203 research outputs found
Finding structure in diversity: A stepwise small-N/medium-N qualitative comparative analysis approach for water resources management research
Drawing particularly on recent debates on, and development of, comparative methods in the field of comparative politics, the paper argues that stepwise small-N/medium-N qualitative comparative analysis (QCA) is a particularly suitable methodological approach for water resources studies because it can make use of the rich but fragmented water resources studies literature for accumulation of knowledge and development of theory. It is suggested that taking an explicit critical realist ontological and epistemological stance allows expansion of the scope of stepwise small-N/medium-N QCA beyond what is claimed for it in Raginâs 'configurational comparative methods (CCM)' perspective for analysing the complexity of causality as 'multiple conjunctural causation'. In addition to explanation of certain sets of 'outcomes' as in CCMâs combinatorial, set-theoretic approach, embedding stepwise small-N/medium-N QCA in a critical realist ontology allows the method to contribute to development of theory on (qualitative differences between) the structures in society that shape water resources use, management and governance
The Narrow-band Ultraviolet Imaging Experiment for Wide-field Surveys (NUVIEWS)-I: Dust scattered continuum
We report on the first results of the Narrow-band Ultraviolet Imaging
Experiment for Wide-field Surveys (NUVIEWS), a sounding rocket experiment
designed to map the far-ultraviolet background in four narrow bands. This is
the first imaging measurement of the UV background to cover a substantial
fraction of the sky. The narrow band responses (145, 155, 161, and 174 nm, 7-10
nm wide) allow us to isolate background contributions from dust-scattered
continuum, H2 fluorescence, and CIV 155 nm emission. In our first flight, we
mapped one quarter of the sky with 5-10 arcminute imaging resolution. In this
paper, we model the dominant contribution of the background, dust-scattered
continuum. Our data base consists of a map of over 10,000 sq. degrees with 468
independent measurements in 6.25 by 6.25 sq. degree bins. Stars and
instrumental stellar halos are removed from the data. We present a map of the
continuum background obtained in the 174 nm telescope. We use a model that
follows Witt, Friedman, and Sasseen (1997: WFS) to account for the
inhomogeneous radiation field and multiple scattering effects in clouds. We
find that the dust in the diffuse interstellar medium displays a moderate
albedo (a=0.55+/-0.1) and highly forward scattering phase function parameter
(g=0.75+/-0.1) over a large fraction of the sky, similar to dust in star
forming regions. We also have discovered a significant variance from the model.Comment: 16 pages, 3 ps figures, submitted to Astrophysical Journal Letter
TCF periodogram's high sensitivity: A method for optimizing detection of small transiting planets
We conduct a methodological study for statistically comparing the
sensitivities of two periodograms for weak signal planet detection in transit
surveys: the widely used Box-Least Squares (BLS) algorithm following light
curve detrending and the Transit Comb Filter (TCF) algorithm following
autoregressive ARIMA modeling. Small depth transits are injected into light
curves with different simulated noise characteristics. Two measures of spectral
peak significance are examined: the periodogram signal-to-noise ratio (SNR) and
a False Alarm Probability (FAP) based on the generalized extreme value
distribution. The relative performance of the BLS and TCF algorithms for small
planet detection is examined for a range of light curve characteristics,
including orbital period, transit duration, depth, number of transits, and type
of noise. The TCF periodogram applied to ARIMA fit residuals with the SNR
detection metric is preferred when short-memory autocorrelation is present in
the detrended light curve and even when the light curve noise had white
Gaussian noise. BLS is more sensitive to small planets only under limited
circumstances with the FAP metric. BLS periodogram characteristics are inferior
when autocorrelated noise is present. Application of these methods to TESS
light curves with small exoplanets confirms our simulation results. The study
ends with a decision tree that advises transit survey scientists on procedures
to detect small planets most efficiently. The use of ARIMA detrending and TCF
periodograms can significantly improve the sensitivity of any transit survey
with regularly spaced cadence.Comment: 30 pages, 13 figures, submitted to AAS Journal
Neutral description and exchange of design computational workflows
Proposed in this paper is a neutral representation of design computational workflows which allows
their exchange and sharing between different project partners and across design stages. This is
achieved by the de-coupling of configuration and execution logic. Thus, the same underlying
workflow can be executed with different (fidelity) models and different software tools as long as the
inputs and outputs of the constituent process are kept the same. To this purpose, an object model is
proposed to define different simulation objects, their scope, and hierarchy in the simulation process.
An XML based computer readable representation of workflows based on the proposed object model, is
also suggested. The application of the proposed representation is demonstrated via a case study
involving the exchange of workflows between two design partners. The case study also demonstrates
how the same workflow can be executed using different execution tools and involving different
fidelity models
Fine-structure diagnostics of neutral carbon toward HE 0515-4414
New high-resolution high signal-to-noise spectra of the damped Lyman
(DLA) system toward the quasi-stellar object HE 0515-4414 reveal
absorption lines of the multiplets 2 and 3 in \ion{C}{i}. The resonance lines
are seen in two components with total column densities of
and , respectively. The comparision of theoretical
calculations of the relative fine-structure population with the ratios of the
observed column densities suggests that the \ion{C}{i} absorbing medium is
either very dense or exposed to very intense UV radiation. The upper limit on
the local UV energy density is 100 times the galactic UV energy density, while
the upper limit on the \ion{H}{i} number density is 110 cm. The
excitation temperatures of the ground state fine-structure levels of
and K, respectively, are consistent with the temperature-redshift
relation predicted by the standard Friedmann cosmology. The cosmic microwave
background radiation (CMBR) is only a minor source of the observed
fine-structure excitation.Comment: 5 pages, 5 figures, uses A&A macro package, gzipped tar archive,
accepted by A&
Do the Broad Emission Line Clouds See the Same Continuum that We See?
Recent observations of quasars, Mrk 335 and the HST quasar composite
spectrum, have indicated that many of them have remarkably soft ionizing
continua (fnu ~ nu^-2.0, 13.6 eV -- 100 eV). We point out that the number of E
> 54.4 eV photons is insufficient to create the observed strengths of the He II
emission lines. While the numbers of photons which energize C IV 1549 and O VI
1034 are sufficient, even the most efficiently emitting clouds for these two
lines must each cover at least 20% -- 40% of the source. If the typical quasar
ionizing continuum is indeed this soft, then we must conclude that the broad
emission line clouds must see a very different (harder) continuum than we see.
The other viable possibility is that the UV -- EUV SED is double-peaked, with
the second peaking near 54 eV, its Wien tail the observed soft X-ray excess.Comment: 11 pages AAS-LATeX aaspp4.sty format, including 1 figure; accepted
for publication in Ap
\u3cem\u3eIUE\u3c/em\u3e Observations of DQ Herculis and its Nebula, and the Nature of the Cold Nova Shells
The nebula ejected in the 1934 outburst of the classical nova DQ Her is remarkable for its unprecedentedly low temperature of Te 500 K as measured by Williams et al. (1978). In this paper, IUE observations are combined with Steward optical spectra. It is confirmed that the gas is quite cold. It is further shown that the gas is ionized by the radiation field of the central object. X-ray, ultraviolet, optical and infrared observations of the underlying binary are combined with the extreme-ultraviolet continuum deduced from the level of ionization of the nebula to obtain a composite energy distribution for the central object. This energy distribution bears no resemblance to that predicted by theoretical models of accretion disks. Photoionization models of the nebula using the deduced continuum, as well as theoretical accretion disk continua, are presented to show that the low electron temperature is the result of the very high metal abundances which characterize nova shells. Infrared fine-structure lines are efficient coolants, and low temperatures are achieved for a wide variety of radiation fields. The implications of these results for nebulae surrounding other old novae are discussed
Broadening of the Iron emission line in MCG-6-30-15 by Comptonization
We show that the Iron K emission line from MCG-6-30-15 could be broadened due
to Comptonization by a surrounding highly ionized cloud with radius cms. We calculate the temperature of the cloud to be \sim0.21\kev,
provided a reasonable estimate of the UV flux is made. The X-ray/-ray
emission observed from the source is compatible with this model. Such a cloud
should be highly ionized and strong absorption edges are not expected from the
source (Fabian et al 1995).
For a \onlyten{6}\msol black hole the size of the could corresponds to
about 300 Schwarzschild radius. The intrinsic line could then be emitted far
from the black hole and gravitational red-shift and Doppler effects would be
negligible. If the black hole mass is much larger than \onlyten{6}\msol,
gravitational/Doppler red-shifts would also contribute significantly to the
broadening.
We argue that the broad red wing observed in the source does not by itself
imply emission from regions close () to the black hole. However,
Comptonization cannot produce a double peak. The presence of such a feature is
a clear sign of inner disk emission influenced by gravitational and Doppler
effects, perhaps broadened by the Comptonization. We note that simultaneous
broad band (2-100 keV) study of this source can also reveal (or rule out) the
presence of such a Comptonizing cloud.Comment: 2 figures. uses aasms4.sty, accepted by ApJ, email:
[email protected]
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