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The Compton Observatory Archive

T. McGlynn, E. Chipman,
J. Jordan, N. Ruggiero,
D. Jennings, and T. Serlemitsos

Compton Observatory Science Support Center
Computer Sciences Corporation


Introduction

The Compton Gamma Ray Observatory was launched in April 1991 as the second of NASA's Great Observatories to look at the universe in the high energy regime from 20 keV to 20 Gev. In July 1992, the Compton Observatory Archive was opened to allow public access to gamma-ray data from the Compton Observatory mission. This article describes the current and future contents of this archive, how astronomers may access the data, and the catalog structures which are used to index the data. The remainder of this introduction will briefly describe the overall mission to help define the terms used in this paper.

The Compton Observatory comprises four experiments:

  • BATSE The Burst and Transient Source Experiment, an all-sky monitor covering energies from approximately 20 keV to 1 Mev using eight detectors each looking at one octant of the sky. Each detector has large area scintillator and a smaller, high energy resolution detector.

  • OSSE The Oriented Scintillation Spectrometer Experiment, four independently pointable, collimated scintillators in the 0.1 to 10 Mev regime with a field of view of approximately 4 x 12 degrees.

  • COMPTEL The Imaging COMPton TELescope, a two-interaction telescope which images approximately 1 steradian of the sky with a resolution of approximately 1.5 degrees in the range 1-30 MeV.

  • EGRET The Energetic Gamma Ray Experiment Telescope, a spark-chamber detector which images roughly a steradian field of view at energies of 30 MeV to 30 GeV. The spatial resolution for individual photons is strongly energy dependent but is better than a degree for the highest energy photons.

    All four experiments in the Compton Observatory operate simultaneously and continuously except for passages through the South Atlantic Anomaly. The imaging instruments, EGRET and COMPTEL, are turned off when they would be observing only the Earth. The EGRET and COMPTEL fields of view are aligned with the spacecraft's Z-axis and the OSSE scintillators can be oriented along an approximately 180 degree arc in the X-Z plane. A typical observation period of the observatory consists of a fixed pointing of the spacecraft for one or more weeks with the OSSE experiment looking at a primary source when possible and then slewing to a secondary source when the primary is occulted by the Earth. When OSSE is looking at a source, the scintillators are nodded on and off the source by a few degrees in a pattern set by the observer to determine the instrumental and sky background. During an observation, the EGRET and COMPTEL instruments may have some nominal target in the center of the field of view, but many other sources may also be contained within the field.

    The overall mission comprises a number of phases. A one month verification phase followed the launch where a number of targets were observed briefly. In Phase 1, which will last from May 1991 through November 1992, the COMPTEL and EGRET instruments are making an all-sky survey. Data from this period was proprietary to the instrument teams. In subsequent phases, an increasing percentage of observing time will be allocated to Guest Investigator proposals. In all cases, the data products are proprietary to the PI's or GI's for a period of one year after they are produced in a usable form.

    Routine data products are produced by the Principal Investigator (PI) teams for their own data and that of the Guest Investigators (GIs). The products are available for analysis for one year by the PI or GI under whose program the observation was taken. Then, as the data become non-proprietary, they are sent to the Compton Observatory Science Support Center (SSC) where they are archived. Each instrument team also maintains an archive of data from its own instrument.

    Characteristics of Compton Data

    This section describes the general characteristics of the data available from each instrument. For some of the instruments, only the low-level data products have been defined and the discussion focuses on these.

    All data archived in the Compton Observatory Archive is available in FITS format. Often this is an encapsulation of the PI-developed internal formats. The FITS binary tables extension is used extensively. The columns are described in the FITS headers, and the numeric format of the data uses the IEEE standards and the big-endian byte order as required by FITS. To accommodate archival researchers who may wish to use the software systems developed by the PI teams, software exists to convert these FITS files back to the original PI formats.

    BATSE

    Three different classes of data are available from the BATSE instrument: Continuous, Pulsar data and Burst data. As the name indicates, continuous data are collected continuously for the BATSE instrument. The low level data comprise fairly high time resolution data in four discriminator channels, and high energy resolution data with a temporal resolution of several minutes. Associated high level products may include sources detected through Earth occultation measurements and bursts which did not meet the trigger criteria. Pulsar data are high energy resolution data which are folded on-board to a specified period. Burst data are collected immediately before, during, and after a gamma ray burst, solar flare or other BATSE trigger. Low level burst data comprise more than a dozen datatypes where there is a general tradeoff between temporal and spectral resolution of each type.

    OSSE

    The primary low-level product of the OSSE experiment are the spectra summed over the nodding interval. OSSE typically nods on and off target in two-minute intervals, so these Spectral DataBase (SDB) products consist of two-minute summed spectra. Most analysis will start with these, but several other products are available for study. The Telemetry Scalar File (TSF) contains information which allows studies with time resolutions smaller than the nodding time. The OSSE Pulsar data detects individual events. Other products give shield rates and other information useful for specialized investigations.

    COMPTEL

    The lowest level COMPTEL data which will be available in the archive comprises individual photon events for COMPTEL. For each event a time, position (p), angle alpha, energy and other instrument parameters will be provided. An individual COMPTEL photon is not well-localized on the sky. Rather, for a given photon, what is known is that original location was from an annulus, where the center and diameter of the annulus are given by p and alpha. The width of the annulus is a degree or two. Higher level products, including skymaps, spectra and light curves generated using sophisticated maximum entropy techniques, will also be archived.

    EGRET

    The lowest level of EGRET data also comprise photon events. However, unlike COMPTEL these have more conventional uncertainties in their location and are amenable to more direct methods for generating maps and other higher level products. The EGRET instrument is almost noise-free, i.e., virtually all EGRET gamma-ray events in the observatory archive will represent real cosmic photons. While a significant fraction of the triggers in the instrument are non-gamma-ray events or Earth albedo gammas, these are marked early in the EGRET standard processing. Maps and spectra, light curves and catalogs of individual sources will also be placed in the archive.

    Schedule of Deliveries

    The pipelines for data delivery to the archive have just begun to flow. Below, a tentative schedule of when we currently anticipate the first data of each type is given.

    	Data			Delivered/Expected

    BATSE Burst July 1992 BATSE Pulsar December 1992 BATSE Continuous December 1992 OSSE SDB October 1992 Other OSSE November 1992 EGRET December 1992 COMPTEL December 1992

    How to access the archive

    The Compton Observatory Archive is open to all astronomers. We prefer electronic data retrieval, but when very large quantities of data are to be retrieved, or for users who do not have electronic connections to the data archive, we shall support archive retrieval to physical media on a limited basis.

    Nodes and accounts

    The archive system is currently located on the node enemy of the SSC cluster. A guest account, gof, is available for initial browsing and occasional use of the archive. Users who anticipate frequent or heavy usage of the archive may wish to obtain a personal account on enemy to facilitate their work. This node is a Unix machine but is accessible through both DECnet (SET HOST ENEMY) and TCP/IP (telnet enemy.gsfc.nasa.gov).

    To use the gof account simply login as gof. No password is required. This captive account will present you with a menu of possible options. Simply select the Archive Data Selector option.

    The Data Selector

    Users who wish to retrieve data log into the archive machine as described above, and start the Data Selector task. The Data Selector allows the user to quickly select archive data according to a variety of selection criteria including instrument, target, target type, position, time of observation and data type.

    The Data Selector is an INGRES forms application. The user enters data in the appropriate fields, and a query is made of the Observation Catalog. The Observation Catalog, described in detail below, maintains an index of the archive and describes all of the data currently available.

    Once a user has selected a set of observations of interest, he or she may elect to retrieve the associated filesets. A fileset is simply a group of one or more files which is retrieved as a unit. When using the gof account, files that are retrieved from the archive are retrieved into directories on enemy which are accessible through anonymous FTP or proxy DECnet copies. The archival researcher can use either route to copy the files back to his or her home computer. Users with accounts on the SSC cluster can retrieve data directly to their own areas.

    Data will also be retrievable through the HEASARC Browse system. While the low-level data currently in the archive is not easily comparable with the high-level data products available from other missions, e.g., Einstein and EXOSAT, future products should be much more compatible.

    Grasp

    Data retrieval is actually performed using the Generic Retrieve/Archive Software Protocol (Grasp). Grasp is a protocol which the SSC has developed to attempt to isolate the data retrieval from the underlying details of the archive implementation. Grasp defines a small set of relatively simple functions that the underlying archive will perform and provides the user with a uniform interface to any of the underlying archives.

    We envisage that the underlying data may reside on several different physical archives both simultaneously and serially during the lifetime of the Compton Observatory. Grasp allows us to isolate our dependencies on this heterogeneous and evolving hardware.

    The Physical Archive

    Data in the Compton Observatory Archive is currently stored in two distinct physical archives, a rewritable magneto-optical disk jukebox with a capacity of 92 GB, and the NSSDC NDADS archive. Generally we anticipate faster response from the local jukebox, but the capacity of the NDADS is far greater. When available, data will be retrieved from the local jukebox but the system will automatically retrieve from the NDADS when necessary.

    For the first year or two of the archive we will keep all data received in our local jukebox. As the total volume of data begins to exceed the capacity of our local jukebox only the data most frequently used will be stored locally. The NSSDC NDADS archive will be used as a backup for the local system and to store data which will not fit in our local system. A few data types will be stored only in NDADS, e.g., telemetry. Also, the NSSDC has requested that data be stored in the NDADS in both the native PI formats as well as in FITS when those data are delivered to the SSC in the native format.

    The Observation Catalog

    The Observation Catalog is a relational database which indexes and describes the contents of the archive. It is this catalog with which a user actually has the greatest interaction when retrieving data.

    The overall organization of the catalog is illustrated in Figure 1 where the relationships between the various catalog tables is given. These tables form two clusters, one about the OBSERVATION table and one about the FILESET table. This first cluster contains information about the actions that the satellite has taken and the scientific programs they are intended to support. The OBSERVATION relation itself contains timing, target and program information about the observations of the satellite. The two pointing tables, POINT and OSSEPOINT, describe the specific spacecraft attitudes, and the PROGRAM relation gives information about accepted Compton Observatory observing programs.

    Figure 1

    The cluster around the FILESET relation describes the data in the archive. FILESET directly indexes the archive, with one entry for each archived fileset. The LEVELINFO and TYPEINFO relations describe the types of filesets. The FILELOG relation maintains a record of when filesets have been retrieved. The FILEINFO and INFOTEXT relations allow comments to be made about filesets, e.g., warnings about flaws in the data or data processing. The FILESET_FILES, KEYWORDS and FIXEDFIELDS tables help control the flow of data into the archive, and the update of catalog fields to reflect new archive data.

    The connection between these two clusters is the DATA relation which describes the filesets used in a given observation. A fileset may be used in several observations and conversely, an observation may generate many filesets.

    Archive Data in the HEASARC

    So far, all Compton Observatory data that has been released into the archive is very low-level and quite instrument specific. These datasets will be visible from within Browse but they will not be accessible visually the way other datasets are, e.g., one cannot display spectra or images of the data. As higher level products are produced and placed in the archive such visual browsing of the Compton Observatory products will be supported.

    The SSC has also begun to look at making Compton Observatory data compatible with the formats used in popular astronomical packages such as XANADU and IRAF. The idiosyncrasies of gamma-ray data make this non-trivial, but we expect to introduce capabilities in this area in 1993. We anticipate that this effort will be especially effective for the high-level Compton data products. The SSC is working with other groups in the OGIP to try to make these products compatible with those of other missions such as ROSAT and ASTRO-D.

    Data analysis of archival data

    While the SSC directly supports data in the archive only in FITS formats, converters to reformat the data to the original Principal Investigator formats are available for all datatypes. Thus PI software can be used to analyze all archival data. Archival researchers can analyze their data at the PI team sites or at the SSC. Generally it is advisable for archival investigators who are just beginning work with Compton Observatory data to discuss their research efforts with the Instrument Specialists and PI teams for the instruments they are working on.

    PI software tools are being collected at the SSC for use by Guest and archival investigators. Currently the BATSE Spectral Analysis Software (BSAS), the OSSE IGORE system, and some EGRET software has been brought up on the SSC cluster. Any archival investigator may request an account to use this software. Archival investigators may also wish to discuss how to port appropriate software to their home sites with the PI teams.

    As discussed above, the SSC is working with the HEASARC to make Compton Observatory data usable within more general software packages. We expect that some capabilities to do spectral analysis within XSPEC will be available this winter, and additional functionality will be developed over the coming year.

    Further reading

    Documents which may be used to get further information on topics discussed above are described below. All documents described as SSC internal document are available by anonymous FTP on the enemy and grossc nodes of the SSC network.

    The Compton GRO Mission

    See various articles in The Compton Observatory Science Workshop, NASA Conference Publication 3137, 1992.

    Data produced by the Compton Observatory

    These are defined in the Compton Observatory Project Data Management Plan. Revision due 1992.

    Some data formats for particular instruments

    For low-level BATSE data: GRO BATSE Flight Software User's Manual, MSFC-MNL-1405, 1991.
    For OSSE SDB data: Spectral Data Base Version 7 Data Structure Description, by Mark Strickman, NRL Document 0926-159 Rev 3.00, 1992.
    The GRASP Interface
    This interface is defined in The Generic Archive/Retrieve Software Protocol, SSC internal document.
    The Observation Database
    The Observation Database, including the mechanisms used for maintenance and update, as well as a more complete description of the Data Selection, is described in The Compton Observatory Observation Catalog, SSC internal document.


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