2,807 research outputs found
Composable Distributed Access Control and Integrity Policies for Query-Based Wireless Sensor Networks
An expected requirement of wireless sensor networks (WSN) is the support of a vast number of users while permitting limited access privileges. While WSN nodes have severe resource constraints, WSNs will need to restrict access to data, enforcing security policies to protect data within WSNs. To date, WSN security has largely been based on encryption and authentication schemes. WSN Authorization Specification Language (WASL) is specified and implemented using tools coded in JavaTM. WASL is a mechanism{independent policy language that can specify arbitrary, composable security policies. The construction, hybridization, and composition of well{known security models is demonstrated and shown to preserve security while providing for modifications to permit inter{network accesses with no more impact on the WSN nodes than any other policy update. Using WASL and a naive data compression scheme, a multi-level security policy for a 1000-node network requires 66 bytes of memory per node. This can reasonably be distributed throughout a WSN. The compilation of a variety of policy compositions are shown to be feasible using a notebook{class computer like that expected to be performing typical WSN management responsibilities
A panel-based investigation into the relationship between stock prices and dividends
This paper investigates the presence of cointegration between stock prices and dividends for a panel of 56 large UK companies. Using new techniques which account for integrated processes in a panel context we demonstrate that stock prices and dividends are cointegrated, with an implied common discount rate of 5.8%
A Proposal to Detect Dark Matter Using Axionic Topological Antiferromagnets
Antiferromagnetically doped topological insulators (A-TI) are among the
candidates to host dynamical axion fields and axion-polaritons; weakly
interacting quasiparticles that are analogous to the dark axion, a long sought
after candidate dark matter particle. Here we demonstrate that using the axion
quasiparticle antiferromagnetic resonance in A-TI's in conjunction with
low-noise methods of detecting THz photons presents a viable route to detect
axion dark matter with mass 0.7 to 3.5 meV, a range currently inaccessible to
other dark matter detection experiments and proposals. The benefits of this
method at high frequency are the tunability of the resonance with applied
magnetic field, and the use of A-TI samples with volumes much larger than 1
mm.Comment: 6 pages, 4 figures. v2 accepted for publication in Physical Review
Letters. Many points clarified, some parameter estimates revise
How do UK-based foreign exchange dealers think their market operates?
This paper summarises the results of a survey of UK based
foreign exchange dealers conducted in 1998. It addresses topics in three
main areas: The microeconomic operation of the foreign exchange
market; the beliefs of dealers regarding the importance, or otherwise, of
macroeconomic fundamental factors in affecting exchange rates;
microstructure factors in FX. We find that heterogeneity of traders’ beliefs
is evident from the results but that it is not possible to explain such
disagreements in terms of institutional detail, rank or trading technique
(e.g. technical analysts versus fundamentalists). As expected, nonfundamental
factors are thought to dominate short horizon changes in
exchange rates, but fundamentals are deemed important over much
shorter horizons that the mainstream empirical literature would suggest.
Finally, market ‘norms’ and behavioural phenomena are very strong in the
FX market and appear to be key determinants of the bid-ask spread
NuSTAR hard X-ray observation of a sub-A class solar flare
We report a NuSTAR observation of a solar microflare, SOL2015-09-01T04.
Although it was too faint to be observed by the GOES X-ray Sensor, we estimate
the event to be an A0.1 class flare in brightness. This microflare, with only 5
counts per second per detector observed by RHESSI, is fainter than any hard
X-ray (HXR) flare in the existing literature. The microflare occurred during a
solar pointing by the highly sensitive NuSTAR astrophysical observatory, which
used its direct focusing optics to produce detailed HXR microflare spectra and
images. The microflare exhibits HXR properties commonly observed in larger
flares, including a fast rise and more gradual decay, earlier peak time with
higher energy, spatial dimensions similar to the RHESSI microflares, and a
high-energy excess beyond an isothermal spectral component during the impulsive
phase. The microflare is small in emission measure, temperature, and energy,
though not in physical size; observations are consistent with an origin via the
interaction of at least two magnetic loops. We estimate the increase in thermal
energy at the time of the microflare to be 2.4x10^27 ergs. The observation
suggests that flares do indeed scale down to extremely small energies and
retain what we customarily think of as "flarelike" properties.Comment: Status: Accepted by the Astrophysical Journal, 2017 July 1
Cosmology of Axions and Moduli: A Dynamical Systems Approach
This paper is concerned with string cosmology and the dynamics of multiple
scalar fields in potentials that can become negative, and their features as
(Early) Dark Energy models. Our point of departure is the "String Axiverse", a
scenario that motivates the existence of cosmologically light axion fields as a
generic consequence of string theory. We couple such an axion to its
corresponding modulus. We give a detailed presentation of the rich cosmology of
such a model, ranging from the setting of initial conditions on the fields
during inflation, to the asymptotic future. We present some simplifying
assumptions based on the fixing of the axion decay constant , and on the
effective field theory when the modulus trajectory is adiabatic, and find the
conditions under which these assumptions break down. As a by-product of our
analysis, we find that relaxing the assumption of fixed leads to the
appearance of a new meta-stable de-Sitter region for the modulus without the
need for uplifting by an additional constant. A dynamical systems analysis
reveals the existence of many fixed point attractors, repellers and saddle
points, which we analyse in detail. We also provide geometric interpretations
of the phase space. The fixed points can be used to bound the couplings in the
model. A systematic scan of certain regions of parameter space reveals that the
future evolution of the universe in this model can be rich, containing multiple
epochs of accelerated expansion.Comment: 27 pages, 12 figures, comments welcome, v2 minor correction
Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has
observed several small microflares in its initial solar pointings. In this
paper, we present the first joint observation of a microflare with NuSTAR and
Hinode/XRT on 2015 April 29 at ~11:29 UT. This microflare shows heating of
material to several million Kelvin, observed in Soft X-rays (SXRs) with
Hinode/XRT, and was faintly visible in Extreme Ultraviolet (EUV) with SDO/AIA.
For three of the four NuSTAR observations of this region (pre-, decay, and post
phases) the spectrum is well fitted by a single thermal model of 3.2-3.5 MK,
but the spectrum during the impulsive phase shows additional emission up to 10
MK, emission equivalent to A0.1 GOES class. We recover the differential
emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving
unprecedented coverage in temperature. We find the pre-flare DEM peaks at ~3 MK
and falls off sharply by 5 MK; but during the microflare's impulsive phase the
emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of
about erg s. As the NuSTAR spectrum is purely
thermal we determined upper-limits on the possible non-thermal bremsstrahlung
emission. We find that for the accelerated electrons to be the source of the
heating requires a power-law spectrum of with a low energy
cut-off keV. In summary, this first NuSTAR microflare
strongly resembles much more powerful flares.Comment: Accepted for publication in ApJ. 14 pages with 12 figures and 1 tabl
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