermal in nature, lending further support to the idea that
Tycho's forward shock is accelerating cosmic rays.Comment: 39 pages, 10 figures, accepted by Ap
The HEAO A-2 survey of Abell clusters and the X-ray luminosity function
The HEAO A-2 all sky data base was surveyed for 2-10KeV X-rau emission from the 225 Abell clusters og galaxies listed in Abell's (1958) catalog which are of distance class four or less, and are within the fraction of the sky surveyed completely by Abell. Thirty-two identifications of clusters with X-ray sources were made, for which 2-10 KeV fluxes and 90% error boxes are presented. Twelve of these identification are new. The X-ray luminosity function was derived for this statistically complete sample and the best exponential fit was found to be f(L) = 20.2 x 10 to the minus 8 power exp (-l(44)/1.9) per cu Mpc 2-10KeV. The relationship between X-ray luminosity and richness was examined and a correlation was found for richness classes 0, 1, and 2. The relationship of X-rau luminosity, Bautz-Morgan type, and Rood-Sastry type was examined. It was found that BM type I's and RS type cD and B have the greatest average luminosity. The contribution of clusters to the X-ray background was calculated from the luminosity function and was found to be 5%, and with 90% certainty, less than 8% in the 2-10 KeV band pass
Reconstructing Deconstruction: High-Velocity Cloud Distance Through Disruption Morphology
We present Arecibo L-band Feed Array 21-cm observations of a sub-complex of
HVCs at the tip of the Anti-Center Complex. These observations show
morphological details that point to interaction with the ambient halo medium
and differential drag within the cloud sub-complex. We develop a new technique
for measuring cloud distances, which relies upon these observed morphological
and kinematic characteristics, and show that it is consistent with H-alpha
distances. These results are consistent with distances to HVCs and halo
densities derived from models in which HVCs are formed from cooling halo gas.Comment: 8 pages, 2 figures, 1 tabe, Accepted to Ap
Quantum Information Processing with Ferroelectrically Coupled Quantum Dots
I describe a proposal to construct a quantum information processor using
ferroelectrically coupled Ge/Si quantum dots. The spin of single electrons form
the fundamental qubits. Small (<10 nm diameter) Ge quantum dots are optically
excited to create spin polarized electrons in Si. The static polarization of an
epitaxial ferroelectric thin film confines electrons laterally in the
semiconductor; spin interactions between nearest neighbor electrons are
mediated by the nonlinear process of optical rectification. Single qubit
operations are achieved through "g-factor engineering" in the Ge/Si structures;
spin-spin interactions occur through Heisenberg exchange, controlled by
ferroelectric gates. A method for reading out the final state, while required
for quantum computing, is not described; electronic approaches involving single
electron transistors may prove fruitful in satisfying this requirement.Comment: 10 pages, 3 figure
The Escape of Ionizing Photons from OB Associations in Disk Galaxies: Radiation Transfer Through Superbubbles
By solving the time-dependent radiation transfer problem of stellar radiation
through evolving superbubbles within a smoothly varying HI distribution, we
estimate the fraction of ionizing photons emitted by OB associations that
escapes the HI disk of our Galaxy into the halo and intergalactic medium (IGM).
We consider both coeval star-formation and a Gaussian star-formation history
with a time spread sigma_t = 2 Myr. We consider both a uniform H I distribution
and a two-phase (cloud/intercloud) model, with a negligible filling factor of
hot gas. We find that the shells of the expanding superbubbles quickly trap or
attenuate the ionizing flux, so that most of the escaping radiation escapes
shortly after the formation of the superbubble. For the coeval star-formation
history, the total fraction of Lyman Continuum photons that escape both sides
of the disk in the solar vicinity is f_esc approx 0.15 +/- 0.05. For the
Gaussian star formation history, f_esc approx 0.06 +/- 0.03, a value roughly a
factor of two lower than the results of Dove & Shull (1994), where superbubbles
were not considered. For a local production rate of ionizing photons Psi_LyC =
4.95 X 10^7 cm^{-2} s^{-1}, the flux escaping the disk is Phi_LyC approx
(1.5-3.0) X 10^6 cm^{-2} s^{-1} for coeval and Gaussian star formation,
comparable to the flux required to sustain the Reynolds layer.Comment: Revised version (expanded), accepted for publication by ApJ, 38
pages, 8 figures, aasms4.sty and aabib.sty files include
Locked down apps versus the social media ecology : why do young people and educators disagree on the best delivery platform for digital sexual health entertainment education?
This article reports on focus groups exploring the best way to reach young men with vulgar comedy videos that provide sexual health information. Young people reported that they found the means by which the material was presented - as a locked down app - to be problematic, and that it would better be delivered through social media platforms such as YouTube. This would make it more 'spreadable'. By contrast, adult sex education stakeholders thought the material should be contained within a locked down, stand-alone app - otherwise it might be seen by children who are too young, and/or young people might misunderstand the messages. We argue that the difference in approach represented by these two sets of opinions represents a fundamental stumbling block for attempts to reach young people with digital sexual health materials, which can be understood through the prism of different cultural forms - education versus entertainment
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