Publications (18)16.08 Total impact
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Article: Statistical Characterization of the Chandra Source Catalog
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ABSTRACT: The first release of the Chandra Source Catalog (CSC) contains ~95,000 X-ray sources in a total area of ~0.75% of the entire sky, using data from ~3,900 separate ACIS observations of a multitude of different types of X-ray sources. In order to maximize the scientific benefit of such a large, heterogeneous data-set, careful characterization of the statistical properties of the catalog, i.e., completeness, sensitivity, false source rate, and accuracy of source properties, is required. Characterization efforts of other, large Chandra catalogs, such as the ChaMP Point Source Catalog (Kim et al. 2007) or the 2 Mega-second Deep Field Surveys (Alexander et al. 2003), while informative, cannot serve this purpose, since the CSC analysis procedures are significantly different and the range of allowable data is much less restrictive. We describe here the characterization process for the CSC. This process includes both a comparison of real CSC results with those of other, deeper Chandra catalogs of the same targets and extensive simulations of blank-sky and point source populations.05/2011; -
Article: TGCat: The Chandra Transmission Grating Data Catalog and Archive
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ABSTRACT: The Chandra Transmission Grating Data Archive and Catalog (TGCat) provides easy access to analysis-ready products, specifically, high-resolution X-ray count spectra and their corresponding calibrations. The web interface makes it easy to find observations of a particular object, type of object, or type of observation; to quickly assess the quality and potential usefulness of the spectra from pre-computed summary plots; or to customize a view with an interactive plotter, optionally combining spectra over multiple orders or observations. Data and responses can be downloaded as a package or as individual files, and the query results themselves can be retrieved as ASCII or Virtual Observatory tables. Portable reprocessing scripts used to create the archive and which use the Chandra X-ray Center's (CXC's) software and other publicly available software are also available, facilitating standard or customized reprocessing from Level 1 CXC archival data to spectra and responses with minimal user interaction.The Astronomical Journal 03/2011; 141(4):129. · 4.03 Impact Factor -
Article: The Chandra Source Catalog
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ABSTRACT: The Chandra Source Catalog (CSC) is a general purpose virtual X-ray astrophysics facility that provides access to a carefully selected set of generally useful quantities for individual X-ray sources, and is designed to satisfy the needs of a broad-based group of scientists, including those who may be less familiar with astronomical data analysis in the X-ray regime. The first release of the CSC includes information about 94,676 distinct X-ray sources detected in a subset of public ACIS imaging observations from roughly the first eight years of the Chandra mission. This release of the catalog includes point and compact sources with observed spatial extents <~ 30''. The catalog (1) provides access to the best estimates of the X-ray source properties for detected sources, with good scientific fidelity, and directly supports scientific analysis using the individual source data; (2) facilitates analysis of a wide range of statistical properties for classes of X-ray sources; and (3) provides efficient access to calibrated observational data and ancillary data products for individual X-ray sources, so that users can perform detailed further analysis using existing tools. The catalog includes real X-ray sources detected with flux estimates that are at least 3 times their estimated 1 sigma uncertainties in at least one energy band, while maintaining the number of spurious sources at a level of <~ 1 false source per field for a 100 ks observation. For each detected source, the CSC provides commonly tabulated quantities, including source position, extent, multi-band fluxes, hardness ratios, and variability statistics, derived from the observations in which the source is detected. In addition to these traditional catalog elements, for each X-ray source the CSC includes an extensive set of file-based data products that can be manipulated interactively. Comment: To appear in The Astrophysical Journal Supplement Series, 53 pages, 27 figures05/2010; -
Article: The Chandra Source Catalog: Source Properties and Data Products
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ABSTRACT: The Chandra Source Catalog (CSC) is breaking new ground in several areas. There are two aspects that are of particular interest to the users: its evolution and its contents. The CSC will be a living catalog that becomes richer, bigger, and better in time while still remembering its state at each point in time. This means that users will be able to take full advantage of new additions to the catalog, while retaining the ability to back-track and return to what was extracted in the past. The CSC sheds the limitations of flat-table catalogs. Its sources will be characterized by a large number of properties, as usual, but each source will also be associated with its own specific data products, allowing users to perform mini custom analysis on the sources. Source properties fall in the spatial (position, extent), photometric (fluxes, count rates), spectral (hardness ratios, standard spectral fits), and temporal (variability probabilities) domains, and are all accompanied by error estimates. Data products cover the same coordinate space and include event lists, images, spectra, and light curves. In addition, the catalog contains data products covering complete observations: event lists, background images, exposure maps, etc. This work is supported by NASA contract NAS8-03060 (CXC).08/2009; -1:187. -
Article: The Chandra Source Catalog: Spectral Properties
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ABSTRACT: The first release of the Chandra Source Catalog (CSC) contains all sources identified from eight years' worth of publicly accessible observations. The vast majority of these sources have been observed with the ACIS detector and have spectral information in 0.5-7 keV energy range. Here we describe the methods used to automatically derive spectral properties for each source detected by the standard processing pipeline and included in the final CSC. Hardness ratios were calculated for each source between pairs of energy bands (soft, medium and hard) using the Bayesian algorithm (BEHR, Park et al. 2006). The sources with high signal to noise ratio (exceeding 150 net counts) were fit in Sherpa (the modeling and fitting application from the Chandra Interactive Analysis of Observations package, developed by the Chandra X-ray Center; see Freeman et al. 2001). Two models were fit to each source: an absorbed power law and a blackbody emission. The fitted parameter values for the power-law and blackbody models were included in the catalog with the calculated flux for each model. The CSC also provides the source energy flux computed from the normalizations of predefined power-law and black-body models needed to match the observed net X-ray counts. In addition, we provide access to data products for each source: a file with source spectrum, the background spectrum, and the spectral response of the detector. This work is supported by NASA contract NAS8-03060 (CXC).08/2009; -1:87. -
Article: X-Ray Emission from a Sample of Young Supernovae
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ABSTRACT: When a massive star produces a powerful stellar wind prior to its supernova event, theory predicts that the collision of the exploded stellar ejecta with the wind leads to a reverse shock, creating soft X-ray emission. To understand the frequency at which luminous young X-ray supernovae occur, we used ROSAT to observe a complete sample of nearby supernovae (vhelio < 1700 km s-1) that occurred in the period 1985.5 through 1994.3, which included eight Type Ia supernovae and 19 Type Ib and Type II events. Three supernovae are detected in this time frame, SN 1987A (LMC), SN 1993J (NGC 3031), and a previously unreported source, SN 1992ad, a Type II supernova in NGC 4411b. No supernova had 0.5-2 keV luminosities exceeding 2 × 1039 ergs s-1, so at the 95% confidence level, the probability of an individual supernova exceeding this luminosity limit is less than 12%. Two of these supernovae had luminosities brighter than 6 × 1038 ergs s-1 and at the 95% confidence level, the probability of a supernova being detected above this luminosity is in the range 8.7%-51%. It is unlikely for young supernovae to be a large component of the Intermediate Luminosity X-Ray Object (IXO or ULX) class, where the luminosity exceeds 2 × 1039 ergs s-1. The rate of successful detections appears to increase for sensitivities in the 1037 ergs s-1 range, especially when obtained close to the time of the event.The Astrophysical Journal 12/2008; 596(1):323. · 6.02 Impact Factor -
Article: The Shape of the Relativistic Iron Kα Line from MCG –6-30-15 Measured with the Chandra High Energy Transmission Grating Spectrometer and the Rossi X-Ray Timing Explorer
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ABSTRACT: We confirm the detection of the relativistically broadened iron Kα emission at 6.4 keV with simultaneous Chandra High Energy Transmission Grating Spectrometer (HETGS) and Rossi X-Ray Timing Explorer Proportional Counter Array observations. Heavily binned HETGS data show a disk line profile with parameters very similar to those previously seen by ASCA. We observe a resolved narrow component with a velocity width of ~4700 km s-1 (FWHM ~ 11,000 km s-1) that is most prominent and is narrower (FWHM ~ 3600 km s-1) when the continuum flux is high. It is plausibly just the blue wing of the broad line. We obtain a stringent limit on the equivalent width of an intrinsically narrow line in the source of 16 eV, indicating little or no contribution due to fluorescence from distant material, such as the molecular torus. Variability studies of the narrow component show a constant iron line flux and variable width indicating the line may be originating from different kinematic regions of the disk.The Astrophysical Journal 12/2008; 570(2):L47. · 6.02 Impact Factor -
Article: Cosmic ray diffusion near the Bohm limit in the Cassiopeia A supernova remnant
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ABSTRACT: Supernova remnants (SNRs) are believed to be the primary location of the acceleration of Galactic cosmic rays, via diffusive shock (Fermi) acceleration. Despite considerable theoretical work the precise details are still unknown, in part because of the difficulty in directly observing nucleons that are accelerated to TeV energies in, and affect the structure of, the SNR shocks. However, for the last ten years, X-ray observatories ASCA, and more recently Chandra, XMM-Newton, and Suzaku have made it possible to image the synchrotron emission at keV energies produced by cosmic-ray electrons accelerated in the SNR shocks. In this article, we describe a spatially-resolved spectroscopic analysis of Chandra observations of the Galactic SNR Cassiopeia A to map the cutoff frequencies of electrons accelerated in the forward shock. We set upper limits on the electron diffusion coefficient and find locations where particles appear to be accelerated nearly as fast as theoretically possible (the Bohm limit).09/2006; -
Article: Models for Nonthermal Photon Spectra
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ABSTRACT: We describe models of nonthermal photon emission from a homogeneous distribution of relativistic electrons and protons. Contributions from the synchrotron, inverse Compton, nonthermal bremsstrahlung and neutral-pion decay processes are computed separately using a common parameterization of the underlying distribution of nonthermal particles. The models are intended for use in fitting spectra from multi-wavelength observations and are designed to be accurate and efficient. Although our applications have focused on Galactic supernova remnants, the software is modular, making it straightforward to customize for different applications. In particular, the shapes of the particle distribution functions and the shape of the seed photon spectrum used by the inverse Compton model are defined in separate modules and may be customized for specific applications. We assess the accuracy of these models by using a recurrence relation and by comparing them with analytic results and with previous numerical work by other authors. Comment: 14 pages, 7 figures, Accepted for publication in ApJ Supplement07/2006; -
Article: Using the Parallel Virtual Machine for Everyday Analysis
CoRR. 01/2005; abs/astro-ph/0510688. -
Article: RXTE, ROSAT and ASCA Observations of G347.3-0.5 (RX J1713.7-3946): Probing Cosmic Ray Acceleration by a Galactic Shell-Type Supernova Remnant
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ABSTRACT: (Abridged) We present an analysis of the X-ray spectrum of the Galactic shell-type supernova remnant (SNR) G347.3-0.5 (RX J1713.7-3946). By performing a joint spectral analysis of data from observations made of G347.3-0.5 using the ROSAT PSPC, the ASCA GIS and the RXTE PCA, we have fit the spectra of particular regions of this SNR (including the bright northwestern and southwestern rims, the northeastern rim and the interior diffuse emission) over the approximate energy range of 0.5 through 30 keV. Based on the parameters of the best fit to the spectra using the SRCUT model, we estimate the maximum energy of cosmic-ray electrons accelerated by the rims of G347.3-0.5 to be 19-25 TeV, assuming a magnetic field strength of 10 microGauss. We present a broadband (radio to gamma-ray) photon energy flux-spectrum for the northwestern rim of the SNR, using a synchrotron-inverse Compton model with a variable magnetic field strength to fit the spectrum. Our fit derived from this model yields a maximum energy of only 8.8 TeV for the accelerated cosmic-ray electrons and a magnetic field strength of 150 microGauss. However, our derived ratio of volumes for TeV emission and X-ray emission (approximately 1000) is too large to be physically acceptable. We argue that neither non-thermal bremsstrahlung nor neutral pion decay can adequately describe the TeV emission from this rim, and therefore the physical process responsible for this emission is currently uncertain. Finally, we compare the gross properties of G347.3-0.5 with other SNRs known to possess X-ray spectra dominated by non-thermal emission. Comment: 46 pages, 16 figures, accepted for publication in the Astrophysical Journal (Volume 593, 10 August 2003 Issue)04/2003; -
Article: MARX 4.0 Technical Manual
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ABSTRACT: This document is part of the MARX software package03/2003; -
Article: Chandra X-Ray Spectroscopy and Imaging of the Galaxy Cluster PKS 0745-191
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ABSTRACT: We present a detailed spectral and spatial analysis of the galaxy cluster PKS 0745-191, using recent observations from the Chandra X-ray Observatory. The data provide information on the temperature and metallicity of the intracluster medium, the distribution of emission throughout the cluster, morphology of the cluster core, and an independent mass estimate which can be compared to that from gravitational lensing. X-ray spectra extracted from the central 300 kpc (~2 arcmin) are well described by a two-temperature plasma and the mean cluster temperature is consistent with previously determined values. The distribution of both temperature and metallicity within the inner 360 kpc (2.3 arcmin) of PKS 0745-191 is probed on scales of 8 arcsec (~20 kpc), yielding a relatively constant abundance throughout the region and a strong temperature gradient (down to ~4-5 keV) within the central 215 kpc. Beta-model fits to the surface brightness profile of PKS 0745-191 indicate that a second beta-model component is required to fit the inner ~10 kpc. Imaging analysis of the cluster core reveals an irregular morphology within the inner 50 kpc, and confirms that the X-ray emission peak lies within 0.5 arcsec (the spatial resolution of Chandra) of both the optical and radio central cD positions. The cluster mass, based on both two temperature and multiphase fits to the X-ray spectrum, is in close agreement with lensing mass estimates of PKS-0745-191. Comment: 30 pages, 4 tables, 10 figures10/2002; -
Article: MARX 3.0 Technical Manual
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ABSTRACT: This document is part of the MARX software package. Copyright (C) 1999, 2000 Massachusetts Institute of Technology. This software was developed by the MIT Center for Space Research under contract SV1-61010 from the Smithsonian Institution. This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. Overview i Overview Part I:02/2002; -
Article: Recent X-Ray Observations of SN1986J with ASCA and ROSAT
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ABSTRACT: We present ASCA and ROSAT observations of SN 1986J covering the period 1991 August to 1996 January. From observations with the ROSAT HRI and PSPC, we find that the 0.5-2.5 keV flux decreased proportional to $t^{-2}$ during this period; the ASCA data are consistent with this result and extend it to the 2-10 keV band. ASCA spectra from 1994 January and 1996 January are consistent with thermal emission from a solar metallicity plasma at an equilibrium temperature kT = 5-7.5 keV, somewhat hotter than that observed from other X-ray supernovae. These spectra also show a clear Fe K emission line at 6.7 keV with FWHM < 20,000 km/s (90% confidence). This limit on the line width is consistent with the reverse shock model of Chevalier & Fransson (1994), but does not rule out the clumpy wind model of Chugai (1993). Comment: 20 pages, 9 postscript figures, latex, uses aastex4.0, submitted to The Astrophysical Journal03/1997; -
Article: Linear stability analysis of spherical accretion flows onto compact objects
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ABSTRACT: The steady state structure and linear stability of spherical accretion flows onto compact objects are investigated over a wide range of accretion rates (ARs) for a variety of cooling functions as a function of the ratio of specific heats (gamma). Both radial and nonradial shock perturbations are considered. The flows become more extended as AR decreases. The cooling function has a significant effect on the extended structure of settling solutions with gamma = 5/3. In contrast, the extended structure of settling solutions with gamma = 4/3 is nearly independent of the form of the cooling function. For both gamma = 4.3 and gamma = 5.3, radial shock oscillation in the fundamental and first overtone modes are destabilized in spherically extended accretion envelopes. The application of these results to postsupernova neutron star accretion flows and to luminosity oscillations in AM Her objects are discussed.09/1992; -
Article: Steady spherical hypercritical accretion onto neutron stars
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ABSTRACT: The present study of hypercritical accretion flows onto neutron stars considers steady-state, spherically symmetric flows whose accretion rate range is characterized by the carrying away of gravitational-accretion energy by neutrinos. The models used encompass pair production, radiation diffusion, and general relativistic effects. While pair pressure dominates throughout the accretion envelope when accretion rates above about 100 solar masses/yr, radiation diffusion becomes important when the accretion rate falls below about 0.001 solar masses/yr. At the lower accretion rates, free fall toward the neutron-star surface stops and an extended, quasi-static, radiation pressure-supported envelope emerges which is probably dynamically unstable.08/1991; -
Article: Low-temperature Galactic fountains
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ABSTRACT: Analytic and one- and two-dimensional hydrodynamic calculations are performed to determine whether or not the Galactic fountain model can explain observations of neutral gas in the Galactic halo. Galactic halo gasdynamics are examined, including thermal conduction, Galactic rotation, and Galactic fountains. Results are presented from numerical modeling of low-temperature Galactic fountains. It is found that the best reproduction of the observations is provided by a model near the transonic regime with a temperature and density at the base of the fountain of 300,000 K and 0.001/cu cm, respectively. The cooling time for the hot gas is 33 million yr and the time for newly formed clouds to return to the disk is 47 million yr, neglecting drag forces.05/1990;
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2008–2011
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Massachusetts Institute of Technology
- Kavli Institute for Astrophysics and Space Research
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