The Compton Gamma-Ray Observatory (CGRO), one of NASA’s Great Observatories, was launched from the Space-Shuttle Atlantis in April of 1991. The CGRO Science Support Center (COSSC), in collaboration with the HEASARC, has been providing support and archival services since then. During the past year, and continuing for the next several months, a gradual reconfiguration of our archive and the systems that make it available has been evolving. We have accomplished the following:
Several new FTOOLS have been developed (see below) and several others are under way and may be available by the publication of this article. Some of these FTOOLS transform CGRO data for use by standard packages like XSPEC. New non-FTOOL utilities available include a Web-based mission timeline and OSSE field-of-view tools. URLs referred to in the text are collated in Appendix A.This article briefly discusses the scientific objectives of the mission and the four CGRO experiments that comprise it. This is followed by a description, experiment by experiment, of available products and the tools for their use. Finally, appendices give useful ancillary information such as the names and affiliations of Principal Investigators, a selection of Web/FTP sites, e-mail addresses, and a short bibliography.
2.1 CGRO Scientific Discoveries
CGRO data have contributed to a number of recent discoveries in the field of high-energy astrophysics both alone and in concert with data from other missions. Each instrument has contributed its share of discoveries as the following highlights show.
The primary objective of BATSE, the Burst And Transient Source Experiment, is the detection, location and study of gamma-ray bursts. BATSE’s large area detectors (LADs) , spectral detectors (SDs) and versatile data system, provide sensitive and comprehensive observations of gamma-ray bursts. The Earth occultation technique detects transient sources above 20 keV and permits studies of variability in stronger sources. Many pulsars can be observed by BATSE using the temporal characteristics of the sources - data are epoch folded onboard or on the ground. BATSE instrument details are on our Web site (see Appendix A) and in the instrument paper (see Appendix B).3.2 EGRET
The Energetic Gamma Ray Experiment Telescope (EGRET) detects gamma rays using a spark chamber for direction measurement and, at its base, a NaI(Tl) calorimeter, the Total Absorption Shower Counter (TASC), for energy measurement. The spark chamber has interleaved tantalum foils and tracking layers where a fraction of incoming gamma rays interacts to produce high-energy positron-electron pairs, which are tracked through subsequent layers and absorbed by the TASC. Reconstruction on the ground of the energies and directions of the positron-electron pairs yields the energies and directions of the incident photons. A plastic scintillator anti-coincidence shield and time-of-flight measurement system prevent false triggers on charged particles or upward-moving events. A complete description of the EGRET experiment can be found at our Web site and in the instrument paper (see Appendices).
Figure 1:Egret All-Sky Map
The spark chamber gas, a noble gas-hydrocarbon mixture, needs to be changed periodically as the gas 'ages' from sparking. Egret was launched with enough extra gas for five refills. Now, well beyond the nominal three-year lifetime of Egrent, only enough gas remains for a partial refill. As a result, in Cycle 8, only approximately six weeks of reduced field-of-view observations will be scheduled.3.3 OSSE The Oriented Scintillation Spectrometer Experiment, consists of four NaI(Tl)-CsI(Tl) phoswichs and permits offset pointing on two-minute timescales for background accumulations and observations of two sources during each orbit. The detectors are generally pointed in co-axial pairs in the x-z plane of the CGRO spacecraft. The instrument can be operated in an on-source/off-source mode to handle background subtraction from point sources and also a scanning mode to map extended sources. The broad field-of-view enables coverage of diffuse emission from the galactic center and galactic plane for observation of nuclear lines associated with radioactive decay of products from explosive nucleosynthesis and from the interaction of cosmic rays and the interstellar medium. OSSE’s slewing capability permits pointing to selected events (typically Gamma-ray or solar flares) detected by BATSE. 3.4 COMPTEL
Interactions in the telescope occur in a two stage process: first a Compton collision occurs in one of seven (low-Z) liquid scintillators and is usually then completely absorbed in one of fourteen (high-Z) NaI(Tl) scintillators. The unusual aspect of this detector is that the location of the gamma-ray on the sky is given by an annulus and not a point. The correlation of many events enables the localization of point sources or the creation of sky maps. Time-of-flight measurements, pulse shape discrimination and anti-coincidence shields are used to reject background events. CompTel has a wide field of view (about 1 steradian) and an angular resolution under optimal conditions of about 1o. Its energy resolution ranges from 5-10% at 1 MeV. During Phase 1 of the mission, COMPTEL completed the first all-sky survey between 0.75-30 MeV (in four energy bands). In addition, COMPTEL can measure energy spectra of solar flares or bright cosmic gamma-ray bursts between 0.1-10 MeV, and neutrons from solar flares.
Figure 2:First COMPTEL map of the plane of the Milky Way Galaxy in the emission of 1.809-MeV gamma-rays from the radioactive decay of 26-Aluminum
The archive contains public-domain data products from each of the four instruments, packaged into FITS or other standard format. Products are listed by experiment below.Reprocessed data is archived on receipt and BATSE trigger and daily data are now on an accelerated delivery schedule to be available from the archive shortly after acquisition and processing. Most other products, however, become public closer to a year after acquisition although a more aggressive schedule is likely in future.
BATSE trigger data (our most requested data set) has recently been placed on disk as well as in the jukebox, permitting faster access and an on-line backup to speed scientific work and reduce strain on the jukeboxes. Other data may be similarly configured in future. During 1998, most CGRO data will become accessible through the W3Browse system maintained by the HEASARC. Also in 1998 much previously inaccessible or reprocessed data will become available for OSSE and COMPTEL -neither of which has previously provided FITS formatted data.Of special interest, the EGRET low-level ('Primary Database') data set will be made available in its entirety for the first time. These data are undergoing FITS wrapping and transfer from the EGRET archive during Spring of 1998 and will be available shortly thereafter. The Primary Database contains raw EGRET data for reconstructing the tracks of Gamma-ray events. It also contains spectra recorded in the TASC throughout the mission and all housekeeping data. FTP access to all CGRO holdings can be made anonymously. Products can be browsed and downloaded in a variety of forms, including tar files. All data except text files are compressed using the GNU utility GZIP. Access to the data - and to much ancillary information - can also be made through our Web site. Detailed descriptions of the GRO data are contained in 'Appendix G' (the technical appendix to the CGRO NRA) at http://heasarc.gsfc.nasa.gov/docs/cgro/cossc/nra/appendix_g.html, which is available on the Web. Requests for assistance in using the data should be addressed to the COSSC at firstname.lastname@example.org/docs/cgro(see Appendix A of this document for electronic access information) 4.1 BATSE
Data are divided into four groups: trigger, daily, occultation, and pulsar. Each group has structured directories beneath it containing data ordered by an appropriate criterion. Typically, several files of data are necessary to form a complete set of data for a particular entity.Trigger data (ordered by trigger number) are the most-requested data type. Each identified trigger is classified as a flare, burst, sgr (soft gamma-ray repeater), tgf (terrestrial gamma flash), or tagged with the name of a known source. Not all triggers pass the acceptance tests, so not all numbers are present. The 4th burst catalog is available on the Web at http://heasarc.gsfc.nasa.gov/docs/cgro/cossc/batse/4Bcatalog/4b_catalog.html A physical copy can be obtained by sending email to email@example.com/docs/cgro. Each trigger directory contains a collection of FITS files that represent the raw data sources and some higher-level products. Table 4.1-1 summarizes these. Spectral (SD) data are prefixed with an 's' (e.g. 'sher'). Not all data types are available for all triggers due to instrument mode differences. Available data taken prior to the trigger may contain the beginning of the triggering event before it satisfied the triggering criteria. Background-type files can be used to remove background signal levels from the triggered period. The BFITS data files - containing burst and background spectral data as a function of time - and the detector response matrices (DRM) - modeling the instrument response to account for scattering and other effects - are extremely useful for gamma-ray burst analysis. Also, the BFITS and DRM files can be converted to PHA-II and RMF format for analysis with XSPEC using available FTOOLS. File types listed in the table are the most useful. Other types exist and advice on their use is obtainable at firstname.lastname@example.org/docs/cgro. OCCULTATION: The BATSE occultation data set is organized by source, and within the source, by TJD. FTOOLS are available to transform these data sets for use by XSPEC by creating summed PHA files and their associated detector response matrices. Light curves may be obtained from these data. PULSAR: The BATSE pulsar data are organized similarly to the OCCULTATION data set with the difference being in the filename extensions: 'olc' files contain on-board folded light curves, 'pll' files contain pulsar low level data. DAILY: BATSE daily data sets are the largest BATSE data set and have, as a result, more directory levels based on TJD to permit their effective management. (The number of directory levels was increased in 1997). These data are the backdrop against which the triggers occur, and contain lower time-resolution data samples from the BATSE detectors. Generally, only two file types are available: CONT and DISCLA. Figure 3: Batse Light Curve for Trigger 143
Table 4.1-1 Summary of BATSE data files and their contents
YYYY = trigger number, XXXX = source name, TJD = Truncated Julian Day, X = detector number
4.2 EGRET data holdings
EGRET data can be obtained via ftp://heasarc.gsfc.nasa.gov/docs/cgro/pub/data/egret/. The high level data products are indexed in W3Browse and also available on CD-ROM (through the beginning of Cycle 6; see http://heasarc.gsfc.nasa.gov/docs/cgro/cossc/egret/egret_cd.html). The EGRET data area is divided into high level and low level products; the high level products are the most useful for analysis. Within the high_level and low_level directory trees, the data products are stored in subdirectories by observatory operating phase (phase1, phase2, etc.) and viewing period (e.g., pnt_3370 for viewing period 337.0).The high level data products include maps of photon counts, instrument exposure, and gamma-ray intensity (counts divided by exposure) binned in 0.5 deg pixels as well as time-ordered lists of photons including arrival times, energies, directions, and a great deal of additional information. Also included is an 'exposure history' file, used by the INTMAP program (see below) to construct exposure and intensity maps. The standard counts, exposure, and intensity maps are in Galactic coordinates except when the viewing direction was close to the Galactic pole; in these cases, the maps are in celestial coordinates. Maps are available for four standard sets of energy ranges as shown below:
Table 4.2-1 EGRET High-Level Data Available For Each Viewing Period (#####)
Energy Ranges of the Standard Maps
Also available under the high_level directory tree is a set of composite counts, exposure, and intensity maps covering the entire sky derived from all viewing periods in Phase 1, Phase 2, etc. as well as the tables from the Second EGRET Source Catalog (Thompson et al.1995).The low_level directory contains EGRET Primary Database files, organized by observatory operating phase (phase1, phase2, etc.) and viewing period (e.g., pnt_3370 for viewing period 337.0). The Primary Database files contain records for each trigger of the EGRET spark chamber, regardless of subsequent rejection. They contain records of background spectra every ~30 sec in the TASC, instrument housekeeping, and the rare gamma-ray events that satisfy the Microsecond Burst trigger requirement, used to record events that appear to result from multiple, nearly simultaneous arrivals of gamma rays. 4.3 OSSE's data holdings
OSSE's archive is undergoing the final stages of a major reorganization and is being gradually rebuilt with the new data products described below. Some are already on line and the archive will grow gradually. Completion is scheduled for Fall, 1998.Raw OSSE data are stored in VAX/VMS indexed files called SDB (for Spectral Data Base) format. This format is designed for efficient processing by the IGORE (Integrated GRO/OSSE Reduction Environment) analysis package which is currently the only software which can perform in-depth analysis of OSSE data. OSSE data are archived as follows:
Table 4.3-1 OSSE data products available
(1) Products are best used with IGORE; (2) deconvolution performed with E-2 deconvolution model;
TT = a form of the target name e.g. GX 1+4 ; VV = Viewing period; xxxx = target designator (primary, secondary etc.); YDOY = 2-digit year and day-of-year (Jan 1 = DOY 1)
Low-level products must be reconverted to their original format to be used with existing analysis tools and converter software is available for this purpose. COSSC staff can assist with the installation of IGORE on VAX platforms only.The OSSE SDB and PDF files are converted to FITS format at the COSSC. High-level data products and more detailed description of their contents are available through the Web: http://heasarc.gsfc.nasa.gov/docs/cgro/cossc/osse/hilev/hilev.html. Other products are available via anonymous FTP (ftp://heasarc.gsfc.nasa.gov/docs/cgro/compton/data/osse/) and can be searched for with W3Browse (capability will grow as archive is replenished during 1998). Figure 4: Map of the 511 keV emission from the galactic center region constructed from OSSE data. The image was constructed by the OSSE team at the Naval Research Laboratory and Northwestern University. It suggests the existence of an 'annihilation fountain' of positrons from the Galactic Center. 4.4 COMPTEL data holdings
The COMPTEL archive contains both native and FITS data. The FITS data is the native binary data product (which, in its pre-FITS form is intended solely for use by COMPASS - COMPTEL's Oracle-based data analysis system) with ancillary information extracted from the COMPASS database used to create the FITS header and properly describe the data. Only recently have these files been made available to the public via WWW and FTP. W3Browse access should be available by 1998 October 1. The FTP archive is organized by observing cycles (phase 1, phase 2, etc.) and by viewing period (e.g. pnt_3370 for viewing period 337.0).
Table 4.4-1 COMPTEL data by type and name
Except as otherwise noted, each file is available on a per viewing period basis. Typically, the standard high level MLM data in four energy bands is useful for quick look purposes as it contains simple FITS skymap images which overlay counts, intensity, and statistical significance for integrated COMPTEL data.
The major FTOOL package at http://heasarc.gsfc.nasa.gov/docs/software/ftools/gro.html contains a wide variety of tools many of which offer capabilities for the CGRO data - which is also accessible to other analysis systems like XSPEC. For specific assistance in using any CGRO product, staff scientists at the CGRO Science Support Center can be contacted via email@example.com/docs/cgro or grohelp@cossc.
5.1 BATSE FTOOLS AND OTHER ANALYSIS TOOLSThere are several FTOOLS designed specifically for BATSE data, and the data are also convertible for use in other analysis/display systems like XSPEC. In addition several non FTOOL capabilities are available (see table in Appendix A for locations):
5.2 EGRET FTOOLSStandard analysis programs provided by the EGRET instrument team are being converted to FTOOLs. ADDMAP - already converted - is used to combine counts, exposure, and intensity maps. MAPGEN is used to generate counts maps for arbitrary coordinate grids and energy ranges and INTMAP is used to derive corresponding exposure and intensity maps. Other programs, such as SPECTRAL for deriving source spectra, and PULSAR for phase binning of photons from gamma-ray pulsars, may also be converted.
5.3 OSSE FTOOLS
The OSSEDRM FTOOL (under development) will generate an OSSE response matrix, for use with XSPEC, for any source in the OSSE field-of-view. This is useful for source-confused regions. We plan no capability outside IGORE (an IDL-based tool for analysis and display of OSSE data available for use on VAX/VMS systems only) for performing a rigorous deconvolution of the spectra of multiple sources in a single field-of-view. The OSSE Guest Investigator Guide is available through the high-level archive Web pages and is filled with examples of OSSE spectral analysis. High-level PHA & RMF products can be analyzed with XSPEC (available from the HEASARC) and IGORE can also be obtained for analysis and display.5.4 COMPTEL FTOOLS
No FTOOLS are planned for COMPTEL and access to the COMPASS environment is therefore necessary (contact COSSC). Availability of the data in FITS format may stimulate the development of further analysis capability using FITS based methods.
Appendix A - Ancillary information
WEB and FTP site information
Email addresses: firstname.lastname@example.org/docs/cgro or email@example.com/docs/cgro
Fishman, G.J., et al. 1991, in Proc. The Compton Observatory Science Workshop, (NASA CP-3137)
Workshop, ed. W.N. Johnson (Greenbelt:NASA),2-52
Hughes, E.B., et al. 1980, IEEE, Trans. Nucl. Sci., NS-27, 364
Kanbach, G., et al. 1988, Space Sci. Rev., 49, 69
Kanbach, G., et al. 1989, in Proc. Gamma-Ray Observatory Science
Workshop, ed. W.N. Johnson (Greenbelt:NASA),2-1
Thompson, D.J., et al. 1993, ApJS, 86, 629
Thompson, D.J., et al. 1993, ApJ, 415, L13
Thompson, D.J., et al. 1995, ApJS, 101, 259
Thompson, D.J., et al. 1996, ApJS, 107, 227
OSSE ReferencesCameron, R.A. et al., NASA Conf. Publication 3137, p3 (1991)
Johnson, W.N. et al. in Proc. Gamma-Ray Observatory Science Workshop, ed W.N. Johnson (Greenbelt, NASA), 2-22 (1989)
Johnson, W.N. et al., 1992 Astron. & Astrophys. Suppl. 97, p21 Copyright 1992, European Southern Observatory (ESO
Kurfess, J.D. et al. in Proc. Gamma-Ray Observatory Science Workshop, ed W.N. Johnson (Greenbelt, NASA), 3-35 (1989)
Kurfess, J.D. Annuals of the New York Academy of Sciences, V759, p236 (1995)
Kurfess, J.D. 3rd Compton Symposium, Astronomy and Astrophysics Supplement Series, 120, p5 (1996)
Purcell, W.R. et al., NASA Conf. Publication 3137, p15 (1991)
van Dijk, Rob, 1996, "Gamma-Ray Observations of X-Ray Binaries with COMPTEL", Ph.D. thesis.
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