Are the Effects of Structure Formation Seen in the Central Metallicity of Galaxy Clusters?

A sample of 46 nearby clusters observed with Chandra is analyzed to produce radial density, temperature, entropy and metallicity profiles, as well as other morphological measurements. The entropy profiles are computed to larger radial extents than in previous Chandra cluster sample analyses. We find that the iron mass fraction measured in the inner 0.15 R500 shows a larger dispersion across the sample of low-mass clusters, than it does for the sample of high-mass clusters. We interpret this finding as the result of the mixing of more haloes in large clusters than in small clusters, which leads to an averaging of the metal content in the large clusters, and thus less dispersion of metallicity for high-mass clusters. This interpretation lends support to the idea that the low-entropy, metal-rich gas of merging haloes reaches clusters' centers, which explains observations of Core-Collapse Supernova products metallicity peaks, and which is seen in hydrodynamical simulations. The gas in these merging haloes would have to reach the centers of clusters without mixing in the outer regions, in order to support our interpretation. On the other hand, metallicity dispersion does not change with mass in the outer regions of clusters, suggesting that most of the outer metals come from a source with a more uniform metallicity level, such as during pre-enrichment. We also measure a correlation between the metal content in low-mass clusters and the degree to which their Intra-Cluster Medium (ICM) is morphologically disturbed, as measured by centroid shift. This suggests an alternative interpretation of the large width of the metallicity distribution in low-mass clusters, whereby a metallicity boost in the center of low-mass clusters is induced as a transitional state, during mergers.Comment: Accepted in ApJ, March 9, 201

Sacrificial charge and the spectral resolution performance of the Chandra Advanced CCD Imaging Spectrometer

Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. The ACIS team is continuing to study the properties of the damage with an emphasis on developing techniques to mitigate charge transfer inefficiency (CTI) and spectral resolution degradation. A post-facto CTI corrector has been developed which can effectively recover much of the lost resolution. Any further improvements in performance will require knowledge of the location and amount of sacrificial charge - charge deposited along the readout path of an event which fills electron traps and changes CTI. We report on efforts by the ACIS Instrument team to characterize which charge traps cause performance degradation and the properties of the sacrificial charge seen on-orbit. We also report on attempts to correct X-ray pulseheights for the presence of sacrificial charge.Comment: 9 pages, 7 figures to be published in Proc. SPIE 485

The Evolution of Structure in X-ray Clusters of Galaxies

Using Chandra archival data, we quantify the evolution of cluster morphology with redshift. Clusters form and grow through mergers with other clusters and groups, and the amount of substructure in clusters in the present epoch and how quickly it evolves with redshift depend on the underlying cosmology. Our sample includes 40 X-ray selected, luminous clusters from the Chandra archive, and we quantify cluster morphology using the power ratio method (Buote & Tsai 1995). The power ratios are constructed from the moments of the X-ray surface brightness and are related to a cluster's dynamical state. We find that, as expected qualitatively from hierarchical models of structure formation, high-redshift clusters have more substructure and are dynamically more active than low-redshift clusters. Specifically, the clusters with z>0.5 have significantly higher average third and fourth order power ratios than the lower redshift clusters. Of the power ratios, $P_3/P_0$ is the most unambiguous indicator of an asymmetric cluster structure, and the difference in $P_3/P_0$ between the two samples remains significant even when the effects of noise and other systematics are considered. After correcting for noise, we apply a linear fit to $P_3/P_0$ versus redshift and find that the slope is greater than zero at better than 99% confidence. This observation of structure evolution indicates that dynamical state may be an important systematic effect in cluster studies seeking to constrain cosmology, and when calibrated against numerical simulations, structure evolution will itself provide interesting bounds on cosmological models.Comment: 42 pages, 6 figures, ApJ accepted. For a version of the paper containing an appendix with images of all of the clusters, see http://www.ociw.edu/~tesla/structure.ps.g

Measuring the Three-Dimensional Structure of Galaxy Clusters. I. Application to a Sample of 25 Clusters

We discuss a method to constrain the intrinsic three-dimensionale shapes of galaxy clusters by combining X-Ray and Sunyaev-Zeldovich observations. The method is applied to a sample of 25 X-Ray selected clusters, with measured Sunyaev-Zeldovich temperature decrements. The sample turns out to be slightly biased, with strongly elongated clusters preferentially aligned along the line of sight. This result demonstrates that X-Ray selected cluster samples may be affected by morphological and orientation effects even if a relatively high threshold signal-to-noise ratio is used to select the sample. A large majority of the clusters in our sample exhibit a marked triaxial structure, with prolate-like shapes being slightly more likely than oblate-like ones; the spherical hypothesis is strongly rejected for most sample members. Cooling flow clusters do not show preferentially regular morphologies.Comment: 13 pages, 9 figures. Accepted by Ap

X-ray Temperature and Mass Measurements to the Virial Radius of Abell 1413 with Suzaku

We present X-ray observations of the northern outskirts of the relaxed galaxy cluster A1413 with Suzaku, whose XIS instrument has the low intrinsic background needed to make measurements of these low surface brightness regions. We excise 15 point sources superimposed on the image above a flux of $1\times 10^{-14}$ \fluxunit (2--10keV) using XMM-Newton and Suzaku images of the cluster. We quantify all known systematic errors as part of our analysis, and show our statistical errors encompasses them for the most part. Our results extend previous measurements with Chandra and XMM-Newton, and show a significant temperature drop to about 3keV at the virial radius, $r_{200}$. Our entropy profile in the outer region ($> 0.5 r_{200}$) joins smoothly onto that of XMM-Newton, and shows a flatter slope compared with simple models, similar to a few other clusters observed at the virial radius. The integrated mass of the cluster at the virial radius is approximately $7.5\times10^{14}M_{\odot}$ and varies by about 30% depending on the particular method used to measure it.Comment: 32pages, 9 figures, accepted for publication in PAS

Temperature dependence of charge transfer inefficiency in Chandra X-ray CCDs

Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. The primary effect of the damage was to increase the charge transfer inefficiency (CTI) of the eight front illuminated CCDs by more than two orders of magnitude. The ACIS instrument team is continuing to study the properties of the damage with an emphasis on developing techniques to mitigate CTI and spectral resolution degradation. We present the initial temperature dependence of ACIS CTI from -120 to -60 degrees Celsius and the current temperature dependence after more than six years of continuing slow radiation damage. We use the change of shape of the temperature dependence to speculate on the nature of the damaging particles.Comment: 9 pages, 8 figures, to appear in Proc. SPIE vol 6276 "High Energy, Optical, and Infrared Detectors for Astronomy II

A Chandra Spectroscopic Survey of Persistent Black Hole Candidates

We present results from observations of persistent black hole candidates with the High Energy Transmission Gratings aboard the Chandra X-ray Observatory. The sources include LMC X-1, LMC X-3, GRS 1758-258, and Cyg X-1. Along with the published results on 1E1740.7-2942, we have completed a high-resolution spectroscopic survey of such systems. The observed X-ray spectra of LMC X-1 and LMC X-3 show no prominent discrete features, while absorption edges (Mg K and Si K) are detected in the spectrum of GRS 1758-258. The edges are likely to be of interstellar origin. In most cases, the X-ray continuum can be described well by models that are often adopted in low-resolution studies of black hole candidates: a multi-temperature disk spectrum plus a Comptonization component. However, the relative contribution of the two components varies greatly among different sources. For instance, only the disk component is present for LMC X-1 and GRS 1758-258, while the Comptonized component is required for other sources. We discuss general issues related to obtaining disk parameters from modeling X-ray continuum.Comment: ApJ, 576, in press (Sept.

Physics of reverse annealing in high-resistivity Chandra ACIS CCDs

After launch, the Advanced CCD Imaging Spectrometer (ACIS), a focal plane instrument on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. An effect of the damage was to increase the charge transfer inefficiency (CTI) of the front illuminated CCDs. As part of the initial damage assessment, the focal plane was warmed from the operating temperature of -100C to +30C which unexpectedly further increased the CTI. We report results of ACIS CCD irradiation experiments in the lab aimed at better understanding this reverse annealing process. Six CCDs were irradiated cold by protons ranging in energy from 100 keV to 400 keV, and then subjected to simulated bakeouts in one of three annealing cycles. We present results of these lab experiments, compare them to our previous experiences on the ground and in flight, and derive limits on the annealing time constants.Comment: 9 pages, to appear in Proc. SPIE 7021, "High Energy, Optical and Infrared Detectors for Astronomy

Long-term trends in radiation damage of Chandra X-ray CCDs

Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. Current operations require ACIS to be protected during radiation belt passages to prevent this type of damage, but there remains a much slower and more gradual increase. We present the history of ACIS charge transfer inefficiency (CTI), and other measures of radiation damage, from January 2000 through June 2005. The rate of CTI increase is low, of order 1e-6 per year, with no indication of step-function increases due to specific solar events. Based on the time history and CCD location of the CTI increase, we speculate on the nature of the damaging particles.Comment: 10 pages, 14 figures to appear in Proc. SPIE vol. 5898 "UV, X-ray, and Gamma-Ray Space Instrumentation for Astronomy XIV