2,154 research outputs found
Integrated assurance assessment of a reconfigurable digital flight control system
The integrated application of reliability, failure effects and system simulator methods in establishing the airworthiness of a flight critical digital flight control system (DFCS) is demonstrated. The emphasis was on the mutual reinforcement of the methods in demonstrating the system safety
LONG-TERM CHANGES IN CANADA GOOSE NEST SUCCESS AND NEST DENSITIES AT AN IOWA WETLAND COMPLEX
Giant Canada geese (Branta canadensis maxima) were extirpated from Iowa by the early 1900s due to unregulated hunting, egg gathering, and wetland drainage in the nineteenth century (Bishop 1978). Ef- forts to reintroduce Canada geese in Iowa began in 1964 (Bishop and Howing 1972) and involved releasing flightless adults and goslings at nearly 30 sites across the state (Zenner and LaGrange 1998a). In 1972, 13 flightless pairs were released at Rice Lake Wildlife Management Area (WMA; Bishop 1978). By 1989, the breeding population of Canada geese at Rice Lake WMA had increased to 420 nesting adults (G. G. Zenner, Iowa Department of Natural Resources, unpublished data). Canada goose nest success and nest densities were documented from 1989–1991 on extant islands at Rice Lake WMA (Zenner and LaGrange 1998b).
Rice Lake WMA (43.379497, –93.472715) is located in north-central Iowa and lies within the southernmost portion of the Prairie Pothole Region. This wetland complex consists of Rice Lake, a 409-ha shallow, natural lake with a maximum depth of 3 m and 20 natural islands ranging in size from 0.04 to 3.9 ha, and Joice Slough, a 73-ha marsh with a maximum depth of 1 m and 15 natural islands ranging in size from 0.02 to 3.19 ha (Zenner and LaGrange 1998b). During 1989–1991, potential Canada goose nest sites included islands, elevated structures, and muskrat houses. Over the course of that study, drought conditions left Joice Slough completely dry and dramatically lowered water levels at Rice Lake, exposing islands to increased predator activity. Despite the drought, nest densities were high (68–158 nests/ha) and nest success ranged from 40–58% (Zenner and LaGrange 1998b)
Astrophysical Fluids of Novae: High Resolution Pre-decay X-ray spectrum of V4743 Sagittarii
Eight X-ray observations of V4743 Sgr (2002), observed with Chandra and
XMM-Newton are presented. The nova turned off some time between days 301.9 and
371, and the X-ray flux subsequently decreased from day 301.9 to 526 following
an exponential decline time scale of days. We use the absorption
lines present in the SSS spectrum for diagnostic purposes, and characterize the
physics and the dynamics of the expanding atmosphere during the explosion of
the nova. The information extracted from this first stage is then used as input
for computing full photoionization models of the ejecta in V4743 Sgr. The SSS
spectrum is modeled with a simple black-body and multiplicative Gaussian lines,
which provides us of a general kinematical picture of the system, before it
decays to its faint phase (Ness et al. 2003). In the grating spectra taken
between days 180.4 and 370, we can resolve the line profiles of absorption
lines arising from H-like and He-like C, N, and O, including transitions
involving higher principal quantum numbers. Except for a few interstellar
lines, all lines are significantly blue-shifted, yielding velocities between
1000 and 6000 km/s which implies an ongoing mass loss. It is shown that
significant expansion and mass loss occur during this phase of the explosion,
at a rate . Our measurements show that the efficiency of the amount of
energy used for the motion of the ejecta, defined as the ratio between the
kinetic luminosity and the radiated luminosity , is
of the order of one.Comment: 25 pages, 9 figures. Accepted in book: Recent Advances in Fluid
Dynamics with Environmental Applications, pp.365-39
Evaluation of HCMM data for assessing soil moisture and water table depth
Soil moisture in the 0-cm to 4-cm layer could be estimated with 1-mm soil temperatures throughout the growing season of a rainfed barley crop in eastern South Dakota. Empirical equations were developed to reduce the effect of canopy cover when radiometrically estimating the soil temperature. Corrective equations were applied to an aircraft simulation of HCMM data for a diversity of crop types and land cover conditions to estimate the soil moisture. The average difference between observed and measured soil moisture was 1.6% of field capacity. Shallow alluvial aquifers were located with HCMM predawn data. After correcting the data for vegetation differences, equations were developed for predicting water table depths within the aquifer. A finite difference code simulating soil moisture and soil temperature shows that soils with different moisture profiles differed in soil temperatures in a well defined functional manner. A significant surface thermal anomaly was found to be associated with shallow water tables
Evaluation of HCMM data for assessing soil moisture and water table depth
Data were analyzed for variations in eastern South Dakota. Soil moisture in the 0-4 cm layer could be estimated with 1-mm soil temperatures throughout the growing season of a rainfed barley crop (% cover ranging from 30% to 90%) with an r squared = 0.81. Empirical equations were developed to reduce the effect of canopy cover when radiometrically estimating the 1-mm soil temperature, r squared = 0.88. The corrective equations were applied to an aircraft simulation of HCMM data for a diversity of crop types and land cover conditions to estimate the 0-4 cm soil moisture. The average difference between observed and measured soil moisture was 1.6% of field capacity. HCMM data were used to estimate the soil moisture for four dates with an r squared = 0.55 after correction for crop conditions. Location of shallow alluvial aquifers could be accomplished with HCMM predawn data. After correction of HCMM day data for vegetation differences, equations were developed for predicting water table depths within the aquifer (r=0.8)
Microscopic mechanisms of dephasing due to electron-electron interactions
We develop a non-perturbative numerical method to study tunneling of a single
electron through an Aharonov-Bohm ring where several strongly interacting
electrons are bound. Inelastic processes and spin-flip scattering are taken
into account. The method is applied to study microscopic mechanisms of
dephasing in a non-trivial model. We show that electron-electron interactions
described by the Hubbard Hamiltonian lead to strong dephasing: the transmission
probability at flux is high even at small interaction strength. In
addition to inelastic scattering, we identify two energy conserving mechanisms
of dephasing: symmetry-changing and spin-flip scattering. The many-electron
state on the ring determines which of these mechanisms will be at play:
transmitted current can occur either in elastic or inelastic channels, with or
without changing the spin of the scattering electron.Comment: 11 pages, 16 figures Submitted to Phys. Rev.
M31N 2008-05d: A M 31 disk nova with a dipping supersoft X-ray light curve
Classical novae (CNe) represent a major class of supersoft X-ray sources
(SSSs) in the central region of our neighbouring galaxy M 31. Significantly
different SSS properties of CNe in the M 31 bulge and disk were indicated by
recent X-ray population studies, which however considered only a small number
of disk novae. We initiated a target of opportunity (ToO) program with
XMM-Newton to observe the SSS phases of CNe in the disk of M 31 and improve the
database for further population studies. We analysed two XMM-Newton ToO
observations triggered in Aug 2011 and Jan 2012, respectively, and extracted
X-ray spectra and light curves. We report the discovery of an X-ray counterpart
to the M 31 disk nova M31N 2008-05d. The X-ray spectrum of the object allows us
to classify it as a SSS parametrised by a blackbody temperature of 32+/-6 eV.
More than three years after the nova outburst, the X-ray light curve of the SSS
exhibits irregular, broad dip features. These dips affect primarily the very
soft part of the X-ray spectrum, which might indicate absorption effects.
Dipping SSS light curves are rarely observed in M 31 novae. As well as
providing an unparalleled statistical sample, the M 31 population of novae with
SSS counterparts produces frequent discoveries of unusual objects, thereby
underlining the importance of regular monitoring.Comment: 6 pages, 4 figures, 1 table; accepted by Astronomy and Astrophysic
A remarkable recurrent nova in M 31: The predicted 2014 outburst in X-rays with Swift
The M 31 nova M31N 2008-12a was recently found to be a recurrent nova (RN)
with a recurrence time of about 1 year. This is by far the fastest recurrence
time scale of any known RNe. Our optical monitoring programme detected the
predicted 2014 outburst of M31N 2008-12a in early October. We immediately
initiated an X-ray/UV monitoring campaign with Swift to study the
multiwavelength evolution of the outburst. We monitored M31N 2008-12a with
daily Swift observations for 20 days after discovery, covering the entire
supersoft X-ray source (SSS) phase. We detected SSS emission around day six
after outburst. The SSS state lasted for approximately two weeks until about
day 19. M31N 2008-12a was a bright X-ray source with a high blackbody
temperature. The X-ray properties of this outburst were very similar to the
2013 eruption. Combined X-ray spectra show a fast rise and decline of the
effective blackbody temperature. The short-term X-ray light curve showed
strong, aperiodic variability which decreased significantly after about day 14.
Overall, the X-ray properties of M31N 2008-12a are consistent with the average
population properties of M 31 novae. The optical and X-ray light curves can be
scaled uniformly to show similar time scales as those of the Galactic RNe U Sco
or RS Oph. The SSS evolution time scales and effective temperatures are
consistent with a high-mass WD. We predict the next outburst of M31N 2008-12a
to occur in autumn 2015.Comment: 13 pages, 7 figures, 3 tables; accepted for publication in A&
GW approximations and vertex corrections on the Keldysh time-loop contour: application for model systems at equilibrium
We provide the formal extension of Hedin's GW equations for single-particle
Green's functions with electron-electron interaction onto the Keldysh time-loop
contour. We show an application of our formalism to the plasmon model of a core
electron within the plasmon-pole approximation. We study in detail the
diagrammatic perturbation expansion of the core-electron/plasmon coupling on
the spectral functions of the so-called S-model which provides an exact
solution, concentrating especially on the effects of self-consistency and
vertex corrections on the GW self-energy. For the S-model, self-consistency is
essential for GW-like calculations to obtain the full spectral information. The
second- order exchange diagram (i.e. a vertex correction) is crucial to obtain
a better spectral description of the plasmon peak and side-band peaks in
comparison to GW-like calculations. However, the vertex corrections are well
reproduced within a non-self-consistent calculation. We also consider
conventional equilibrium GW calculations for the pure jellium model. We find
that with no second-order vertex correction, we cannot obtain the full set of
plasmon side-band peaks. Finally, we address the issues of formal connection
for the Dyson equations of the time-ordered Green's function and the Keldysh
Green's functions at equilibrium in the cases of zero and finite temperature.Comment: Published in PRB November 22 201
Nova M31N 2007-12b: Supersoft X-rays reveal an intermediate polar?
For the He/N nova M31N 2007-12b, we analyzed XMM-Newton EPIC and Chandra
HRC-I observations of our monitoring program performed at intervals of ten days
and added results of a XMM-Newton target of opportunity observation and Swift
XRT observations. The supersoft source (SSS) emission started between 21 and 30
d after the optical outburst and ended between 60 and 120 d after outburst,
making M31N 2007-12b one of the few novae with the shortest SSS phase known.
The X-ray spectrum was supersoft and can be fitted with a white dwarf (WD)
atmosphere model with solar abundances absorbed by the Galactic foreground. The
temperature of the WD atmosphere seems to increase at the beginning of the SSS
phase from ~70 to ~80 eV. The luminosity of M31N 2007-12b during maximum was at
the Eddington limit of a massive WD and dropped by ~30% in the observation 60 d
after outburst. The radius of the emission region is ~6x10^8 cm. In the four
bright state observations, we detected a stable 1110 s pulsation, which we
interpret as the WD rotation period. In addition, we detect dips in three
observations that might represent a 4.9 h or 9.8 h binary period of the system.
Nova envelope models with <50% mixing between solar-like accreted material and
the degenerate core of the WD can be used to describe the data. We derive a WD
mass of 1.2 Msun, as well as an ejected and burned mass of 2.0x10^{-6} Msun}
and 0.2x10^{-6} Msun, respectively. The observed periodicities indicate that
nova M31N 2007-12b erupted in an intermediate polar (IP) system. The WD
photospheric radius seems to be larger than expected for a non-magnetic WD but
in the range for magnetic WDs in an IP system. (abridged)Comment: 10 pages, 5 figures, A&A accepte
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