EGRETDATA - CGRO/EGRET Photon Lists and Maps

Overview

The Energetic Gamma-Ray Experiment Telescope (EGRET), which operated from 1991-April until 2000-May, was sensitive to photons in the energy range from about 30 MeV to over 20 GeV, the highest energies accessible by the CGRO instruments, and, like COMPTEL, was an imaging instrument. This catalog includes photon lists in qvp files, with one file for each EGRET viewing period. Photons which pass an energy-dependent cut on zenith angle (to exclude earth-albedo gamma rays) are processed into maps of photon counts for a standard set of energy ranges. Exposure and intensity (counts divided by exposure) maps are generated for each counts map. The counts maps typically include photons detected up to 40 degrees from the instrument axis. Most analysis has been done using data within 30 degrees of the instrument axis, where the point-spread function is narrower and the effective area greater than further off axis. All EGRET data files in the archive are the final versions, produced after the end of the mission. More detailed information about EGRET, EGRET data, and EGRET data analysis software can be found at:

    https://heasarc.gsfc.nasa.gov/docs/cgro/egret/egret_doc.html

References

Bertsch, D.L., et al. 1989, in Proc. Gamma-Ray Observatory Science
    Workshop, ed. W.N. Johnson (Greenbelt:NASA), 2-52
Hartman, R. C., et al. 1999, ApJS, 123, 79
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

Provenance

The data contained in this database table was supplied by the CGRO Science Support Center. The contents this table was last updated in May 2001. Galactic coordinates were added to the table by the HEASARC in August 2005.

Data Products

EGRET data products are currently available via the Browse Web interface. All files are in FITS format unless otherwise noted. The various data product types are briefly summarized below. A more comprehensive description is available at https://heasarc.gsfc.nasa.gov/docs/cgro/egret/egret_doc.html.

Observation-Specific Files: Users wanting a quicklook image of the EGRET observation, should examine the EGRET Quicklook (GIF) Map file. For most other users, the standard maps contained in the EGRET Photon Count Map files or the EGRET Intensity Map files will likely be the most suitable data products to examine. Those experienced users desiring to do more detailed screening and/or timing analysis will need to get the EGRET Event List (qvp) file. Most users will not need the EGRET Exposure History (exphst) files. All of these observation-specific files are described in more detail below. The file-naming convention used for them is such that they all contain a 4-digit set of characters that specify the viewing period (usually but not always preceded by the characters 'vp'), e.g., an EGRET Event List file for viewing period 2.0 would have the file name of qvp0020f.fits. Files containing standard maps that have been produced for the standard EGRET energy ranges will further contain a 4-digit set of characters of the form 'g001', 'g002', 'g003', or 'g004' that is used to specify the energy ranges included, e.g.,

                   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

EGRET Quicklook (GIF) Map File: This file has a file name of the type vp####.gif, and contains a GIF image of the EGRET intensity map for energies integrated above 100 MeV.

EGRET Photon Counts and Map Files: These files have file names of the type counts_vp####_g00#.fits and contain the standard EGRET maps of the photon counts for the various standard EGRET energy ranges.

EGRET Exposure Map Files: These files have file names of the type exposr_vp####_g00#.fits and contain the EGRET instrumental exposure maps for the various standard EGRET energy ranges.

EGRET Intensity Map Files: These files have file names of the type intens_vp####_g00#.fits and contain the EGRET gamma-ray intensity maps for the various standard EGRET energy ranges.

EGRET Photon List File (EGRET Events File): This file has a file name of the type qvp####f.fits and contains a time-ordered list of all photons detected during the viewing period. This includes earth albedo photons and wide-angle photons that are not useful for higher-level analysis.

EGRET Exposure History File: This file has a file name of the type exphst_v01p####.fits and contains the exposure history information. This file is used by the EGRET program INTMAP.

EGRET Low-Level Data (Primary Database Files): These files have names like pd_19990511_144958_150037.fits, where the first set of numbers is the year-month-day, the second set is the start time in hours-minutes-seconds UT, and the third set is the end time in hours-minutes-seconds UT, and are FITS versions of the EGRET Primary Database Files that were created by the CGRO Science Support Center. The different kinds of records in the original files have been segregated into different binary table extensions. Within each extension, the records remain time-ordered. The extensions are Gamma_Ray_Event, TASC_Burst_Spectra, Housekeeping, and Microsec_Burst. If no Microsecond Burst records were in the original file, the Microsec_Burst HISTORY header is not written. Since the typical duration of a PD file is several hours, there will be, in general, many such files for a given viewing period.

General Files: There are also available various other general EGRET data products including (i) a set of EGRET Combined Data files that contain composite counts, exposure, and intensity maps covering the entire sky and derived from all viewing periods, Phase 1, Phase 2, etc; (ii) a set of EGRET Diffuse Model Data files that contain an all-sky model of the diffuse gamma-ray emission in the form of FITS maps for the standard EGRET energy ranges; (iii) an EGRET/CGRO Timeline file which contains the valid observation times for each viewing period, and is used by the EGRET programs INTMAP and MAPGEN; and (iv) a set of EGRET calibration files for energy dispersion, point-spread function, and sensitive area.

Parameters

VP
The viewing period of the observation. E.g., viewing period 207.0 translates to a value of 2070 for this parameter.

Proposal_Cycle
A number corresponding to the proposal review and observation scheduling round for CGRO.

RA
Right Ascension of the EGRET pointing in the specified equinox; notice that the original epoch of the positions was J2000.

Dec
Declination of the EGRET pointing in the specified equinox; notice that the original epoch of the positions was J2000.

LII
The Galactic Longitude of the EGRET pointing.

BII
The Galactic Latitude of the EGRET pointing.

Start_Time
The date and time of the beginning of the observation.

End_Time
The date and time of the completion of the observation.

Wide_FOV_Flag
Flag which specifies whether EGRET was in the wide field of view or narrow field of view observation mode; the latter was used extensively later in the mission to conserve spark chamber gas by limiting the number of event triggers. (Y=Wide, N=Narrow)

ObsID
This is a unique tag derived from the viewing period number that distinguishes each EGRET data set from the other data sets in the archive.

Contact Person

Questions regarding the EGRETDATA database table can be addressed to the HEASARC Help Desk.

Page Author: Browse Software Development Team
Last Modified: Monday, 16-Sep-2024 17:26:48 EDT