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Compton Gamma-Ray Observatory Science Support Center

Introduction to EGRET, EGRET Data Products, and EGRET Data Analysis


  1. Instrument Characteristics
  2. Scientific Objectives and Discoveries
  3. EGRET Data Products
  4. EGRET Analysis and Display Software

1. Instrument Characteristics

EGRET, the Energetic Gamma-Ray Experiment Telescope on CGRO, detected gamma rays in the 20 MeV-30 GeV range. It had a very large field of view, approximately 80° in diameter, although the instrument point-spread function and the effective area degrade signficantly beyond 30° off-axis. The effective area on-axis was more than 1000 cm2 between 100 MeV and 3 GeV. The angular resolution was strongly energy dependent, with a 67% confinement angle of 5.5° at 100 MeV, falling to 0.5° at 5 GeV on axis; bright gamma-ray sources can be localized with approximately 10' accuracy. The energy resolution of EGRET was 20-25% over most of its range of sensitivity. Absolute arrival times for photons were recorded with approximately 50 µs accuracy.

EGRET Instrument EGRET detected gamma rays using a spark chamber for direction measurement and a NaI(Tl) calorimeter, the Total Absorption Shower Counter (TASC), for energy measurement. The spark chamber had interleaved tantalum foils and tracking layers. A fraction of the incoming gamma rays (about 35% above 200 MeV) interacted in the foils to produce high-energy positron-electron pairs, which were tracked through subsequent layers of the spark chamber and absorbed by the TASC at the bottom of the tracker. Reconstruction of the energies and directions of the positron-electron pairs yielded the energies and directions of the incident photons. Background signals from cosmic rays in the magnetosphere have intensities approximately 105 times greater than the celestial gamma-ray emission and therefore EGRET was designed to reject this background very efficiently. The spark chamber was surrounded on the top and sides with a plastic scintillator dome to veto events from charged particles entering from above and the instrument also included a time-of-flight coincidence system to identify down-going events before the spark chamber is triggered. After a trigger, the ions in the spark chamber gas created by the passage of the positron and electron were accelerated by high voltages, creating avalanches of secondary ionizations. The resulting pulses of charge were recorded in the tracking layers for analysis on the ground.

During routine operations, data were also recorded from the anticoincidence dome (raw count rate) and the TASC (spectra). The anticoincidence dome event rate, with 0.25 sec resolution, may be used in the study of gamma-ray bursts and solar flares. TASC spectra, accumulated approximately every 30 sec, permit spectroscopy between 0.6 and 140 MeV. These spectra are useful to search for gamma-ray bursts with no associated BATSE trigger, for example, when BATSE is reading out a previous event, and for the study of the high-energy portion of the solar flare gamma-ray spectrum. If a BATSE trigger signal is received, spectra are recorded in four commandable time intervals (normally 0 to 1 sec, 1 to 3 sec, 3 to 7 sec, and 7 to 23 sec).

The spark chamber gas, a noble gas-hydrocarbon mixture that quenches the high voltage sparks, needed to be changed periodically to limit performance degradation as the gas 'ages' (over the course of approximately a year) from sparking. EGRET was launched with an onboard supply sufficent for five gas refills, more than enough for the 2-year design duration of the mission. Beginning near the end of Phase 3, EGRET was been turned off during viewing periods that did not contain high-priority EGRET targets, or operated in reduced field-of-view mode (with the time-of-flight system configured to trigger the spark chamber only for photons nearly on axis) to conserve the gas. After Cycle 4, all 5 gas changes had been made, and enough gas remained only for a partial refill. In Cycle 8, more than seven years into the CGRO mission, only approximately six weeks of reduced field-of-view observations with EGRET were scheduled. The last viewing periods in Cycle 9 were obtained in full field-of-view mode after the re-entry date of CGRO had been planned.

2. Scientific Objectives and Discoveries

The principal scientific objectives of the EGRET instrument were to perform an all-sky survey of high-energy gamma-ray emission and make detailed studies of high-energy gamma-ray emitting sources.

Major discoveries with EGRET include the identification of blazars, a type of active galaxy classified from optical and radio observations, as prodigious gamma-ray emitters. EGRET has detected dozens of blazars and found them to be quite variable in flux, with flares occurring on time scales of days or hours. EGRET has detected the high-energy tails of several gamma-ray bursts, including in one notable case GeV photons more than one hour after a burst at lower energies. EGRET observations of the LMC and SMC were used to confirm that cosmic rays are Galactic and not metagalactic or universal. EGRET data were used to confirm the well-known, enigmatic source Geminga as a radio-quiet pulsar, the first detected in gamma-rays. With its efficient rejection of background, EGRET has obtained the first sensitive map of the diffuse gamma-ray emission of the Milky Way, emission which is associated with cosmic-ray interactions with interstellar gas and photons, and made a reliable measurement of the isotropic, presumably extragalactic diffuse emission.

3. EGRET Data Products

EGRET data can be obtained via All EGRET data are now publicly available. In 2001 May, the EGRET team delivered to the COSSC the final versions of all data products and these have been incorporated into the archive. The high level data products are indexed in W3Browse. The earlier release of the data is also available available on CD-ROM (through the beginning of Cycle 6). 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° 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:

NOTE that with the final delivery of EGRET data products, several changes were made in the files and filenames in the archive. Briefly, the changes are:

  1. 'WVP' files are no longer included. Formerly the event lists had been split between QVP and WVP files, with the former including events within 30 deg of the instrument axis and the latter containing events at wider angles (which are generally less useful for analysis). Now all events are in the QVP file.
  2. 'QVP' file events are no longer filtered by zenith angle or energy. Formerly, the events had been subjected to the standard cuts on zenith angle (to reduce the earth albedo gamma-ray background) and to the limits of the standard EGRET energy bands. Now all photons detected in the viewing period are included. The 'f' in the file names indicates 'final version.'
  3. 'RDF' files are no longer included. These were a combination of the QVP and exposure history files and contained no additional information.
  4. The GIF preview images have been enhanced.

High-Level Data Available For Each Viewing Period
Filename Description
qvp_vp####f.fits Time-ordered list of all photons detected during the viewing period.
counts_vp####_g00#.fits Maps of photon counts
exposr_vp####_g00#.fits Maps of instrumental exposure
intens_vp####_g00#.fits Maps of gamma-ray intensity
exphst_v01p####.fits Exposure history file for INTMAP

Energy Ranges of the Standard Maps
Extension Energy ranges of maps in file (MeV)
g001 30-50, 50-70, 70-100, 100-150, 150-300, 300-500, 500-1000, 1000-2000, 2000-4000, and 4000-10000
g002 30-100 and 100-10000
g003 30-300 and 300-1000
g004 30-1000 and 1000-10000

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 will contain 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, whether or not the event was subsequently rejected. They also contain records of background spectra recorded every ~30 sec in the TASC, instrument housekeeping information, and the rare gamma-ray events that statisified the Microsecond Burst trigger requirement, used to record events that appear to result from multiple, nearly simultaneous arrivals of gamma rays. As of this writing, delivery of the Primary Database files by the EGRET instrument team is well underway, complete into Cycle 6.

All of the high level data products are produced by the EGRET instrument team. FITS headers for the QVP and exposure history files, as well as the GIF preview images, are generated at the COSSC.

4. EGRET Analysis and Display Software

Listed below are the standard analysis programs provided by the EGRET instrument team. Many of them require the PGPLOT and XView libraries, and some may have Sun OS dependencies. Several of the standard programs will be converted to FTOOLS, and as such will be platform independent. Source code and documentation for most of these programs are also available on CD-ROM.

The limited number of programs currently available as FTOOLS are listed in a separate section below. For each program, documentation is available with the source code. Some support is also available from the CGRO Science Support Center.

Utility programs:

  • POINTEXPOSE - C, FORTRAN, XView, X11 - Calculates the sensitive area and livetimes as a function of time for any point in the sky, useful for variability analyses. (manual, software)
  • PTEXPO - C, FORTRAN, XView, X11 - Calculates the exposure for a given time range for any point in the sky. (manual, software)
  • QUICKLOOK - FORTRAN, FITSIO, PGPLOT, and X11 - Views and selects individual photon events. (manual, software)
  • SKYMAP - IDL - Views and manipulates EGRET FITS maps (manual, software)
  • SKYUTIL - C, XView, X11 - Performs various useful tasks associated with point sources and candidate sources. (software)
  • TRANSMAP - C, FORTRAN, XView, X11 - Reprojects maps between spherical coordinate systems. (manual, software)
  • XPOSE - C, FORTRAN, XView, X11 - Calculates the exposure as a function of time for any point in the sky, useful for variability analyses. (manual, software)

Point Source Analysis:

  • ADDMAP - FORTRAN, XView, X11 (an FTOOL version also exists; see below) - Combines counts, exposure, and intensity maps for different viewing periods. (manual, software)
  • INTMAP - FORTRAN, XView, X11 - Creates exposure and intensity maps using counts maps written by MAPGEN. (manual, software)
  • LIKE - FORTRAN, PGPLOT, and X11 - Performs maximum likelihood searches for point sources in EGRET data using counts and exposure maps and EGRET calibration files. (manual, software)
  • MAPGEN - FORTRAN, X11 - Allows the user to create a custom counts map, e.g. for point source analysis with non-standard energy, spatial, or time binnings. (manual, software)
  • SHOW - FORTRAN, FITSIO, PGPLOT, and X11 - Simple map analysis. (software)
  • SPECTRAL - FORTRAN - Makes spectral fits for point sources, taking into account EGRET's response using forward folding. (manual, software)

Timing Analysis:

  • KBURST - FORTRAN, IDL - Analyzes EGRET photons data for transient sources (up to time scales of orbits) in a given region. (manual, software)
  • PULSAR - C, XView, X11 - The basic pulsar analysis program. Will look for a pulsed signal from known pulsars using the JPL ephemeris data and the Princeton pulsar catalog. (manual, software)
  • TBURST - FORTRAN, XView, X11 - Analyzes EGRET TASC data for transient sources. (manual, software)
  • SEARCH - C, FORTRAN - Period searches for gamma-ray sources. (manual, software)

FTOOLS for EGRET Data Analysis:

The FTOOLS require the FITSIO library. At the time of this writing, the following EGRET FTOOLS are available:

  • EADDMAP - Add or subtract two EGRET maps
  • ECONVPHA - Converts EGRET .spec files to XSPEC .pha files
  • ECONVRMF - Converts EGRET .resp files to XSPEC .rmf files
  • FESDB2RDF - Reads QVP and EXP (optional) data files and writes a FITS data file in OGIP standard format

Ancillary data products:

  • calibration files - Energy dispersion (EDP), point-spread function (PSF), and sensitive area (SAR) files. These contain pre-launch calibration information for various instrument modes. They are needed, e.g., by INTMAP.
  • diffuse model - All-sky model of the diffuse gamma-ray emission, used by LIKE when searching for point sources. The model is in the form of FITS maps for the standard EGRET energy ranges.
  • pulsar timing files - Files needed by pulsar and/or quicklook for epoch folding.
  • timeline - EGRET CGRO timeline file which contains the valid observation times for each viewing period (required, e.g., by INTMAP).

Basic EGRET references

Instrument and calibration:
Bertsch, D.L., et al. 1989, in Proc. Gamma-Ray Observatory Science 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
Esposito, J.A., et al. 1999, ApJS, 123, 203

EGRET source catalog:
Hartman, R.C., et al. 1999, ApJS, 123, 79H

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This page was last modified on Monday, 15-Mar-2010 12:49:19 EDT.

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