66 research outputs found
HEXTE Studies of Sco X-1 Spectra: Detections of Hard X-Ray Tails Beyond 200 keV
Using the HEXTE experiment on-board the RXTE satellite, we performed a search
for hard X-ray tails in Sco X-1 spectra. We found strong evidence for the
presence of such a non-thermal component on several occasions. Using the
PCA/RXTE we were able to track the position of the source along the Z diagram,
and we observed that the presence of the hard X-ray tail is not confined to a
particular region. However, we found a correlation between the power law index
of the non-thermal component and the position of the source in the Z diagram,
suggesting that the hard X-ray spectrum (i.e., E > 50 keV) becomes flatter as
the mass accretion rate increases. We were also able to study the temporal
variation of the appearance/absence of the hard X-ray component. With our
derived luminosities, we were also able to test the idea that X-ray
luminosities can be used to distinguish between X-ray binary systems containing
neutron stars and black holes.Comment: 6 pages, 2 figures; Presented at the 33rd COSPAR Scientific Assembly
(Warsaw, Poland, 2000 July); Accepted for publication in Advances in Space
Researc
Expulsion of Magnetic Flux Lines from the Growing Superconducting Core of a Magnetized Quark Star
The expulsion of magnetic flux lines from a growing superconducting core of a
quark star has been investigated. The idea of impurity diffusion in molten
alloys and an identical mechanism of baryon number transport from hot
quark-gluon-plasma phase to hadronic phase during quark-hadron phase transition
in the early universe, micro-second after big bang has been used. The
possibility of Mullins-Sekerka normal-superconducting interface instability has
also been studied.Comment: Thoroughly revised version. Accepted for Astrophysics & Space Scienc
Extragalactic Inverse Compton Light from Dark Matter Annihilation and the Pamela Positron Excess
We calculate the extragalactic diffuse emission originating from the
up-scattering of cosmic microwave photons by energetic electrons and positrons
produced in particle dark matter annihilation events at all redshifts and in
all halos. We outline the observational constraints on this emission and we
study its dependence on both the particle dark matter model (including the
particle mass and its dominant annihilation final state) and on assumptions on
structure formation and on the density profile of halos. We find that for
low-mass dark matter models, data in the X-ray band provide the most stringent
constraints, while the gamma-ray energy range probes models featuring large
masses and pair-annihilation rates, and a hard spectrum for the injected
electrons and positrons. Specifically, we point out that the all-redshift,
all-halo inverse Compton emission from many dark matter models that might
provide an explanation to the anomalous positron fraction measured by the
Pamela payload severely overproduces the observed extragalactic gamma-ray
background.Comment: Version accepted for publication in JCAP, one new figure and text
added; 19 pages, 5 figure
Nonthermal phenomena in clusters of galaxies
Recent observations of high energy (> 20 keV) X-ray emission in a few
clusters of galaxies broaden our knowledge of physical phenomena in the
intracluster space. This emission is likely to be nonthermal, probably
resulting from Compton scattering of relativistic electrons by the cosmic
microwave background (CMB) radiation. Direct evidence for the presence of
relativistic electrons in some 50 clusters comes from measurements of extended
radio emission in their central regions. We briefly review the main results
from observations of extended regions of radio emission, and Faraday rotation
measurements of background and cluster radio sources. The main focus of the
review are searches for nonthermal X-ray emission conducted with past and
currently operating satellites, which yielded appreciable evidence for
nonthermal emission components in the spectra of a few clusters. This evidence
is clearly not unequivocal, due to substantial observational and systematic
uncertainties, in addition to virtually complete lack of spatial information.
If indeed the emission has its origin in Compton scattering of relativistic
electrons by the CMB, then the mean magnetic field strength and density of
relativistic electrons in the cluster can be directly determined. Knowledge of
these basic nonthermal quantities is valuable for the detailed description of
processes in intracluster gas and for the origin of magnetic fields.Comment: 23 pages, 7 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 5; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
A multilaboratory comparison of calibration accuracy and the performance of external references in analytical ultracentrifugation.
Analytical ultracentrifugation (AUC) is a first principles based method to determine absolute sedimentation coefficients and buoyant molar masses of macromolecules and their complexes, reporting on their size and shape in free solution. The purpose of this multi-laboratory study was to establish the precision and accuracy of basic data dimensions in AUC and validate previously proposed calibration techniques. Three kits of AUC cell assemblies containing radial and temperature calibration tools and a bovine serum albumin (BSA) reference sample were shared among 67 laboratories, generating 129 comprehensive data sets. These allowed for an assessment of many parameters of instrument performance, including accuracy of the reported scan time after the start of centrifugation, the accuracy of the temperature calibration, and the accuracy of the radial magnification. The range of sedimentation coefficients obtained for BSA monomer in different instruments and using different optical systems was from 3.655 S to 4.949 S, with a mean and standard deviation of (4.304 ± 0.188) S (4.4%). After the combined application of correction factors derived from the external calibration references for elapsed time, scan velocity, temperature, and radial magnification, the range of s-values was reduced 7-fold with a mean of 4.325 S and a 6-fold reduced standard deviation of ± 0.030 S (0.7%). In addition, the large data set provided an opportunity to determine the instrument-to-instrument variation of the absolute radial positions reported in the scan files, the precision of photometric or refractometric signal magnitudes, and the precision of the calculated apparent molar mass of BSA monomer and the fraction of BSA dimers. These results highlight the necessity and effectiveness of independent calibration of basic AUC data dimensions for reliable quantitative studies
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