45 research outputs found
Statistical Uncertainties in Temperature Diagnostics for Hot Coronal Plasma Using the ASCA SIS
Statistical uncertainties in determining the temperatures of hot (0.5 keV to
10 keV) coronal plasmas are investigated. The statistical precision of various
spectral temperature diagnostics is established by analyzing synthetic ASCA
Solid-state Imaging Spectrometer (SIS) CCD spectra. The diagnostics considered
are the ratio of hydrogen-like to helium-like line complexes of
elements, line-free portions of the continuum, and the entire spectrum. While
fits to the entire spectrum yield the highest statistical precision, it is
argued that fits to the line-free continuum are less susceptible to atomic data
uncertainties but lead to a modest increase in statistical uncertainty over
full spectral fits. Temperatures deduced from line ratios can have similar
accuracy but only over a narrow range of temperatures. Convenient estimates of
statistical accuracies for the various temperature diagnostics are provided
which may be used in planning ASCA SIS observations.Comment: postscript file of 8 pages+3 figures; 4 files tarred, compressed and
uuencoded. To appear in the Astrophysical Journal Letters; contents copyright
1994 American Astronomical Societ
Self-Consistent and Time-Dependent Solar Wind Models
We describe the first results from a self-consistent study of Alfven waves for the time-dependent, single-fluid magnetohydrodynamic (MHD) solar wind equations, using a modified version of the ZEUS MHD code. The wind models we examine are radially symmetrical and magnetized; the initial outflow is described by the standard Parker wind solution. Our study focuses on the effects of Alfven waves on the outflow and is based on solving the full set of the ideal nonlinear MHD equations. In contrast to previous studies, no assumptions regarding wave linearity, wave damping, and wave-flow interaction are made; thus, the models naturally account for the back-reaction of the wind on the waves, as well as for the nonlinear interaction between different types of MHD waves. Our results clearly demonstrate when momentum deposition by Alfven waves in the solar wind can be sufficient to explain the origin of fast streams in solar coronal holes; we discuss the range of wave amplitudes required to obtained such fast stream solutions
Galaxy Cluster Shapes and Systematic Errors in H0 Measured by the Sunyaev-Zel'dovich Effect
Imaging of the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters combined
with cluster plasma x-ray diagnostics can measure the cosmic distance scale to
high redshift. Projecting the inverse-Compton scattering and x-ray emission
along the cluster line-of-sight introduces systematic errors in the Hubble
constant, H0, because the true shape of the cluster is not known. I present a
study of the systematic errors in the value of H0, as determined by the x-ray
and SZ properties of theoretical samples of triaxial isothermal ``beta'' model
clusters, caused by projection effects and observer orientation. I calculate
estimates for H0 for each cluster based on their large and small apparent
angular core radii and their arithmetic mean. I demonstrate that the estimates
for H0 for a sample of 25 clusters have 99.7% confidence intervals for the mean
estimated H0 analyzing the clusters using either their large or mean angular
core radius are within 14% of the ``true'' (assumed) value of H0 (and enclose
it), for a triaxial beta model cluster sample possessing a distribution of
apparent x-ray cluster ellipticities consistent with that of observed x-ray
clusters. This limit on the systematic error in H0 caused by cluster shape
assumes that each sample beta model cluster has fixed shape; deviations from
constant shape within the clusters may introduce additional uncertainty or bias
into this result.Comment: Accepted for publication in the Astrophysical Journal, 24 March 1998;
4 pages, 2 figure
Using binary stars to bound the mass of the graviton
Interacting white dwarf binary star systems, including helium cataclysmic
variable (HeCV) systems, are expected to be strong sources of gravitational
radiation, and should be detectable by proposed space-based laser
interferometer gravitational wave observatories such as LISA. Several HeCV star
systems are presently known and can be studied optically, which will allow
electromagnetic and gravitational wave observations to be correlated.
Comparisons of the phases of a gravitational wave signal and the orbital light
curve from an interacting binary white dwarf star system can be used to bound
the mass of the graviton. Observations of typical HeCV systems by LISA could
potentially yield an upper bound on the inverse mass of the graviton as strong
as km (
eV), more than two orders of magnitude better than present solar system derived
bounds.Comment: 21 pages plus 4 figures; ReVTe
A Multiphase Model for the Intracluster Medium
Constraints on the clustered mass density \Omega_m of the universe derived
from the mean intracluster gas fraction of X-ray clusters may be biased by a
single-phase assumption for the thermodynamic structure of the intracluster
medium (ICM). We propose a descriptive model for multiphase structure in which
a spherically symmetric ICM contains isobaric density perturbations with
radially dependent variance \sigma^2(r)=\sigma_c^2 (1+r^2/r_c^2)^{-\eps}. The
model extends the work of Gunn & Thomas (1996) which assumed radially
independent density fluctuations thoughout the ICM. Fixing the X-ray emission
profile and emission weighted temperature, we explore two independently
observable signatures of the model in the {\sigma_c,\eps} space. For
bremsstrahlung dominated emission, the central Sunyaev--Zeldovich (SZ)
decrement in the multiphase case is increased over the single-phase case and
multiphase X-ray spectra in the range 0.1-20 keV are flatter in the continuum
and exhibit stronger low energy emission lines than their single-phase
counterpart. We quantify these effects for a fiducial 10^8 K cluster and
demonstrate how the combination of SZ and X-ray spectroscopy can be used to
identify a preferred location in the {\sigma_c,\eps} plane. From these
parameters, the correct value of mean ICM gas fraction in the multiphase model
results, allowing an unbiased estimate of \Omega_m to be recovered. The
consistency of recent determinations of the Hubble constant from SZ and X-ray
observations with values determined by other methods suggests that biases in
ICM gas fractions are small, \ltsim 20%.Comment: Nine pages, submitted to Monthly Notices of the RAS. Seven postscript
figures incoporate
Uses of continuum radiation in the AXAF calibration
X-ray calibration of the Advanced X-ray Astrophysics Facility (AXAF) observatory at the MSFC X-Ray Calibration Facility (XRCF) made novel use of the x-ray continuum from a conventional electron-impact source. Taking advantage of the good spectral resolution of solid-state detectors, continuum measurements proved advantageous in calibrating the effective area of AXAF's High-Resolution Mirror Assembly (HRMA) and in verifying its alignment to the XRCF's optical axis
Joint analysis of X-ray and Sunyaev Zel'dovich observations of galaxy clusters using an analytic model of the intra-cluster medium
We perform a joint analysis of X-ray and Sunyaev Zel'dovich (SZ) effect data
using an analytic model that describes the gas properties of galaxy clusters.
The joint analysis allows the measurement of the cluster gas mass fraction
profile and Hubble constant independent of cosmological parameters. Weak
cosmological priors are used to calculate the overdensity radius within which
the gas mass fractions are reported. Such an analysis can provide direct
constraints on the evolution of the cluster gas mass fraction with redshift. We
validate the model and the joint analysis on high signal-to-noise data from the
Chandra X-ray Observatory and the Sunyaev-Zel'dovich Array for two clusters,
Abell 2631 and Abell 2204.Comment: ApJ in pres
Thioredoxin is involved in endothelial cell extracellular transglutaminase 2 activation mediated by celiac disease patient IgA
Purpose: To investigate the role of thioredoxin (TRX), a novel regulator of extracellular transglutaminase 2 (TG2), in celiac patients IgA (CD IgA) mediated TG2 enzymatic activation. Methods: TG2 enzymatic activity was evaluated in endothelial cells (HUVECs) under different experimental conditions by ELISA and Western blotting. Extracellular TG2 expression was studied by ELISA and immunofluorescence. TRX was analysed by Western blotting and ELISA. Serum immunoglobulins class A from healthy subjects (H IgA) were used as controls. Extracellular TG2 enzymatic activity was inhibited by R281. PX12, a TRX inhibitor, was also employed in the present study. Results: We have found that in HUVECs CD IgA is able to induce the activation of extracellular TG2 in a dose-dependent manner. Particularly, we noted that the extracellular modulation of TG2 activity mediated by CD IgA occurred only under reducing conditions, also needed to maintain antibody binding. Furthermore, CD IgA-treated HUVECs were characterized by a slightly augmented TG2 surface expression which was independent from extracellular TG2 activation. We also observed that HUVECs cultured in the presence of CD IgA evinced decreased TRX surface expression, coupled with increased secretion of the protein into the culture medium. Intriguingly, inhibition of TRX after CD IgA treatment was able to overcome most of the CD IgA-mediated effects including the TG2 extracellular transamidase activity. Conclusions: Altogether our findings suggest that in endothelial cells CD IgA mediate the constitutive activation of extracellular TG2 by a mechanism involving the redox sensor protein TRX
On the Internal Structure of Relativistic Jets
A magnetohydrodynamic model is constructed for a cylindrical jet immersed in
an external uniform magnetic field. It is shown that, as in the force-free
case, the total electric current within the jet can be zero. The particle
energetics and the magnetic field structure are determined in a self-consistent
way; all jet parameters depend on the physical conditions in the external
medium. In particular, we show that a region with subsonic flow can exist in
the central jet regions. In actual relativistic jets, most of the energy is
transferred by the electromagnetic field only when the magnetization parameter
is sufficiently large, . We also show that, in general, the
well-known solution with a central core, ,
can not be realized in the presence of an external medium.Comment: 19 pages, 2 figure
Linkage analysis of HLA and candidate genes for celiac disease in a North American family-based study
BACKGROUND: Celiac disease has a strong genetic association with HLA. However, this association only explains approximately half of the sibling risk for celiac disease. Therefore, other genes must be involved in susceptibility to celiac disease. We tested for linkage to genes or loci that could play a role in pathogenesis of celiac disease. METHODS: DNA samples, from members of 62 families with a minimum of two cases of celiac disease, were genotyped at HLA and at 13 candidate gene regions, including CD4, CTLA4, four T-cell receptor regions, and 7 insulin-dependent diabetes regions. Two-point and multipoint heterogeneity LOD (HLOD) scores were examined. RESULTS: The highest two-point and multipoint HLOD scores were obtained in the HLA region, with a two-point HLOD of 3.1 and a multipoint HLOD of 5.0. For the candidate genes, we found no evidence for linkage. CONCLUSIONS: Our significant evidence of linkage to HLA replicates the known linkage and association of HLA with CD. In our families, likely candidate genes did not explain the susceptibility to celiac disease