Radiation mechanisms and geometry of Cygnus X-1 in the soft state

We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in 1996 May and June. The spectra consist of a dominant soft component below ~2 keV and a power-law-like continuum extending to at least ~800 keV. We interpret them as emission from an optically-thick, cold accretion disc and from an optically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of total available power is dissipated in the corona. We model the soft component by multi-colour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCA/RXTE data yield the most probable black hole mass of about 10 solar masses and an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure dominated, accretion-disc solution branch. The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50 keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. The compactness of the corona is between 2 and 7, and a presence of a significant population of electron-positron pairs is ruled out. We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuum is accompanied by a broad iron K-alpha line.Comment: 18 pages, 12 figures, 2 landscape tables in a separate file. Accepted to MNRA

SMOTE: Synthetic Minority Over-sampling Technique

An approach to the construction of classifiers from imbalanced datasets is described. A dataset is imbalanced if the classification categories are not approximately equally represented. Often real-world data sets are predominately composed of "normal" examples with only a small percentage of "abnormal" or "interesting" examples. It is also the case that the cost of misclassifying an abnormal (interesting) example as a normal example is often much higher than the cost of the reverse error. Under-sampling of the majority (normal) class has been proposed as a good means of increasing the sensitivity of a classifier to the minority class. This paper shows that a combination of our method of over-sampling the minority (abnormal) class and under-sampling the majority (normal) class can achieve better classifier performance (in ROC space) than only under-sampling the majority class. This paper also shows that a combination of our method of over-sampling the minority class and under-sampling the majority class can achieve better classifier performance (in ROC space) than varying the loss ratios in Ripper or class priors in Naive Bayes. Our method of over-sampling the minority class involves creating synthetic minority class examples. Experiments are performed using C4.5, Ripper and a Naive Bayes classifier. The method is evaluated using the area under the Receiver Operating Characteristic curve (AUC) and the ROC convex hull strategy

Automatic radioxenon analyzer for CTBT monitoring

Over the past 3 years, with support from US DOE`s NN-20 Comprehensive Test Ban Treaty (CTBT) R&D program, PNNL has developed and demonstrated a fully automatic analyzer for collecting and measuring the four Xe radionuclides, {sup 131m}Xe(11.9 d), {sup 133m}Xe(2.19 d), {sup 133}Xe (5.24 d), and {sup 135}Xe(9.10 h), in the atmosphere. These radionuclides are important signatures in monitoring for compliance to a CTBT. Activity ratios permit discriminating radioxenon from nuclear detonation and that from nuclear reactor operations, nuclear fuel reprocessing, or medical isotope production and usage. In the analyzer, Xe is continuously and automatically separated from the atmosphere at flow rates of about 7 m{sup 3}/h on sorption bed. Aliquots collected for 6-12 h are automatically analyzed by electron-photon coincidence spectrometry to produce sensitivities in the range of 20-100 {mu}Bq/m{sup 3} of air, about 100-fold better than with reported laboratory-based procedures for short time collection intervals. Spectral data are automatically analyzed and the calculated radioxenon concentrations and raw gamma- ray spectra automatically transmitted to data centers

Creative Assessment in Programming: Diversity and Divergence

Negative stereotypes persist in computing, and align poorly with research into the motivations of female students. In particular, female students are more inclined to want to work creatively and have a positive impact through their work. However programming assignments are often tightly constrained and rather pointless in themselves so are doubly unattractive. Alongside this, concerns are often raised about plagiarism in programming assignments, particularly when the assessment process is automated. We attempt to address both of these issues by designing more creative programming assignments, allowing students to engage in work aligned with whatever their interests are. By providing a more divergent assessment, automated plagiarism detectors are much more effective because the likelihood of false positives is much lower than in more constrained, convergent assessments. We also show how to combine this with partial automation of assessment. To examine this approach we compare the results of two subsequent years of delivery of the same second-year undergraduate programming module, and find that, using more creative assessments, female students average scores were substantially increased so that they outperform male students. While the results are not quite statistically significant (according to 2-way ANOVA), they demonstrate potential that could be verified with a larger sample

X-ray Signatures of an Ionized Reprocessor in the Seyfert galaxy Ton S 180

We discuss the hard X-ray properties of the Seyfert galaxy Ton S 180, based upon the analysis of ASCA data. We find the X-ray flux varied by a factor ~2 on a time scale of a few thousand seconds. The source showed significantly higher amplitude of variability in the 0.5-2 keV band than in the 2-10 keV band. The continuum is adequately parameterized as a Gamma ~ 2.5 power-law across the 0.6--10 keV band . We confirm the recent discovery of an emission line of high equivalent width, due to Fe K-shell emission from highly-ionized material. These ASCA data show the Fe line profile to be broad and asymmetric and tentatively suggest it is stronger during the X-ray flares, consistent with an origin from the inner parts of an accretion disk. The X-ray spectrum is complex below 2 keV, possibly due to emission from a blend of soft X-ray lines, which would support the existence of an ionized reprocessor, most likely due to a relatively high accretion rate in this source.Comment: 24 pages, 8 figures. LaTeX with encapsulated postscript. To appear in the Astrophysical Journa

Pionium Production in the Cooler

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

The Density of Coronal Plasma in Active Stellar Coronae

We have analyzed high-resolution X-ray spectra of a sample of 22 active stars observed with the High Energy Transmission Grating Spectrometer on {\em Chandra} in order to investigate their coronal plasma density. Densities where investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII. While Si XIII lines in all stars of the sample are compatible with the low-density limit, Mg XI lines betray the presence of high plasma densities ($> 10^{12}$ cm$^{-3}$) for most of the sources with higher X-ray luminosity ($> 10^{30}$ erg/s); stars with higher $L_X$ and $L_X/L_{bol}$ tend to have higher densities at high temperatures. Ratios of O VII lines yield much lower densities of a few $10^{10}$ cm$^{-3}$, indicating that the ``hot'' and ``cool'' plasma resides in physically different structures. Our findings imply remarkably compact coronal structures, especially for the hotter plasma emitting the Mg XI lines characterized by coronal surface filling factor, $f_{MgXI}$, ranging from $10^{-4}$ to $10^{-1}$, while we find $f_{OVII}$ values from a few $10^{-3}$ up to $\sim 1$ for the cooler plasma emitting the O VII lines. We find that $f_{OVII}$ approaches unity at the same stellar surface X-ray flux level as solar active regions, suggesting that these stars become completely covered by active regions. At the same surface flux level, $f_{MgXI}$ is seen to increase more sharply with increasing surface flux. These results appear to support earlier suggestions that hot $10^7$ K plasma in active coronae arises from flaring activity, and that this flaring activity increases markedly once the stellar surface becomes covered with active regions.Comment: 53 pages, 19 figures, accepted for publication in Astrophysical Journal. A version of the paper with higher quality figures is available from http://www.astropa.unipa.it/Library/preprint.htm

Pionium Production in the Cooler

This research was sponsored by the National Science Foundation Grant NSF PHY-931478