10,415 research outputs found
The Effects of Clumping and Substructure on ICM Mass Measurements
We examine an ensemble of 48 simulated clusters to determine the effects of
small-scale density fluctuations and large-scale substructure on X-ray
measurements of the intracluster medium (ICM) mass. We measure RMS density
fluctuations in the ICM which can be characterized by a mean mass-weighted
clumping factor C = /^2 between 1.3 and 1.4 within a density
contrast of 500 times the critical density. These fluctuations arise from the
cluster history of accretion shocks and major mergers, and their presence
enhances the cluster's luminosity relative to the smooth case. We expect,
therefore, that ICM mass measurements utilizing models which assume uniform
density at a given radius carry a bias of order sqrt(C) = 1.16. We verify this
result by performing ICM mass measurements on X-ray images of the simulations
and finding the expected level of bias.
The varied cluster morphologies in our ensemble also allow us to investigate
the effects of departures from spherical symmetry on our measurements. We find
that the presence of large-scale substructure does not further bias the
resulting gas mass unless it is pronounced enough to produce a second peak in
the image of at least 1% the maximum surface brightness. We analyze the subset
of images with no secondary peaks and find a bias of 9% and a Gaussian random
error of 4% in the derived mass.Comment: To appear in ApJ
QED self-energy contribution to highly-excited atomic states
We present numerical values for the self-energy shifts predicted by QED
(Quantum Electrodynamics) for hydrogenlike ions (nuclear charge ) with an electron in an , 4 or 5 level with high angular momentum
(). Applications include predictions of precision transition
energies and studies of the outer-shell structure of atoms and ions.Comment: 20 pages, 5 figure
Calculation of the Electron Self Energy for Low Nuclear Charge
We present a nonperturbative numerical evaluation of the one-photon electron
self energy for hydrogenlike ions with low nuclear charge numbers Z=1 to 5. Our
calculation for the 1S state has a numerical uncertainty of 0.8 Hz for hydrogen
and 13 Hz for singly-ionized helium. Resummation and convergence acceleration
techniques that reduce the computer time by about three orders of magnitude
were employed in the calculation. The numerical results are compared to results
based on known terms in the expansion of the self energy in powers of (Z
alpha).Comment: 10 pages, RevTeX, 2 figure
A Chandra Study of the Effects of a Major Merger on the Structure of Abell 2319
We present an analysis of a Chandra observation of the massive, nearby galaxy
cluster Abell 2319. A sharp surface brightness discontinuity--suggested by
previous, lower angular resolution X-ray imaging--is clearly visible in the
ACIS image. This roughly 300kpc feature suggests that a major merger is taking
place with a significant velocity component perpendicular to the line of sight.
The cluster emission-weighted mean temperature is 11.8+/-0.6kev, somewhat
higher than previous temperature measurements. The Chandra temperature map of
A2319 reveals substructure resembling that anticipated based on hydrodynamic
simulations of cluster mergers. The merger feature shows a pressure change
across the surface brightness discontinuity by a factor of <=2.5. The higher
density side of the front has a lower temperature, suggesting the presence of a
cold front similar to those in many other merging clusters. The velocity of the
front is roughly sonic.
We compare bulk properties of the ICM and galaxies in A2319 to the same
properties in a large sample of clusters as a way of gauging the effects of the
major merger. Interestingly, by comparing A2319 to a sample of 44 clusters
studied with the ROSAT PSPC we find that the X-ray luminosity, isophotal size,
and ICM mass are consistent with the expected values for a cluster of its
temperature; in addition, the K-band galaxy light is consistent with the
light--temperature scaling relation derived from a sample of about 100 clusters
studied with 2MASS. Together, these results indicate either that the merger in
A2319 has not been effective at altering the bulk properties of the cluster, or
that there are large but correlated displacements in these quantities.Comment: 11 pages, 8 figures, ApJ Submitte
Analysis of No-Difference Findings in Evaluation Research
Conclusions of no difference are becoming increasingly important in evaluation research. We delineate three major uses of no-difference findings and analyze their meanings. (1) No-differ ence findings in randomized experiments can be interpreted as support for conclusions of the absence of a meaningful treatment effect, but only if the proper analytic methods are used. (2) Statistically based conclusions in quasi-experiments do not allow causal statements about the treatment impact but do provide a metric to judge the size of the resulting difference. (3) Using no-difference findings to conclude equivalence on control variables is inefficient and potentially misleading. The final section of the article presents alternative methods by which conclusions of no difference may be supported when applicable. These methods include the use of arbitrarily high alpha levels, interval estimation, and power analysis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67182/2/10.1177_0193841X8901300604.pd
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