The Chandra Source Catalog, presently being developed by the Chandra X-ray Center, will be the definitive catalog of
all X-ray sources detected by the Chandra X-ray Observatory. The catalog interface will provide users with a simple
mechanism to perform advanced queries on the data content of the archival holdings on a source-by-source basis for
X-ray sources matching user-specified search criteria, and is intended 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.
For each detected X-ray source, the catalog will record commonly tabulated quantities that can be queried, including
source position, dimensions, multi-band fluxes, hardness ratios, and variability statistics, derived from all of the
observations that include the source within the field of view. However, in addition to these traditional catalog elements,
for each X-ray source the catalog will include an extensive set of file-based data products that can be manipulated
interactively by the catalog user, including source images, event lists, light curves, and spectra from each observation in
which a source is detected.
In this paper, we emphasize the design and development of the Chandra Source Catalog. We describe the evaluation
process used to plan the data content of the catalog, and the selection of the tabular properties and file-based data
products to be provided to the user. We discuss our approach for managing catalog updates derived from either
additional data from new observations or from improvements to calibrations and/or analysis algorithms.
KEYWORDS: Calibration, Error analysis, Monte Carlo methods, Principal component analysis, Data modeling, Sensors, Statistical analysis, Data analysis, Point spread functions, Astronomy
Unlike statistical errors, whose importance has been well established in astronomical applications, uncertainties
in instrument calibration are generally ignored. Despite wide recognition that uncertainties in calibration can
cause large systematic errors, robust and principled methods to account for them have not been developed, and
consequently there is no mechanism by which they can be incorporated into standard astronomical data analysis.
Here we present a framework where they can be encoded such that they can be brought within the scope of
analysis. We describe this framework, which is based on a modified MCMC algorithm, and propose a format
standard derived from experience with effective area measurements of the ACIS-S detector on Chandra that can
be applied to any instrument or method of codifying systematic errors. Calibration uncertainties can then be
propagated into model parameter estimates to produce error bars that include systematic error information.
Antonella Fruscione, Jonathan McDowell, Glenn Allen, Nancy Brickhouse, Douglas Burke, John Davis, Nick Durham, Martin Elvis, Elizabeth Galle, Daniel Harris, David Huenemoerder, John Houck, Bish Ishibashi, Margarita Karovska, Fabrizio Nicastro, Michael Noble, Michael Nowak, Frank Primini, Aneta Siemiginowska, Randall Smith, Michael Wise
The CIAO (Chandra Interactive Analysis of Observations) software package was first released in 1999 following the launch of the Chandra X-ray Observatory and is used by astronomers across the world to analyze Chandra data as well as data from other telescopes. From the earliest design discussions, CIAO was planned as a general-purpose scientific data analysis system optimized for X-ray astronomy, and consists mainly of command line tools (allowing easy pipelining and scripting) with a parameter-based interface layered on a flexible data manipulation I/O library. The same code is used for the standard Chandra archive pipeline, allowing users to recalibrate their data in a consistent way. We will discuss the lessons learned from the first six years of the software's evolution. Our initial approach to documentation evolved to concentrate on recipe-based "threads" which have proved very successful. A multi-dimensional abstract approach to data analysis has allowed new capabilities to be added while retaining existing interfaces. A key requirement for our community was interoperability with other data analysis systems, leading us to adopt standard file formats and an architecture which was as robust as possible to the input of foreign data files, as well as re-using a number of external libraries. We support users who are comfortable with coding themselves via a flexible user scripting paradigm, while the availability of tightly constrained pipeline programs are of benefit to less computationally-advanced users. As with other analysis systems, we have found that infrastructure maintenance and re-engineering is a necessary and significant ongoing effort and needs to be planned in to any long-lived astronomy software.
The Chandra Data Model (CDM) library was developed to support data analysis for the Chandra X-ray Observatory, one of NASA's orbiting Great Observatories. The library and its associated tools are designed to be multi-mission and can be used to manipulate a wide variety of astronomical data. Much of the library's power comes from its use of virtual files, which provide a flexible command-line user interface.
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