71 research outputs found
Status of LDEF activation measurements and archive
We review the status of induced radioactivity measurements for the LDEF spacecraft which includes studies of the nuclide, target, directional and depth dependences of the activation. Analysis of the data has focused on extraction of the specific activities for many materials to develop a global picture of the low Earth orbital environment to which the LDEF was subjected. Preliminary comparisons of data in a previous review showed that it was possible to make meaningful intercomparisons between results obtained at different facilities. Generally these comparisons were good and gave results to within 10-20 percent, although some analysis remains. These results clearly provide constraints for recent calculations being performed of the radiation environment of the LDEF. We are not anticipating a period of production of final activation results. An archive is being prepared jointly between NASA/Marshall and Eastern Kentucky University which will include gamma ray spectra and other intermediate results
Commensurate antiferromagnetic ordering in Ba(Fe{1-x}Co{x})2As2 determined by x-ray resonant magnetic scattering at the Fe K-edge
We describe x-ray resonant magnetic diffraction measurements at the Fe K-edge
of both the parent BaFe2As2 and superconducting Ba(Fe0.953Co0.047)2As2
compounds. From these high-resolution measurements we conclude that the
magnetic structure is commensurate for both compositions. The energy spectrum
of the resonant scattering is in reasonable agreement with theoretical
calculations using the full-potential linear augmented plane wave method with a
local density functional.Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev. B Rapid
Com
Gamma ray monitoring of a AGN and galactic black hole candidates by the Compton Gamma Ray Observatory
The Compton Gamma-Ray Observatory's Burst and Transient Source Experiment (BATSE) has a powerful capability to provide nearly uninterrupted monitoring in the 25 keV-10 MeV range of both active galactic nuclei (AGN) and galactic black hole candidates (GBHC) such as Cygnus X-1, using the occultation of cosmic sources by the Earth. Since the Crab is detected by the BATSE Large Area Detectors with roughly 25(sigma) significance in the 15-125 keV range in a single rise or set, a variation by a factor of two of a source having one-tenth the strength of Cygnus X-1 should be detectable within a day. Methods of modeling the background are discussed which will increase the accuracy, sensitivity, and reliability of the results beyond those obtainable from a linear background fit with a single rise or set discontinuity
A Sequence of Outbursts from the Transient X-Ray Pulsar GS 0834-430
GS 0834-430, a 12.3 s accretion-powered pulsar, has been observed in seven outbursts with the BATSE large-area detectors on the Compton Gamma Ray Observatory. The first five outbursts observed by BATSE occurred at intervals of about 107 days, while the final two outbursts were separated by about 140 days. The photon energy spectrum, measured by Earth occultation in the 20-100 keV band, can be fitted by a power law with photon index α ≈ -3.7 or by an exponential spectrum with temperature kT ≈ 15 keV, with some variations within outbursts. The source has a low pulse fraction, ≾ 0.15 in the 20-50 keV band. We have observed significant temporal and energy-dependent variations in epoch folded pulse profiles. Because the intrinsic torque effects for this system are at least comparable to orbital effects, pulse timing analysis did not produce a unique orbital solution. However, confidence regions for the orbital elements yielded the following 1 σ limits: orbital period P_(orb) = 105.8 ± 0.4 days and eccentricity 0.10 ≾ e ≾ 0.17. GS 0834-430 is most likely a Be/X-ray binary
Effects of transition metal substitutions on the incommensurability and spin fluctuations in BaFe2As2 by elastic and inelastic neutron scattering
The spin fluctuation spectra from nonsuperconducting Cu-substituted, and
superconducting Co-substituted, BaFe2As2 are compared quantitatively by
inelastic neutron scattering measurements and are found to be indis-
tinguishable. Whereas diffraction studies show the appearance of incommensurate
spin-density wave order in Co and Ni substituted samples, the magnetic phase
diagram for Cu substitution does not display incommensu- rate order,
demonstrating that simple electron counting based on rigid-band concepts is
invalid. These results, supported by theoretical calculations, suggest that
substitutional impurity effects in the Fe plane play a signifi- cant role in
controlling magnetism and the appearance of superconductivity, with Cu
distinguished by enhanced impurity scattering and split-band behavior.Comment: 5 pages, 5 figures, Major change in the manuscrip
Pressure-induced collapsed-tetragonal phase in SrCo2As2
We present high-energy x-ray diffraction data under applied pressures up to p
= 29 GPa, neutron diffraction measurements up to p = 1.1 GPa, and electrical
resistance measurements up to p = 5.9 GPa, on SrCo2As2. Our x-ray diffraction
data demonstrate that there is a first-order transition between the tetragonal
(T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6
GPa at T = 7 K. The pressure for the onset of the cT phase and the range of
coexistence between the T and cT phases appears to be nearly temperature
independent. The compressibility along the a-axis is the same for the T and cT
phases whereas, along the c-axis, the cT phase is significantly stiffer, which
may be due to the formation of an As-As bond in the cT phase. Our resistivity
measurements found no evidence of superconductivity in SrCo2As2 for p <= 5.9
GPa and T >= 1.8 K. The resistivity data also show signatures consistent with a
pressure-induced phase transition for p >= 5.5 GPa. Single-crystal neutron
diffraction measurements performed up to 1.1 GPa in the T phase found no
evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K.
Spin-polarized total-energy calculations demonstrate that the cT phase is the
stable phase at high pressure with a c/a ratio of 2.54. Furthermore, these
calculations indicate that the cT phase of SrCo2As2 should manifest either
A-type antiferromagnetic or ferromagnetic order.Comment: 6 pages, 5 figure
Discovery of the 198 s X-ray Pulsar GRO J2058+42
GRO J2058+42, a transient 198 second x-ray pulsar, was discovered by the
Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray
Observatory (CGRO), during a "giant" outburst in 1995 September-October. The
total flux peaked at about 300 mCrab (20-50 keV) as measured by Earth
occultation. The pulse period decreased from 198 s to 196 s during the 46-day
outburst. The pulse shape evolved over the course of the outburst and exhibited
energy dependent variations. BATSE observed five additional weak outbursts from
GRO J2058+42, each with two week duration and peak pulsed flux of about 15
mCrab (20-50 keV), that were spaced by about 110 days. An observation of the
1996 November outburst by the Rossi X-ray Timing Explorer (RXTE) Proportional
Counter Array (PCA) localized the source to within a 4\arcmin radius error
circle (90 % confidence) centered on R.A. = 20h 59m.0, Decl. = 41 deg 43 arcmin
(J2000). Additional shorter outbursts with peak pulsed fluxes of about 8 mCrab
were detected by BATSE halfway between the first four 15 mCrab outbursts. The
RXTE All-Sky Monitor detected all 8 weak outbursts with approximately equal
durations and intensities. GRO J2058+42 is most likely a Be/X-ray binary that
appears to outburst at periastron and apastron. No optical counterpart has been
identified to date and no x-ray source was present in the error circle in
archival ROSAT observations.Comment: 19 pages including 10 figures. Accepted for publication in the
Astrophysical Journa
GX 339-4: the distance, state transitions, hysteresis and spectral correlations
We study X-ray and variability and distance of GX 339-4. We derive d>7 kpc,
based on recent determination of the binary parameters. We study data from the
Ginga/ASM, the CGRO/BATSE, and the RXTE/ASM, PCA and HEXTE. From 1987 to 2004,
GX 339-4 underwent 15 outbursts and went through all known states of black-hole
binaries. We also present the PCA data from the initial stage of the 2004
outburst. We then study colour-colour and colour-flux correlations. In the hard
state, there is a strong anticorrelation between the 1.5-5 and 3-12 keV
spectral slopes, which we explain by thermal Comptonization of disc photons.
There is also a softening of the spectrum above 3 keV with the increasing flux
that becomes stronger with increasing energy up to 200 keV. This indicates an
anticorrelation between the electron temperature and luminosity, explained by
hot accretion models. In addition, we see a variable broad-band slope with a
pivot at 200 keV. We confirm the presence of pronounced hysteresis, with the
hard-to-soft state transitions occurring at much higher (and variable)
luminosities than the soft-to-hard transitions. We fit the ASM data with a
model consisting of an outer accretion disc and a hot inner flow. State
transitions are associated then with variations in the disc truncation radius,
which we fit as 6GM/c^2 in the soft state and several times that in the hard
state. The disappearence of the inner disc takes place at a lower accretion
rate than its initial appearance due to the dependence of the transitions on
the source history. We provide further evidence against the X-ray emission in
the hard state being nonthermal synchrotron, and explain the observed
radio-X-ray correlation by the jet power being correlated with the accretion
power.Comment: MNRAS, in press (a substantially revised version, including new data
from the Feb. 2004 outburst of GX 339-4
The Main Belt Comets and ice in the Solar System
We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies
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