Spectra of cosmic X-ray sources

X-ray measurements provide the most direct probes of astrophysical environments with temperatures exceeding one million K. Progress in experimental research utilizing dispersive techniques (e.g., Bragg and grating spectroscopy) is considerably slower than that in areas utilizing photometric techniques, because of the relative inefficiency of the former for the weak X-ray signals from celestial sources. As a result, the term "spectroscopy" as applied to X-ray astronomy has traditionally satisfied a much less restrictive definition (in terms of resolving power) than it has in other wavebands. Until quite recently, resolving powers of order unity were perfectly respectable, and still provide (in most cases) the most useful spectroscopic data. In the broadest sense, X-ray photometric measurements are spectroscopic, insofar as they represent samples of the overall electromagnetic continua of celestial objects

Spectral variability in early-type binary X-ray systems

Theoretical models for the ionization of trace elements in a strong stellar wind by a compact binary X-ray source and for the resulting orbital phase dependence of the emergent soft X-ray spectra and the profiles of ultraviolet resonance lines are presented. Model results agree qualitatively with the X-ray and ultraviolet spectra of the system 4U 0900-40/HD 77581 and explain the suppression of the absorption profiles of the Si IV upsilon 1394 and C IV upsilon 1548 lines when the X-ray sources is in front of the star. The model predicts that the absorption profiles of the N V upsilon 1239 and O VI upsilon 1032 lines will be enhanced rather than suppressed during this orbital phase. We predict phase-dependent linear polarization in the resonance lines profiles. Future observations of these phase dependent effects in early-type binary X-ray systems may be used to investigate the dynamics of stellar winds and their interactions with the X-ray source

Supershells and Propagating Star Formation

Stellar winds and repeated supernovae from an OB association will create a cavity of coronal gas in the interstellar medium, with radius greater than 100 pc, surrounded by a dense, expanding shell of cool interstellar gas. If the association has a typical initial mass function, its supernovae explosions will inject energy into the supershell at a nearly constant rate for about 5 x 10^7 yr. The supershell loses its interior pressure and enters the snowplow phase when radiative cooling becomes important or when the shell bursts through the gas disk of a galaxy, typically after a few times 10^7 yr and with a radius -100-300 pc. At approximately the same time, the supershell becomes gravitationally unstable, forming giant molecular clouds which are sites for new star formation. There is widespread evidence for supershells in the Milky Way and other spiral and irregular galaxies from 21 cm emission-line surveys, optical emission-line surveys, and studies of supernova remnants. The gravitational instability of the supershells provides a physical mechanism for induced star formation and may account for bursts of star formation, especially in irregular galaxies

Statistical Time-Dependent Model for the Interstellar Gas

We present models for temperature and ionization structure of low, uniform-density (n ~ 0.3 cm^-3) interstellar gas in a galactic disk which is exposed to soft X-rays from supernova outbursts occurring randomly in space and time. The structure was calculated by computing the time record of temperature and ionization at a given point by Monte Carlo simulation. The calculation yields probability distribution functions for ionized fraction x, temperature T, and their various observable moments. These time-dependent models predict a bimodal temperature distribution of the gas with structure in x, T that agrees with various observations. Cold regions in the low-density gas may have the appearance of clouds in 21-cm absorption. The time-dependent model, in contrast to the steady-state model, predicts large fluctuations in ionization rate and the existence of cold (T ~ 30° K), ionized (x ~ 0.1) regions

An Analysis of How the Gender and Race of School Principals Influence Their Perception of Multicultural Education

The purpose of this study was to investigate secondary school principals’ perceptions of multicultural education in a rural southeastern state. The researchers wanted to ascertain whether or not the race or gender of school principals have a role in how those principals view multicultural education in theory (its theoretical value). For the purpose of this study, multicultural education in theory was defined as the belief that multicultural education is for all students, elevates students’ self-esteem, is embedded in cultural pluralism, and recognizes the social, political, and economic community and societal constructs on students of color (Fernandez, 1996). Three hundred and two secondary school principals were surveyed in a designated southeastern state. A significant difference was discovered with regard to the gender of the school principals and their perceptions of the theoretical value of multicultural education

Changes and Transformations in the Philosophy of Character Education in the 20th Century

Character education is a “hot-button” issue in many educational circles. It can be both controversial and divisive. Though there is generous support from politicians, educators, and parents, character education is still somewhat mired by its lack of scope. This examination proposes that current character education proponents can find new frameworks for implementation by observing the history of character education. Twentieth century character education efforts are examined with regard to sociohistorical influences and philosophical vicissitudes. Careful consideration is given to the interaction and contributions of the school and society in terms of promoting and developing character education

Predicting individual contrast sensitivity functions from acuity and letter contrast sensitivity measurements.

Contrast sensitivity (CS) is widely used as a measure of visual function in both basic research and clinical evaluation. There is conflicting evidence on the extent to which measuring the full contrast sensitivity function (CSF) offers more functionally relevant information than a single measurement from an optotype CS test, such as the Pelli-Robson chart. Here we examine the relationship between functional CSF parameters and other measures of visual function, and establish a framework for predicting individual CSFs with effectively a zero-parameter model that shifts a standard-shaped template CSF horizontally and vertically according to independent measurements of high contrast acuity and letter CS, respectively. This method was evaluated for three different CSF tests: a chart test (CSV-1000), a computerized sine-wave test (M&S Sine Test), and a recently developed adaptive test (quick CSF). Subjects were 43 individuals with healthy vision or impairment too mild to be considered low vision (acuity range of -0.3 to 0.34 logMAR). While each test demands a slightly different normative template, results show that individual subject CSFs can be predicted with roughly the same precision as test-retest repeatability, confirming that individuals predominantly differ in terms of peak CS and peak spatial frequency. In fact, these parameters were sufficiently related to empirical measurements of acuity and letter CS to permit accurate estimation of the entire CSF of any individual with a deterministic model (zero free parameters). These results demonstrate that in many cases, measuring the full CSF may provide little additional information beyond letter acuity and contrast sensitivity

Educational Salvation: Integrating Critical Spirituality in Educational Leadership

Improving education for students in K-12 urban settings remains a slow-paced and difficult task, with many successes in student learning being episodic at best. Disconnect between government mandates to improve schools and persistent societal issues of poverty and inequity act to increase stress on teachers and educational leaders working in urban schools. Drawing upon the strengths of the African-American community and its collective historical experiences, this study explores creative ways to integrate spirituality in the education of students in urban schools. The authors begin by addressing the contextual and structural issues facing urban schools. They then explain the benefits of integrating the four elements of critical spirituality—critical self-reflection; deconstructive interpretation; performative creativity; and, transformative action—in educational leadership to enhance their work in urban communities

High-Resolution X-ray Spectroscopy of SNR 1987A: Chandra LETG and HETG Observations in 2007

We present an extended analysis of the deep Chandra LETG and HETG observations of the supernova remnant 1987A (SNR 1987A) carried out in 2007. The global fits to the grating spectra show that the temperature of the X-ray emitting plasma in the slower shocks in this system has remained stable for the last three years, while that in the faster shocks has decreased. This temperature evolution is confirmed by the first light curves of strong X-ray emission lines and their ratios. On the other hand, bulk gas velocities inferred from the X-ray line profiles are too low to account for the post-shock plasma temperatures inferred from spectral fits. This suggests that the X-ray emission comes from gas that has been shocked twice, first by the blast wave and again by shocks reflected from the inner ring of SNR 1987A. A new model that takes these considerations into account gives support to this physical picture.Comment: 36 pages, 10 figures, Accepted for publication in Ap