Symposium Proceedings

Galactic Diffuse Emission and Surveys

Deep Chandra Observations of the Arches and Quintuplet Clusters at the Galactic Center

Hui Dong, Q. Daniel Wang (Dept. of Astronomy, Univ. of Massachusetts, Amherst)

The Arches and Quintuplet clusters provide excellent local testbeds for our understanding of very young massive stellar systems and their interaction with galactic nuclear environment. We present an 100 ks on-axis ACIS-I observation of these two Galactic clusters. We detect several bright point-like sources that are clearly associated with the clusters. Some of the sources are positionally coincident with known radio sources with strong stellar winds. The sources also have similar X-ray spectra, which are dominantly thermal and show an unusually strong 6.7-keV emission line. The sources most likely represent the colliding wind close binaries of very massive stars. Diffuse X-ray emission is unambigously detected and extends at least to a distance of 2 pc from the cluster centers. In the cluster cores, the diffuse emission shows evidence of hot gas cavities around some massive stars and filamental structures. These substructures most likely represent the wind-wind interaction on cluster scales, providing potentially very useful diagnostics of collisionless shock dynamics and the formation of the cluster winds. The diffuse X-ray spectra also show remarkably different spectra from those point-like sources. In particular, the diffuse X-ray spectrum of the Arches clluster shows a strong 6.4-keV emission line and little evidence for the 6.7-keV line. But the bulk of the 6.4-keV emission line arises in regions away from the cluster core. The origin of this line emission is still uncertain. The understanding of the two clusters in our Galactic center will provide important hints about what might be happening in active starforming nuclear regions of other galaxies.

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A Search for Serendipitous Chandra Periodic and Transient Systems

Anna Hourihane (Univ. College Cork), Jeremy Drake (Harvard-Smithsonian Center for Astrophysics), Paul Callanan (Univ. College Cork), Dong-Woo Kim, Peter Ratzlaff (Harvard-Smithsonian Center for Astrophysics)

We present the results of a search for serendipitous X-ray sources in Chandra fields that show either strong variability or detectable periodicity in their X-ray emission. Our search concentrates on fields toward lower Galactic latitudes for which the exposure time exceeds 25ks. Sources bright enough for time-variability analysis are subject to FFT and Bayesian Blocks analysis to seek out promising candidates for more detailed study. We describe field selection criteria and data analysis methods, and present the findings of this initial phase of the study.

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A Non-equilibrium Ionization Code and its Applications

Li Ji, Q. Daniel Wang, John Kwan (UMass, Amherst)

We have developed a non-equilibrium ionization code based on updated atomic data. A version of the code has been optimized so that the calculation can be done efficiently and accurately enough for comparison with X-ray CCD spectra. We also self-consistently include recombination into highly excited levels, which is important in some non-equilibrium cases but has generally been ignored in the past. We will show example applications to illustrate the characteristics of the code and its combination with various models of gas dynamics. In particular, we will present results from an application to massive stellar cluster winds.

ChESS: ChaMP Extended Stellar Survey

Vinay Kashyap, Nancy Evans, Deron Pease, Amy Mossman (SAO), Richard Gray (Appalachian State Univ.), Jeremy Drake, Belinda Wilkes, Paul Green, Dong-Woo Kim (SAO), ChaMP Collaboration

We present a catalog of stellar X-ray sources serendipitously detected in Chandra data from 1999-2001. We have searched for optical matches to X-ray sources from the Chandra Multi-wavelength Project (ChaMP) using the Tycho, SDSS, and ChaMP optical follow-up catalogs. We find over 250 stellar X-ray sources covering a magnitude range V = 10 - 19. This forms an X-ray selected, but otherwise unbiased, sample of field stars covering a wide range of spectral types, ages, metallicities, and rotation rates. We have classified the spectral types of those stars for which we have optical spectra, and use this set to calibrate the colors of the remainder of the sample. As expected, the majority of the sources are M type stars. We confront the observed sample with predictions from modeled Galactic populations, and discuss the X-ray properties of this sample.

This work was supported by NASA contract NAS8-39073 to the CXC.

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The Metal Abundance in the Interstellar Medium from XMM RGS Spectroscopy of AGN

Edward Lloyd-Davies, Joel Bregman (Univ. of Michigan)

The star formation history of the galaxy has left a substantial imprint on the interstellar medium in the form pollution from metals generated by supernovae. The abundance of metals in the ISM is an extremely important probe of past supernova activity. Optical measurements of metal abundance in the ISM can be uncertain due to corrections for ionization and dust. However absorption at X-ray wavelengths is relatively insensitive to these effects, in principle allowing much greater accuracy. We present the analysis of ISM absorption in XMM-Newton Reflection Grating Spectrometer observations of a sample of bright AGN. We find a systematic increase in measured absorption with observation date which we attribute to an undetermined instrumental problem (possibly analogous to the contamination build up on the Chandra ACIS). To minimize the corrections necessary for this effect we initially consider only sources with high absorption columns and derive O and Fe abundances for these sight-lines. We comment on the implication of these results for models of the star formation history in the Milky Way.

BEHR: Bayesian Estimation of Hardness Ratios

Taeyoung Park (Harvard), Vinay Kashyap, Andreas Zezas, Aneta Siemiginowska (SAO), David van Dyk (UC Irvine), Alanna Connors (Eureka Scientific), Craig Heinke (Northwestern)

We present a generalized and statistically coherent scheme of computing hardness ratios and associated error bars. In this scheme, we model the observed counts as a non-homogeneous Poisson process and exploit sophisticated Bayesian approaches (e.g., Gibbs sampling) to calculate hardness ratios, accounting for local background contamination and effective area variations. We apply this scheme to the simple counts ratio [S/H] as well as its variants, colors [log(S/H)] and fractional difference hardness ratios [(H-S)/(H+S)]. We also perform simulations to compare the new Bayesian methods with the classical method, thereby illustrating that (a) the former provides more accurate estimates of the uncertainties, (b) the mode of the posterior probability distribution function (pdf) is a robust estimator of the hardness ratio, and (c) the pdfs of the colors are the best behaved in the low counts limit.

We apply this method to identify candidate qLMXB's in the globular cluster Terzan 5.

This project is part of the California-Harvard AstroStatistics Collaboration. The authors gratefully acknowledge funding for this project partially provided by NSF grant DMS-01-04129 and by NASA Contract NAS8-39073, and NASA grants NCC2-1350 and NAG5-13056.

Chandra Observations of Dust Grains

Randall Smith (NASA GSFC/JHU)

Chandra observations of X-ray halos have begun to show the power of this technique to measure interstellar grain size distributions and their line of sight positions. I will discuss existing observations of halos around XRBs, including recent observation using the HRC-I, and what they can uniquely tell us about IS grains.

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Probing the Spatial Distribution of the Interstellar Dust Medium by High Angular Resolution X-Ray Halos of Point Sources

Jingen Xiang (Harvard-Smithsonian Center for Astrophysics/MSU), Shuang Nan Zhang (Tsinghua THCA)

The excellent angular and good energy resolution coupled with broad energy coverage, the Chandra ACIS is so far the best instrument for studying the X-ray halos. But the direct images of bright sources obtained with ACIS usually suffer from severe pileup which prevents us from obtaining the halos in small angles. We first improve the method proposed by Yao et al to resolve the X-ray dust scattering halos of point sources from the zeroth order data in CC-mode or the first order data in TE mode with the Chandra HETG/ACIS. Using this method we studied the X-ray dust scattering halos around 17 bright X-ray point sources using Chandra data, including a re-analysis of Cygnus X-1. Using the interstellar grain models of WD01 model and MRN model to fit the halo profiles, we get the hydrogen column densities and the spatial distributions of the scattering dust grains along the line of sights (LOS) to these sources. We find there is a good linear correlation not only between the scattering hydrogen column density from WD01 model and the one from MRN model, but also between NH derived from spectral fits and the one derived from the grain models WD01 and MRN (except for GX 301-2 and Vela X-1). Finally we discuss the possibility of testing the model that the black hole can be formed from the direct collapse of a massive star without supernova using the statistical distribution of the dust density nearby the X-ray binaries.

ChaMPlane Optical Survey

Ping Zhao, Jonathan Grindlay, JaeSub Hong, Silas Laycock, Xavier Koenig, Eric Schlegel, Maureen van den Berg (Harvard-Smithsonian Center for Astrophysics)

The Chandra Multiwavelength Plane (ChaMPlane) survey is a project to systematically identify and analyze the serendipitous X-ray sources in deep galactic plane fields imaged by the Chandra X-ray Observatory in order to determine the populations of accretion-powered binaries in the Galaxy. ChaMPlane consists of an X-ray survey and an optical survey, which is also one of NOAO's Long-term Survey Programs. We have successfully completed this 5-year ChaMPlane Optical Survey, using the NOAO 4-m telescopes with the Mosaic cameras and V, R, I and H$\alpha$ filters at CTIO and KPNO. These Mosaic images provide ground to identify Chandra optical counterparts for spectroscopic follow-up in order to determine the nature of the Chandra sources. The survey produced 65 Mosaic fields covering about 23 square degrees and 154 ACIS observations on 105 distinct Chandra fields in the Galactic plane during Chandra Cycles 1-6. Using 6 Mosaic pointings, we mapped out 2.2 square degrees around the Galactic center to cover 58 Chandra ACIS observations. This is so far the deepest optical survey towards the Galactic center.