Daniel Dewey

The Chandra High-Energy Transmission Grating: Design, Fabrication, Ground Calibration, and 5 Years in Flight

United States. National Aeronautics and Space Administration. George C. Marshall Space Flight Center (Contract NAS8-38249)

A MILLION-SECOND CHANDRA VIEW OF CASSIOPEIA A

We introduce a million second observation of the supernova remnant Cassiopeia A with the Chandra X-Ray Observatory. The bipolar structure of the Si-rich ejecta (northeast jet and southwest counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

Geometry-defining processors for engineering design and analysis

Critical issues of interactive three-dimensional geometry definition and high-speed parallel computation are addressed in a unified fashion by Geometry-Defining Processors (GDPs). GDPs are microprocessors housed in three-dimensional physical polyhedral packages which can be easily manually assembled or reconfigured to construct approximate scale models of physical objects or domains. An individual GDP communicates with neighboring GDPs in an assembly through optical ports associated with the faces of its package. An assembly of communicating GDPs is able to bothdefine a system geometry and, operating as an optimally connected parallel processor,solve the associated continuum partial differential equations required for design evaluation. Combining simplicity-of-use with efficient computational capabilities, the GDP design system should prove useful in numberous engineering applications.

CHANDRA HIGH-RESOLUTION X-RAY SPECTRUM OF SUPERNOVA REMNANT 1E 0102.2-7219

Chandra High Energy Transmission Grating Spectrometer observations of the supernova remnant (SNR) 1E 0102.2-7219 in the Small Magellanic Cloud reveal a spectrum dominated by X-ray emission lines from hydrogen-like and helium-like ions of oxygen, neon, magnesium, and silicon, with little iron. The dispersed spectrum shows a series of monochromatic images of the source in the light of individual spectral lines. Detailed examination of these dispersed images reveals Doppler shifts within the SNR, indicating bulk matter velocities on the order of 1000 km s-1. These bulk velocities suggest an expanding ringlike structure with additional substructure, inclined to the line of sight. A two-dimensional spatial/velocity map of the SNR shows a striking spatial separation of redshifted and blueshifted regions and indicates a need for further investigation before an adequate three-dimensional model can be found. The radii of the ringlike images of the dispersed spectrum vary with ionization stage, supporting an interpretation of progressive ionization due to passage of the reverse shock through the ejecta. Plasma diagnostics with individual emission lines of oxygen are consistent with an ionizing plasma in the low-density limit and provide temperature and ionization constraints on the plasma. Assuming a pure metal plasma, the mass of oxygen is estimated at ~6 M☉, consistent with a massive progenitor.

An Overview of the Performance of the Chandra X-ray Observatory

The Chandra X-ray Observatory is the X-ray component of NASAs Great Observatory Program which includes the recently launched Spitzer Infrared Telescope, the Hubble Space Telescope (HST) for observations in the visible, and the Compton Gamma-Ray Observatory (CGRO) which, after providing years of useful data has reentered the atmosphere. All these facilities provide, or provided, scientific data to the international astronomical community in response to peer-reviewed proposals for their use. The Chandra X-ray Observatory was the result of the efforts of many academic, commercial, and government organizations primarily in the United States but also in Europe. NASAs Marshall Space Flight Center (MSFC) manages the project and provides project science; Northrop Grumman Space Technology (NGST – formerly TRW) served as prime contractor responsible for providing the spacecraft, the telescope, and assembling and testing the observatory; and the Smithsonian Astrophysical Observatory (SAO) provides technical support and is responsible for ground operations including the Chandra X-ray Center (CXC). Telescope and instrument teams at SAO, the Massachusetts Institute of Technology (MIT), the Pennsylvania State University (PSU), the Space Research Institute of the Netherlands (SRON), the Max-Planck Institüt für extraterrestrische Physik (MPE), and the University of Kiel also provide technical support to the Chandra Project. We present here a detailed description of the hardware, its on-orbit performance, and a brief overview of some of the remarkable discoveries that illustrate that performance.

TGCat *: THE CHANDRA TRANSMISSION GRATING DATA CATALOG AND ARCHIVE

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.

Bursting SN 1996cr's bubble: hydrodynamic and X-ray modelling of its circumstellar medium

SN1996cr is one of the five closest supernovae (SNe) to explode in the past 30 yr. Due to its fortuitous location in the Circinus galaxy at ~3.7 Mpc, there is a wealth of recently acquired and serendipitous archival data available to piece together its evolution over the past decade, including a recent 485-ks Chandra high-energy transmission grating spectrum. In order to interpret these data, we have explored hydrodynamic simulations, followed by computations of simulated spectra and light curves under non-equilibrium ionization conditions, and directly compared them to the observations. Our simulated spectra manage to fit both the X-ray continuum and lines at four epochs satisfactorily, while our computed light curves are in good agreement with additional flux-monitoring data sets. These calculations allow us to infer the nature and structure of the circumstellar medium (CSM), the evolution of the SN shock wave, and the abundances of the ejecta and surrounding medium. The data imply that SN 1996cr exploded in a low-density medium before interacting with a dense shell of material about 0.03 pc away from the progenitor star. We speculate that the shell could be due to the interaction of a blue supergiant or Wolf-Rayet wind with a previously existing red supergiant (RSG) wind. The shock wave has now exited the shell and is expanding in the medium exterior to it, possibly the undisturbed continuation of the dense RSG wind. The narrow lines that earned SN 1996cr its IIn designation possibly arise from dense, shocked clumps in the CSM. Although the possibility for a luminous blue variable progenitor for this Type IIn SN cannot be completely excluded, it is inconsistent with much of the data. These calculations allow us to probe the stellar mass-loss in the very last phases ( 10 6 yr), and provide another means to deducing the progenitor of the SN.

HIGH-RESOLUTION X-RAY SPECTROSCOPY OF SNR 1987A: CHANDRA LETG AND HETG OBSERVATIONS IN 2007

We present an extended analysis of the deep Chandra LETG and HETG observations of the supernova remnant 1987A (SNR 1987A) carried out in 2007. The global fits to the grating spectra show that the temperature of the X-ray emitting plasma in the slower shocks in this system has remained stable for the last three years, while that in the faster shocks has decreased. This temperature evolution is confirmed by the first light curves of strong X-ray emission lines and their ratios. On the other hand, bulk gas velocities inferred from the X-ray line profiles are too low to account for the postshock plasma temperatures inferred from spectral fits. This suggests that the X-ray emission comes from gas that has been shocked twice, first by the blast wave and again by shocks reflected from the inner ring of SNR 1987A. A new model that takes these considerations into account gives support to this physical picture.

X-ray calibration of the AXAF Low Energy Transmission Grating Spectrometer: effective area

The low energy transmission grating spectrometer (LETGS) on board the Advanced X-ray Astrophysics Facility provides high resolution dispersive spectroscopy between 70 eV and more than 7 keV. The LETG contains 180 grating modules, each equipped with 3 grating facets. The freestanding gold gratings have 1008 lines per mm. Early 1997, the AXAF telescope underwent extended calibrations in the long beam X-Ray Calibration Facility at the NASA/Marshall Space Flight Center. As part of the telescope, also the performance of the LETGS with respect of spectral resolving power and effective area was measured. At more than 50 individual energies we have measured the grating efficiency or the effective area of the spectrometer, respectively. All these energies were chosen in order to cover the numerous spectral features due to absorption edges of filters, detector coatings, mirror reflectivities, and grating efficiency variations. Although preliminary, the performance of the gratings is close to the predictions made on the basis of subassembly measurements of individual grating elements. In particular, the first order efficiency is about 15% (both sides including vignetting effects) outside the energy regime of partial transparency of the grating wires; inside the efficiency gains from constructive interference effects. Both first diffraction orders are symmetric within less than 1%. The second order is suppressed by a factor of about 200 relative to the first order.

Raytracing with MARX: x-ray observatory design, calibration, and support

MARX is a portable ray-trace program that was originally developed to simulate event data from the trans- mission grating spectrometers on-board the Chandra X-ray Observatory (CXO). MARX has since evolved to include detailed models of all CXO science instruments and has been further modified to serve as an event simulator for future X-ray observatory design concepts. We first review a number of CXO applications of MARX to demonstrate the roles such a program could play throughout the life of a mission, including its design and calibration, the production of input data products for the development of the various software pipelines, and for observer proposal planning. We describe how MARX was utilized in the design of a proposed future X-ray spectroscopy mission called ÆGIS (Astrophysics Experiment for Grating and Imaging Spectroscopy), a mission concept optimized for the 0.2 to 1 keV soft X-ray band. ÆGIS consists of six independent Critical Angle Transmission Grating Spectrometers (CATGS) arranged to provide a resolving power of 3000 and an effective area exceeding 1000 cm2 across its passband. Such high spectral resolution and effective area will permit ÆGIS to address many astrophysics questions including those that pertain to the evolution of Large Scale Structure of the universe, and the behavior of matter at very high densities. The MARX ray-trace of the ÆGIS spectrometer yields quantitative estimates of how the spectrometer’s performance is affected by misalignments between the various system elements, and by deviations of those elements from their idealized geometry. From this information, we are able to make the appropriate design tradeoffs to maximize the performance of the system.