786 research outputs found
INTEGRAL High Energy Observations of 2S 0114+65
We report the first INTEGRAL timing and spectral analysis of the high mass
X-ray binary source 2S 0114+65 at high energies (5-100 keV). The pulse period
was found at 2.668 hr with a high pulsed fraction, ~80% in both the 20-40 keV
and 40-80 keV energy bands. The spin-up trend over ~8 years was measured to be
-8.9 * 10^{-7}. The hard X-ray spectrum obtained with JEM-X/ISGRI is well
described by a high energy exponential cut-off power law model where the
estimated luminosity is 1.8 * 10^{36} erg/s in the 5-100 keV energy band, for a
source distance of 7.2 kpc. We tentatively identify a cyclotron resonance
scattering feature at ~22 keV with one harmonic, implying a magnetic field of
2.5 * 10^{12} G.Comment: 4 pages, 5 figures, accepted to A&A Letter
Electric organ discharge diversity in the genus Gymnotus: anatomo-functional groups and electrogenic mechanisms
Previous studies describe six factors accounting for interspecific diversity of electric organ discharge (EOD) waveforms in Gymnotus. At the cellular level, three factors determine the locally generated waveforms: (1) electrocyte geometry and channel repertoire; (2) the localization of synaptic contacts on electrocyte surfaces; and (3) electric activity of electromotor axons preceding the discharge of electrocytes. At the organismic level, three factors determine the integration of the EOD as a behavioral unit: (4) the distribution of different types of electrocytes and specialized passive tissue forming the electric organ (EO); (5) the neural mechanisms of electrocyte discharge coordination; and (6) post-effector mechanisms. Here, we reconfirm the importance of the first five of these factors based on comparative studies of a wider diversity of Gymnotus than previously investigated. Additionally, we report a hitherto unseen aspect of EOD diversity in Gymnotus. The central region of the EO (which has the largest weight on the conspecific-received field) usually exhibits a negative-positive-negative pattern where the delay between the early negative and positive peaks (determined by neural coordination mechanisms) matches the delay between the positive and late negative peaks (determined by electrocyte responsiveness). Because delays between peaks typically determine the peak power frequency, this matching implies a co-evolution of neural and myogenic coordination mechanisms in determining the spectral specificity of the intraspecific communication channel. Finally, we define four functional species groups based on EO/EOD structure. The first three exhibit a heterogeneous EO in which doubly innervated electrocytes are responsible for a main triphasic complex. Group I species exhibit a characteristic cephalic extension of the EO. Group II species exhibit an early positive component of putative neural origin, and strong EO auto-excitability. Group III species exhibit an early, slow, negative wave of abdominal origin, and variation in EO auto-excitability. Representatives of Group IV generate a unique waveform comprising a main positive peak followed by a small, load-dependent negative component
Microlensing in the double quasar SBS1520+530
We present the results of a monitoring campaign of the double quasar
SBS1520+530 at Maidanak observatory from April 2003 to August 2004. We obtained
light curves in V and R filters that show small-amplitude \Delta m~0.1 mag
intrinsic variations of the quasar on time scales of about 100 days. The data
set is consistent with the previously determined time delay of \Delta
t=(130+-3) days by Burud et al. (2002). We find that the time delay corrected
magnitude difference between the quasar images is now larger by (0.14+-0.03)
mag than during the observations by Burud et al. (2002). This confirms the
presence of gravitational microlensing variations in this system.Comment: 6 pages, 7 figures. Accepted for publication in A&
The Impact of Exploring Computer Science in Wisconsin: Where Disadvantage is an Advantage
Assessing the impact of regional or statewide interventions in primary and secondary school (K-12) computer science (CS) education is difficult for a variety of reasons. Qualitative survey data provide only a limited view of impacts, but quantitative data can be notoriously difficult to acquire at scale from large numbers of classrooms, schools, or local educational authorities. In this paper, we use several publicly available data sources to glean insights into public high school CS enrollments across an entire U.S. state. Course enrollments with NCES course codes and local descriptors, school-level demographic data, and school geographic attendance boundaries can be combined to highlight where CS offerings persist and thrive, how CS enrollments change over time, and the ultimate quantitative impact of a statewide intervention. We propose a more appropriate level of data aggregation for these types of quantitative studies than has been undertaken in previous work while demonstrating the importance of a contextual aggregation process. The results of our disparate impact analysis for the first time quantify the impact of a statewide Exploring Computer Science (ECS) program rollout on economic groups across the region. Our blueprint for this analysis can serve as a template to guide and assess large-scale K-12 CS interventions wherever detailed project evaluation methods cannot scale to encompass the entire study area, especially in cases where attribute heterogeneity is a significant issue
Six supersoft X-ray binaries: system parameters and twin-jet outflows
A comparison is made between the properties of CAL 83, CAL 87, RX
J0513.9-6951, 1E 0035.4-7230 (SMC 13), RX J0019.8+2156, and RX J0925.7-4758,
all supersoft X-ray binaries. Spectra with the same resolution and wavelength
coverage of these systems are compared and contrasted. Some new photometry is
also presented. The equivalent widths of the principal emission lines of H and
He II differ by more than an order of magnitude among these sources, although
those of the highest ionization lines (e.g. O VI) are very similar. In
individual systems, the velocity curves derived from various ions often differ
in phasing and amplitude, but those whose phasing is consistent with the light
curves (implying the lines are formed near the compact star) give masses of
and for the degenerate and mass-losing
stars, respectively. This finding is in conflict with currently prevailing
theoretical models for supersoft binaries. The three highest luminosity sources
show evidence of "jet" outflows, with velocities of .
In CAL 83 the shape of the He II 4686\AA profile continues to show evidence
that these jets may precess with a period of days.Comment: 27 pages including 5 tables, plus 6 figures. To appear in Ap
The eclipsing supersoft X-ray binary CAL 87
We present and discuss 25 spectra obtained in November 1996, covering all
phases of the CAL 87 binary system. These spectra are superior both in
signal-to-noise and wavelength coverage to previously published data so that
additional spectral features can be measured. Photometry obtained on the same
nights is used to confirm the ephemeris and to compare with light curves from
previous years. Analysis of the color variation through the orbital cycle has
been carried out using archival MACHO data. When a barely resolved red field
star is accounted for, there is no (V-R)-color variation, even through eclipse.
There have been substantial changes in the depth of minimum light since 1988;
it has decreased more than 0.5 mag in the last several years. The spectral
features and radial velocities are also found to vary not only through the
0.44-day orbit but also over timescales of a year or more. Possible
interpretations of these long-term changes are discussed. The 1996 spectra
contain phase-modulated Balmer absorption lines not previously seen, apparently
arising in gas flowing from the region of the compact star. The changes in
emission-line strengths with orbital phase indicate there are azimuthal
variations in the accretion disk structures. Radial velocities of several lines
give different amplitudes and phasing, making determination of the stellar
masses difficult. All solutions for the stellar masses indicate that the
companion star is considerably less massive than the degenerate star. The
Balmer absorption-line velocities correspond to masses of ~1.4Msun for the
degenerate star and ~0.4Msun for the mass donor. However, the strong He II
emission lines indicate a much more massive accreting star, with Mx>4Msun.Comment: 18 pages including tables, plus10 figures. To appear in Ap
Shear and Ellipticity in Gravitational Lenses
Galaxies modeled as singular isothermal ellipsoids with an axis ratio
distribution similar to the observed axis ratio distribution of E and S0
galaxies are statistically consistent with both the observed numbers of
two-image and four-image lenses and the inferred ellipticities of individual
lenses. However, no four-image lens is well fit by the model (typical
), the axis ratio of the model can be significantly
different from that of the observed lens galaxy, and the major axes of the
model and the galaxy may be slightly misaligned. We found that models with a
second, independent, external shear axis could fit the data well (typical
), while adding the same number of extra parameters to
the radial mass distribution does not produce such a dramatic improvement in
the fit. An independent shear axis can be produced by misalignments between the
luminous galaxy and its dark matter halo, or by external shear perturbations
due to galaxies and clusters correlated with the primary lens or along the line
of sight. We estimate that the external shear perturbations have no significant
effect on the expected numbers of two-image and four-image lenses, but that
they can be important perturbations in individual lens models. However, the
amplitudes of the external shears required to produce the good fits are larger
than our estimates for typical external shear perturbations (10-15% shear
instead of 1-3% shear) suggesting that the origin of the extra angular
structure must be intrinsic to the primary lens galaxy in most cases.Comment: 38 pages, 9 figures, submitted to Ap
A 2.3-Day Periodic Variability in the Apparently Single Wolf-Rayet Star WR 134: Collapsed Companion or Rotational Modulation?
We present the results of an intensive campaign of spectroscopic and
photometric monitoring of the peculiar Wolf-Rayet star WR 134 from 1989 to
1997.
This unprecedentedly large data set allows us to confirm unambiguously the
existence of a coherent 2.25 +/- 0.05 day periodicity in the line-profile
changes of He II 4686, although the global pattern of variability is different
from one epoch to another. This period is only marginally detected in the
photometric data set. Assuming the 2.25 day periodic variability to be induced
by orbital motion of a collapsed companion, we develop a simple model aiming at
investigating (i) the effect of this strongly ionizing, accreting companion on
the Wolf-Rayet wind structure, and (ii) the expected emergent X-ray luminosity.
We argue that the predicted and observed X-ray fluxes can only be matched if
the accretion on the collapsed star is significantly inhibited. Additionally,
we performed simulations of line-profile variations caused by the orbital
revolution of a localized, strongly ionized wind cavity surrounding the X-ray
source. A reasonable fit is achieved between the observed and modeled
phase-dependent line profiles of He II 4686. However, the derived size of the
photoionized zone substantially exceeds our expectations, given the observed
low-level X-ray flux. Alternatively, we explore rotational modulation of a
persistent, largely anisotropic outflow as the origin of the observed cyclical
variability. Although qualitative, this hypothesis leads to greater consistency
with the observations.Comment: 34 pages, 16 figures. Accepted by the Astrophysical Journa
Kraft lignin: a novel alternative to oil spill cleanup recycling industrial waste
Oil spills occur frequently due to negligence or through acts of vandalism or war. Marine oil spills damage marine, coastal, and ter¬restrial habitats, as well as impact coastal economies. In order to palliate such issues, researchers are working to develop cost ef¬fective and environmentally safe methods of adsorbing oil from oil spills. In this paper, a complex chemical compound found in the secondary cell walls of plants and ex¬tracted as a waste product of the pulping process known as kraft lignin was tested as an adsorbing material for oil spills. Lignin is light, and its high surface energy and wet¬tability allow it to interact with charged oil droplets. It was found that lignin can absorb 80% of the carbon introduced into water by oil spills within 12 hours of treatment. This study demonstrates a novel method for resolving oil spills using kraft lignin. After it is saturated with oil, lignin can be inciner¬ated, and its high calorific value can be used to produce energy from what would other¬wise be a waste product.Les fuites de pétrole sont produites fréquemment à cause de la négligence, des actes de vandalisme ou de la guerre. Les fuites de pétrole en mer en¬dommagent les écosystèmes maritimes, côtières et terrestres, et affectent les économies côtières. Pour atténuer de tels problèmes, les chercheurs tentent de développer des méthodes rentables et écologiques pour absorber le pétrole des fuites. Dans ce rapport, un composé chimique compli¬qué trouvé dans la paroi cellulaire secondaire des plantes et extrait comme produit résiduaire du processus de réduction en pâte, connu sous le nom de lignine ‘kraft’, a été testé comme matériel absorbant pour les fuites de pétrole. Les pro¬priétés de la lignine, y compris sa légèreté, son énergie de surface élevée et sa mouillabilité, la permettent d’interagir avec les gouttes de pétrole chargées. La lignine est capable d’absorber 80% du charbon introduit dans l’eau par les fuites de pétrole en 12 heures de traitement ou moins. Cette étude démontre une solution originalepour l’absorption du pétrole des fuites pétro-lières. Après avoir été saturée avec le pétrole, la lignine peut être incinérée, et sa valeur calori-fique élevée permet la production d’énergie à partir de ce qui aurait été autrement gaspillé
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