Arnold H. Rots

DETERMINING THE ABSOLUTE ASTROMETRIC ERROR IN CHANDRA SOURCE CATALOG POSITIONS

Although relative errors can readily be calculated, the absolute astrometric accuracy of the source positions in the Chandra Source Catalog (CSC), version 1.0, is a priori unknown. However, the cross-match with stellar objects from the Sloan Digital Sky Survey offers the opportunity to compare the apparent separations of the cross-matched pairs with the formally calculated errors. The analysis of these data allowed us to derive a value of 016 for the residual absolute astrometric error in CSC positions. This error will be added to the published position errors in the CSC from now on, starting with CSC, version 1.1.

The Chandra X-ray Observatory calibration database (CalDB): building, planning, and improving

The calibration database implemented for the Chandra X-ray Observatory is the most detailed and extensive CalDB of its kind to date. Built according to the NASA High Energy Astrophysics Science Archive Research Center (HEASARC) CalDB prescription, the Chandra CalDB provides indexed, selectable calibration data for detector responses, mirror effective areas, grating efficiencies, instrument geometries, default source aim points, CCD characteristics, and quantum efficiencies, among many others. The combined index comprises approximately 500 entries. A standard FTOOLS parametric interface allows users and tools to access the index. Unique dataset selection requires certain input calibration parameters such as mission, instrument, detector, UTC date and time, and certain ranged parameter values. The goals of the HEASARC CalDB design are (1) to separate software upgrades from calibration upgrades, (2) to allow multi-mission use of analysis software (for missions with a compliant CalDB) and (3) to facilitate the use of multiple software packages for the same data. While we have been able to meet the multivariate needs of Chandra with the current CalDB implementation from HEASARC, certain requirements and desirable enhancements have been identified that raise the prospect of a developmental rewrite of the CalDB system. The explicit goal is to meet Chandras specific needs better, but such upgrades may also provide significant advantages to CalDB planning for future missions. In particular we believe we will introduce important features aiding in the development of mission-independent analysis software. We report our current plans and progress.

Chandra Publication Statistics

In this study we develop and propose publication metrics, based on an analysis of data from the Chandra bibliographic database, that are more meaningful and less sensitive to observatory-specific characteristics than are the traditional metrics. They fall in three main categories: speed of publication, fraction of observing time published, and archival usage. Citation of results is a fourth category, but lends itself less well to definite statements. For Chandra, the median time from observation to publication is 2.36 years; after about 7 years 90% of the observing time is published; after 10 years 70% of the observing time is published more than twice; and the total annual publication output of the mission is 60-70% of the cumulative observing time available, assuming a two-year lag between data retrieval and publication.

STATISTICAL CHARACTERIZATION OF THE CHANDRA SOURCE CATALOG

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 ~3900 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 or the 2 Mega-second Deep Field Surveys, 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.

Telescope bibliographies: an essential component of archival data management and operations

Assessing the impact of astronomical facilities rests upon an evaluation of the scientic discoveries which their data have enabled. Telescope bibliographies, which link data products with the literature, provide a way to use bibliometrics as an impact measure for the underlying observations. In this paper we argue that the creation and maintenance of telescope bibliographies should be considered an integral part of an observatorys operations. We review the existing tools, services, and work ows which support these curation activities, giving an estimate of the eort and expertise required to maintain an archive-based telescope bibliography.

Meaningful metrics for observatory publication statistics

Observatories have wrestled for decades with the questions how to measure their importance to the astronomical community, what their scientific impact is, and how their performance in that respect compares to that of other observatories. There is a general sense that the answer is to be found in the publication record - specifically, in the refereed journal articles. However, simple parameters (such as the number of papers) are not helpful, because in isolation (applied to a single observatory) they are meaningless, while in comparison between observatories they are subject to external influences that all but invalidate the comparisons. We were fortunate in having the Chandra X-ray Observatorys bibliographic database with its rich variety of metadata available as a resource for experimenting with more sophisticated metrics. Out of this project we propose a modest set that contains meaningful information when viewed in the isolation of a single observatory as well as in comparison with other observatories. Even so, we urge users not to draw conclusions on the basis of the face value of the comparisons, but only after a serious analysis of potential causes for any differences or similarities. We have designed our metrics to provide useful information in three main areas of interest: speed of publication; fraction of observing time published; and archival usage. The basic measured parameters are the percentage of available observing time published as a function of the datas age, at a few specific age values; the median time it takes to publish observations; and similar parameters for multiple publications of the same observations. Citation of results is a fourth category, but it does not lend itself well to comparisons and defies the search for definite statements.

Observatory bibliographies: not just for statistics anymore

Creating and maintaining an observatory bibliography is labor intensive, but the results can be used for more than accounting purposes. The information gathered during the curating process can be used by data discovery and research tools as well; the key is to collect sufficient metadata during the publication classification phase. The Chandra X-ray Center has taken this approach from the inception of its bibliography and we now have an interconnected web of links which lead researchers to the Chandra Data Archive from many sources. We provide links between datasets and astronomical publications to the Astrophysics Data System (ADS) so users of the ADS can directly access Chandra data associated with a publication. Those same links are used by WebChaser, the Chandra data access tool, so users can directly access articles associated with the data they are reviewing. We are expanding our exchange with the ADS to include details about the observations, proposals and bibliographic classifications related to the data in publications. This information will be used by the ADS to provide new semantic literature search capabilities. These interactions with the ADS and WebChaser have improved scientists’ ability to discover Chandra data in meaningful ways. In this paper we will cover how the Chandra bibliography has grown over the years and the many ways we have used our bibliography metadata for statistics, user services, and data discovery aids.

Chandra data archive operations: lessons learned

We present a discussion of the lessons learned from establishing and operating the Chandra Data Archive (CDA). We offer an overview of the archive, what preparations were done before launch, the transition to operations, actual operations, and some of the unexpected developments that had to be addressed in running the archive. From this experience we highlight some of the important issues that need to be addressed in the creation and running of an archive for a major project. Among these are the importance of data format standards; the integration of the archive with the rest of the mission; requirements throughout all phases of the mission; operational requirements; what to expect at launch; the user interfaces; how to anticipate new tasks; and overall importance of team management and organization.

Observatory bibliographies: a vital resource in operating an observatory

The Chandra Data Archive (CDA) maintains an extensive observatory bibliography. By linking the published articles with the individual datasets analyzed in the paper, we have the opportunity to join the bibliographic metadata (including keywords, subjects, objects, data references from other observatories, etc.) with the meta- data associated with the observational datasets. This rich body of information is ripe for far more sophisticated data mining than the two repositories (publications and data) would afford individually. Throughout the course of the mission the CDA has investigated numerous questions regarding the impact of specific types of Chandra programs such as the relative science impact of GTO, GO, and DDT programs or observing, archive, and theory programs. Most recently the Chandra bibliography was used to assess the impact of programs based on the size of the program to examine whether the dividing line between standard and large projects should be changed and whether another round of X-ray Visionary Programs should be offered. Traditionally we have grouped observations by proposal when assessing the impact of programs. For this investigation we aggregated observations by pointing and instrument configuration such that objects observed multiple times in the mission were considered single observing programs. This change in perspective has given us new ideas for assessing the science impact of Chandra and for presenting data to our users. In this paper we present the methodologies used in the recent study, some of its results, and most importantly some unexpected insights into assessing the science impact of an observatory.

Planning and developing the Chandra Source Catalog

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.