10,257 research outputs found
X-ray variability of AGNs in the soft and the hard X-ray bands
We investigate the X-ray variability characteristics of hard X-ray selected
AGNs (based on Swift/BAT data) in the soft X-ray band using the RXTE/ASM data.
The uncertainties involved in the individual dwell measurements of ASM are
critically examined and a method is developed to combine a large number of
dwells with appropriate error propagation to derive long duration flux
measurements (greater than 10 days). We also provide a general prescription to
estimate the errors in variability derived from rms values from unequally
spaced data. Though the derived variability for individual sources are not of
very high significance, we find that, in general, the soft X-ray variability is
higher than those in hard X-rays and the variability strengths decrease with
energy for the diverse classes of AGN. We also examine the strength of
variability as a function of the break time scale in the power density spectrum
(derived from the estimated mass and bolometric luminosity of the sources) and
find that the data are consistent with the idea of higher variability at time
scales longer than the break time scale.Comment: 17 pages, 15 Postscript figures, 3 tables, accepted for publication
in Ap
Consumption, silicosis, and the social construction of industrial disease.
In the wake of the bacterial revolution after Robert Koch identified the tuberculosis bacillus, medical and public health professionals classified the various forms of consumption and phthisis as a single disease--tuberculosis. In large measure, historians have adopted that perspective. While there is undoubtedly a great deal of truth in this conceptualization, we argue that it obscures almost as much as it illuminates. By collapsing the nineteenth-century terms phthisis and consumption into tuberculosis, we maintain that historians have not understood the effect of non-bacterial consumption on working-class populations who suffered from the symptoms of coughing, wasting away, and losing weight. In this essay, we explore how, in the nineteenth century, what we now recognize as silicosis was referred to as miners' "con," stonecutters' phthisis, and other industry-specific forms of phthisis and consumption. We examine how the later and narrower view of the bacterial origins of tuberculosis limited the medical professions' ability to diagnose and understand diseases caused by industrial dust. This paper explores the contention that developed at the turn of the century over occupational lung disease and tuberculosis and the circumstances that led to the unmasking of silicosis as a disease category
Risk and Utility in Portfolio Optimization
Modern portfolio theory(MPT) addresses the problem of determining the optimum
allocation of investment resources among a set of candidate assets. In the
original mean-variance approach of Markowitz, volatility is taken as a proxy
for risk, conflating uncertainty with risk. There have been many subsequent
attempts to alleviate that weakness which, typically, combine utility and risk.
We present here a modification of MPT based on the inclusion of separate risk
and utility criteria. We define risk as the probability of failure to meet a
pre-established investment goal. We define utility as the expectation of a
utility function with positive and decreasing marginal value as a function of
yield. The emphasis throughout is on long investment horizons for which
risk-free assets do not exist. Analytic results are presented for a Gaussian
probability distribution. Risk-utility relations are explored via empirical
stock-price data, and an illustrative portfolio is optimized using the
empirical data.Comment: 10 pages, 1 figure, presented at 2002 Conference on Econophysics in
Bali Indonesi
Portfolio Optimization Using SPEA2 with Resampling
Proceeding of: Intelligent Data Engineering and
Automated Learning – IDEAL 2011: 12th International Conference, Norwich, UK, September 7-9, 2011The subject of financial portfolio optimization under real-world constraints is a difficult problem that can be tackled using multiobjective evolutionary algorithms. One of the most problematic issues is the dependence of the results on the estimates for a set of parameters, that is, the robustness of solutions. These estimates are often inaccurate and this may result on solutions that, in theory, offered an appropriate risk/return balance and, in practice, resulted being very poor. In this paper we suggest that using a resampling mechanism may filter out the most unstable. We test this idea on real data using SPEA2 as optimization algorithm and the results show that the use of resampling increases significantly the reliability of the resulting portfolios.The authors acknowledge financial support granted by the Spanish Ministry of Science under contract TIN2008-06491-C04-03 (MSTAR) and Comunidad de Madrid (CCG10- UC3M/TIC-5029).Publicad
Education in Kansas receives good marks
A Kansas poll shows different results than the national Gallup Poll on public education
Evidence for a Truncated Accretion Disc in the Low Luminosity Seyfert Galaxy, NGC 7213?
We present the broad-band 0.6-150 keV Suzaku and Swift BAT spectra of the low
luminosity Seyfert galaxy, NGC 7213. The time-averaged continuum emission is
well fitted by a single powerlaw of photon index Gamma = 1.75 and from
consideration of the Fermi flux limit we constrain the high energy cutoff to be
350 keV < E < 25 MeV. Line emission from both near-neutral iron K_alpha at 6.39
keV and highly ionised iron, from Fe_(xxv) and Fe_(xxvi), is strongly detected
in the Suzaku spectrum, further confirming the results of previous observations
with Chandra and XMM-Newton. We find the centroid energies for the Fe_(xxv) and
Fe_(xxvi) emission to be 6.60 keV and 6.95 keV respectively, with the latter
appearing to be resolved in the Suzaku spectrum. We show that the Fe_(xxv) and
Fe_(xxvi) emission can result from a highly photo-ionised plasma of column
density N_(H) ~ 3 x 10^(23) cm^(-2). A Compton reflection component, e.g.,
originating from an optically-thick accretion disc or a Compton-thick torus,
appears either very weak or absent in this AGN, subtending < 1 sr to the X-ray
source, consistent with previous findings. Indeed the absence of either neutral
or ionised Compton reflection coupled with the lack of any relativistic Fe K
signatures in the spectrum suggests that an inner, optically-thick accretion
disc is absent in this source. Instead, the accretion disc could be truncated
with the inner regions perhaps replaced by a Compton-thin Radiatively
Inefficient Accretion Flow. Thus, the Fe_(xxv) and Fe_(xxvi) emission could
both originate in ionised material perhaps at the transition region between the
hot, inner flow and the cold, truncated accretion disc on the order of 10^(3) -
10^(4) gravitational radii from the black hole. The origin for the unresolved
neutral Fe K_alpha emission is then likely to be further out, perhaps
originating in the optical BLR or a Compton-thin pc-scale torus.Comment: 15 pages, 11 figures, accepted for publication by MNRA
IT Workforce: Point-Counterpoint Dialogue between Academic Professionals and IT Leaders
The challenges of developing the IT workforce are greater than ever. The challenges include recruiting talented students into MIS and IT majors, retaining talented candidates in the IT/MIS field, and developing programs that prepare entry-level and experienced IT professionals in the right skill sets to meet the needs of employers. The dialogue we propose will enable both academic professionals and industry leaders to talk about the challenges they face and to exchange viewpoints about how to meet these challenges
Microtubule-targeting agents can sensitize cancer cells to ionizing radiation by an interphase-based mechanism
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