Supernova Remnant 1987A: Opening the Future by Reaching the Past

We report an up-turn in the soft X-ray light curve of supernova remnant (SNR) 1987A in late 2003 (~6200 days after the explosion), as observed with the Chandra X-ray Observatory. Since early 2004, the rapid increase of the 0.5-2 keV band X-ray light curve can no longer be described by the exponential density distribution model with which we successfully fitted the data between 1990 and 2003. Around day ~6200, we also find that the fractional contribution to the observed soft X-ray flux from the decelerated shock begins to exceed that of the fast shock and that the X-ray brightening becomes "global" rather than "spotty". We interpret these results as evidence that the blast wave has reached the main body of the dense circumstellar material all around the inner ring. This interpretation is supported by other recent observations, including a deceleration of the radial expansion of the X-ray remnant, a significant up-turn in the mid-IR intensities, and the prevalence of the optical hot spots around the entire inner ring, all of which occur at around day 6000. In contrast to the soft X-ray light curve, the hard band (3-10 keV) X-ray light curve increases at a much lower rate which is rather similar to the radio light curve. The hard X-ray emission may thus originate from the reverse shock where the radio emission is likely produced. Alternatively, the low increase rate of the hard X-rays may simply be a result of the continuous softening of the overall X-ray spectrum.Comment: AASTex preprint style 12 pages including 1 table and 4 figures, Accepted by ApJ

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

An exploratory analysis of revision behavior development of L2 writers on an intensive English for academic purposes program using Bayesian methods

Revision is a fundamental part of the writing process and is particularly important in the production of high-quality academic writing. This study is an exploratory examination of changes in revision behavior, as measured by keystroke logging software, at the beginning (T1) and end (T2) of a one-month intensive English for Academic Purposes (EAP) course on n = 39 undergraduate and postgraduate students. Bayes Factors (BFs) are utilized as measures of strength of evidence for changes in behavior. In this paper, we examine the application of a Bayesian Hypothesis Testing (BHT) approach and its implications specifically for exploratory studies, i.e., studies with relatively small samples intended to search data for emergent patterns. The results show that, in most cases, we have moderate evidence against any change in behavior over time. Based on this evidence, we conclude that the experimental parameters of further exploratory work into the development of revisions should be modified to maximize the chance of finding patterns in the data from which to generate any confirmatory hypotheses

The Place of the Bifactor Model in Confirmatory Factor Analysis Investigations Into Construct Dimensionality in Language Testing.

For practical and theoretical purposes, tests of second language (L2) ability commonly aim to measure one overarching trait, general language ability, while simultaneously measuring multiple sub-traits (e.g., reading, grammar, etc.). This tension between measuring uni- and multi-dimensional constructs concurrently can generate vociferous debate about the precise nature of the construct(s) being measured. In L2 testing, this tension is often addressed through the use of a higher-order factor model wherein multidimensional traits representing subskills load on a general ability latent trait. However, an alternative modeling framework that is currently uncommon in language testing, but gaining traction in other disciplines, is the bifactor model. The bifactor model hypothesizes a general factor, onto which all items load, and a series of orthogonal (uncorrelated) skill-specific grouping factors. The model is particularly valuable for evaluating the empirical plausibility of subscales and the practical impact of dimensionality assumptions on test scores. This paper compares a range of CFA model structures with the bifactor model in terms of theoretical implications and practical considerations, framed for the language testing audience. The models are illustrated using primary data from the British Council's Aptis English test. The paper is intended to spearhead the uptake of the bifactor model within the cadre of measurement models used in L2 language testing

Bridge Builders to Health Equity: The High School Community Health Worker Training Program

Background: During the summers of 2016- 2019, Morehouse School of Medicine (MSM) with the support of community partners, developed, piloted and implemented an innovative Community Health Worker Training Program for High School students (HSCHW). Training high school community health workers can impact health and social disparities in underserved populations and demonstrates pipeline of future workers in community health and other health careers. The program has achieved promising outcomes, and access to the program model has been requested many universities and community-based organizations. Method: The HSCHW training consists of summer classroom instruction, field instruction and monthly continuing education during the school year. The students are trained in CHW core competencies through a combination of online curriculum, interactive classroom experiences, and field activities provided by community partners, multidisciplinary MSM clinical instructors, and graduate student interns. Results: 77 high school students from metropolitan Atlanta High Schools and rural Columbus, GA have been trained as CHWs in the HSCHW program. Students are ages 15-18 and are rising sophomores to recent high school graduates. All participants (100%) were from economically disadvantaged communities. Sixteen school-based and community-based health improvement projects have been developed and implemented by teams of trained HSCHWs. Additionally, over 300 individuals (family and community members) received monthly health monitoring by trained HSCHWs. Conclusions: Overall, the HSCHW program models the engagement of community partners in translational educational initatives to support community transformation. It can be a giant leap toward improved population health and achieving health equity in underserved communities. It has had a tremendous impact on the youth themselves, their families, and the communities in which they live. In communities with disparate prevalence of chronic disease and unhealthy behaviors, early exposure may mitigate exacerbation or early onset of disease among the participants and their families. High School Community Health Workers are bridges to health equity

The cyclo-synchrotron process and particle heating through the absorption of photons

We propose a new approximation for the cyclo-synchrotron emissivity of a single electron. In the second part of this work, we discuss a simple application for our approximation, and investigate the heating of electrons through the self-absorption process. Finally, we investigate the self-absorbed part of the spectrum produced by a power-law population of electrons. In comparison to earlier approximations, our formula provides a few significant advantages. Integration of the emissivity over the whole frequency range, starting from the proper minimal emitting frequency, gives the correct cooling rate for any energy particle. Further, the spectrum of the emission is well approximated over the whole frequency range, even for relatively low particle energies (beta << 0.1), where most of the power is emitted in the first harmonic. In order to test our continuous approximation, we compare it with a recently derived approximation of the first ten harmonics. Finally, our formula connects relatively smooth to the synchrotron emission at beta=0.9. We show that the self-absorption is a very efficient heating mechanism for low energy particles, independent of the shape of the particle distribution responsible for the self-absorbed synchrotron emission. We find that the energy gains for low energy particles are always higher than energy losses by cyclo-synchrotron emission. We show also that the spectral index of the self-absorbed part of the spectrum at very low frequencies differs significantly from the well known standard relation I(nu) ~ nu^(5/2).Comment: 9 pages, 4 figures, accepted for publication in A&

A Chandra View of The Morphological And Spectral Evolution of Supernova Remnant 1987A

We present an update on the results of our monitoring observations of the X-ray remnant of supernova (SN) 1987A with the {\it Chandra X-Ray Observatory}. As of 2002 December, we have performed a total of seven observations of SN 1987A. The high angular resolution images from the latest data reveal developments of new X-ray bright spots in the northwestern and the southwestern portions of the remnant as well as changes on the eastern side. The latest 0.5-2 keV band flux ($f_X$ $\sim$ 6 $\times$ 10$^{-13}$ ergs cm$^{-2}$ s$^{-1}$) is four times brighter than three years earlier. The overall X-ray emission is primarily from the blast wave shock with $kT$ $\sim$ 2.4 keV. As the blast wave approaches the dense circumstellar material, the contribution from the decelerated slow shock ($kT$ $\sim$ 0.22 keV) to the observed X-ray emission is becoming significant. The increase of this slow shock contribution over the last two years is particularly noticeable in the western half of the remnant. These results indicate that the shock front is now reaching the main body of the inner circumstellar ring. Based on the best-fit two-shock spectral model, we derive approximate densities of the X-ray-emitting regions ($n_e$ $\sim$ 235 cm$^{-3}$ for the fast shock and $n_e$ $\sim$ 7500 cm$^{-3}$ for the slow shock). We obtain an upper limit on the observed X-ray luminosity of any embedded point source ($L_X$ $\le$ 1.5 $\times$ 10$^{34}$ ergs s$^{-1}$) in the 2$-$10 keV band. The X-ray remnant continues to expand linearly at a rate of 4167 km s$^{-1}$.Comment: 22 pages (ApJ preprint style), 7 Figures, Accepted by ApJ (scheduled on July 20, 2004), for high-quality Fig 1 and Fig 2, please contact [email protected]

Observing Supernova 1987A with the Refurbished Hubble Space Telescope

Observations with the Hubble Space Telescope (HST), conducted since 1990, now offer an unprecedented glimpse into fast astrophysical shocks in the young remnant of supernova 1987A. Comparing observations taken in 2010 using the refurbished instruments on HST with data taken in 2004, just before the Space Telescope Imaging Spectrograph failed, we find that the Ly-a and H-a lines from shock emission continue to brighten, while their maximum velocities continue to decrease. We observe broad blueshifted Ly-a, which we attribute to resonant scattering of photons emitted from hotspots on the equatorial ring. We also detect NV~\lambda\lambda 1239,1243 A line emission, but only to the red of Ly-A. The profiles of the NV lines differ markedly from that of H-a, suggesting that the N^{4+} ions are scattered and accelerated by turbulent electromagnetic fields that isotropize the ions in the collisionless shock.Comment: Science, accepted. Science Express, 02 Sept 2010. 5 figures. Supporting online material can be found at http://www.sciencemag.org/cgi/content/full/sci;science.1192134/DC