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Compton Gamma-Ray Observatory Science Support Center

EGRET Technical Information

[From Appendix G of the NRA for the CGRO Guest Investigator Program for Cycle 6.]


  1. Introduction and Scientific Objectives
  2. The EGRET Instrument
  3. Instrument Parameters and Capabilities
  4. Data Reduction and Analysis
  5. EGRET Guest Investigator Opportunities
  6. Feasibility Evaluation for EGRET Observations
  7. Specification of EGRET Observations
  8. Data Products to be Provided to EGRET Guest Investigators
  9. Use of EGRET During Cycle 6 and Beyond

A. Introduction and Scientific Objectives

The broad objective of the Energetic Gamma Ray Experiment Telescope (EGRET) is to make a major advance in high energy (20 MeV to about 30 GeV) gamma-ray astrophysics using a gamma-ray telescope with more than an order of magnitude greater sensitivity and better angular and energy resolution than instruments previously flown. The study of the gamma-ray sky reveals the sites of the most energetic interactions occurring in astrophysics. Because these interactions are generally associated with the dynamic, non-thermal processes in nature, gamma-ray astrophysics provides an excellent opportunity to learn about the forces of change occurring throughout the universe. In addition, since high energy gamma rays have a very low interaction cross section, they have a very high penetrating power and can reach the Earth from essentially any part of the Galaxy or universe.

Beginning with compact objects and proceeding to the Galaxy, other galaxies, and the universe, the specific objectives of this experiment are:

  1. To search for localized gamma-ray sources in the energy range from 20 MeV to 30 GeV and to measure the intensity, energy spectrum, position, and possible time variations of each;
  2. To improve the knowledge of the locations and nature of known high energy gamma-ray sources not identified with objects seen at other frequencies;
  3. To examine supernova remnants and search for evidence of cosmic-ray particle acceleration revealed by high energy gamma rays, and, if there are cosmic rays at detectable levels, to study their expansion;
  4. To search for high energy gamma-ray bursts, to study the spectra of low energy gamma-ray bursts, and to contribute to the observation of gamma rays from solar flares;
  5. To obtain a detailed picture of the diffuse high energy gamma-ray emission from the Galaxy, which in turn will provide information of fundamental importance in the study of the dynamics of the Galaxy. (This study should provide a high contrast picture of Galactic structure, including spiral arm segments, clouds, the Galactic center, and the extended disk);
  6. To determine the relative importance of cosmic ray electrons and cosmic ray nucleons throughout the Galaxy by studying the gamma-ray spectrum above 20 MeV and, by combining the results with the continuum radio data, to obtain a better understanding of the Galactic magnetic fields;
  7. To detect and examine the high energy gamma-ray emission from other galaxies, including both normal galaxies and active galaxies; and
  8. To study the diffuse celestial radiation in the energy range above 20 MeV including its energy spectrum and the degree of uniformity, both on small and broad scales (These measurements are of particular importance in relation to cosmological models.).

B. The EGRET Instrument

EGRET is designed to cover the energy range from 20 MeV to about 30 GeV. The instrument uses a multilevel thin-plate spark chamber system to detect gamma rays by the electron-positron pair production process. A calorimeter using NaI(Tl) is placed beneath the instrument to provide good energy resolution over a wide dynamic range. The instrument is covered by a plastic scintillator anticoincidence dome to prevent triggering on events not associated with gamma rays. The combination of high energies and good spatial resolution in this instrument provides the best source positions of any Compton GRO instrument.

The EGRET Instrument
Figure IV-1. The EGRET Instrument.
Click for larger view

The telescope is shown schematically in Figure IV-1. A gamma ray entering the telescope within the acceptance angle has a reasonable probability (about 35% above 200 MeV) of converting into an electron-positron pair in one of the thin plates between the spark chambers in the upper portion of the telescope. If at least one particle of the pair is detected by the directional time-of-flight coincidence system as a downward moving particle, and if there is no signal in the large anticoincidence scintillator surrounding the upper portion of the telescope, the track imaging system is triggered, providing a digital picture of the gamma-ray event, and the analysis of the energy signal from the NaI(Tl) detector is initiated. Incident charged particles are rejected by the anticoincidence dome. Low energy backward-moving charged particles which do not reach the anticoincidence dome are rejected by the time-of-flight measurement. Events other than the desired gamma rays, such as those gamma rays interacting in the thin pressure vessel inside the anticoincidence scintillator, are rejected in the subsequent data analysis.

The directional telescope consists of two levels of a four by four scintillator array with selected elements of each array in a time-of-flight coincidence. The upper spark chamber assembly consists of 28 spark chamber modules interleaved with twenty-seven 0.02 radiation length plates in which the gamma ray may convert into an electron pair. The initial direction is usually determined from the upper spark chamber data. The lower spark chamber assembly, between the two time-of-flight scintillator planes, allows the electron trajectories to be followed, provides further information on the division of energy between the electrons, permits seeing the separation of the two electrons for very high energy gamma rays, and shows the entry points of the electrons into the NaI detector.

The energy of the gamma ray is determined in large part from measurements made in an eight radiation-length thick, 76 cm x 76 cm square NaI(Tl) scintillator crystal below the lower time-of-flight scintillator plane. Spark chamber measurements of Coulomb scattering in the thin plates and position information in the spark chamber system also aid in the energy determination. The energy resolution of the experiment is about 20% (FWHM) over the central part of the energy range. The resolution is degraded to about 25% above several GeV due to incomplete absorption in the NaI calorimeter, and at energies below about 100 MeV where ionization losses in the spark chamber plates comprise an appreciable portion of the total energy. In addition, some particles may completely miss the calorimeter. These losses can be partially recovered through an analysis of the scattering characteristics.

The anticoincidence dome event rate is determined every 1/4 second. These data may be used in the study of gamma-ray bursts and solar flares. Also, the large NaI(Tl) crystal may be used for spectroscopy between 0.6 and 140 MeV independently of the spark chamber system. If a BATSE trigger signal is received, spectra are recorded in four commandable time intervals (normally 0 to 1 sec, 1 to 3 seconds, 3 to 7 seconds, and 7 to 23 seconds) and then sent in the EGRET telemetry stream over the next 35 minutes. In addition, a spectrum between 0.6 and 140 MeV is accumulated every 33 seconds. These spectra are useful for the study of gamma-ray bursts with no associated BATSE trigger, (e.g. when BATSE is reading out a previous event) and for the study of the high-energy portion of the solar flare gamma-ray spectrum. A model of the spacecraft mass and its effect on the gamma-ray spectrum for a specific direction will be available for these studies.

The EGRET operating mode may be altered in two ways. In the common mode, an energy deposition of 7 MeV in the NaI(Tl) calorimeter is required for an event. This mode increases instrument live time by reducing the rate of triggers. It does reduce the sensitivity somewhat (see Table IV-1). For an observation where low energy sensitivity is desired in spite of the poor angular accuracy, it would be reasonable to remove the calorimeter coincidence requirement. The event rate may be reduced by limiting the time-of-flight telescope tile combinations which cause a trigger. The 16 tiles in each of two planes form 96 recognized sub-telescopes which point in 9 different directions, each approximately 0.1 steradians in size. These 9 direction groupings are utilized to limit triggers due to earth albedo gamma rays, but simultaneously maximize celestial exposure. It would be possible to observe with a subset of these direction modes. Vertical only would give the best efficiency. The field of view would be greatly reduced and not available to others.

The principal characteristics of the EGRET instrument are summarized in Section C. The Co-Principal Investigators of EGRET are Dr. Carl E. Fichtel of the NASA/Goddard Space Flight Center (GSFC) and Dr. Klaus Pinkau of the Max Planck Institute for Plasma Physics. Co-Investigators are at the Goddard Space Flight Center, Stanford University, the Max Planck Institute for Extraterrestrial Physics, Grumman Aerospace Corporation and Hampden-Sydney College.

C. Instrument Parameters and Capabilities

  1. Type: spark chambers, NaI(Tl) crystals, and plastic scintillators.
  2. Energy Range: 20 MeV to about 30 GeV.
  3. Energy Resolution: approximately twenty percent over the central part of the energy range.
  4. Total Detector Area: approximately 6400 cm2
  5. Effective Area: approximately 1500 cm2 between 200 MeV and 1000 MeV, falling at higher and lower energies.
  6. Point Source Sensitivity: varies with the spectrum and location of the source and the observing time. Under optimum conditions, well off the galactic plane, it should be approximately 6 x 10-8 cm-2s-1 for E > 100 MeV for a full two week exposure.
  7. Source Position Location: Varies with the nature of the source intensity, location, and energy spectrum from 5 - 30 arcmin.
  8. Field of View: approximately a gaussian shape with a half width at half maximum of about 20. Note that the full field of view will not generally be used.
  9. Timing Accuracy: 0.1 ms absolute
  10. Weight: about 1830 kg (4035 lbs)
  11. Size: 2.25 m x 1.65 m diameter
  12. Power: 190 W (including heater power)

D. Data Reduction and Analysis

The EGRET instrument is a wide field telescope even when only one of its subtelescope directional modes is used. Each observed gamma ray is detected individually. Even though the instrument is designed to detect high energy gamma rays, much less than half of the events resulting from spark chamber triggers satisfy the rigid rules for finally accepted celestial gamma ray events. It is the EGRET Science Team's responsibility, together with their EGRET team of data analysts and programmers, to separate the gamma rays from other triggers and determine within errors the direction and energy of each gamma ray based on carefully quality controlled procedures and the calibration data. An entire observation is analyzed as a set since there is no way to select gamma rays or subsets prior to analysis. The direction will be given in terms of galactic, celestial and Earth coordinates. It is the responsibility of the EGRET Science Team to make a determination of the diffuse celestial radiation, galactic and extragalactic, to the best of their ability for all regions observed. They are also responsible for determining source strengths and spectra in a consistent, carefully reviewed fashion and maintaining a catalog. They will also provide upper limits, when it seems appropriate, for a potential source. When a Guest Observer source has been named and the data from the appropriate observing period analyzed, he or she will be given the information on the relevant photons and the estimated source strength and spectrum or upper limit. These data will be withheld from the public domain including the catalog for the duration of the proprietary period after it is given to the Guest Observer.

The telemetered data from EGRET are organized into packets of 1,786 bytes that span a time interval of 2.048 seconds. Each packet contains a header with the time of day to the nearest second, and spacecraft position, pointing, and earth-center coordinate information. Following the header, housekeeping data consisting, for example, of voltages, currents, pressures, temperatures, command verification, counter rates, and instrument operating modes are included. Spectral data from the Total Absorption Shower (~ 0.6 to ~140 MeV) Counter (TASC) are routinely sent with each set of 16 packets. Likewise, burst data, when present, are encoded into a series of 1,024 packets. The remainder of the packet data consists of individual event information. Events are associated with a triggering of the spark chamber and consist primarily of coordinate readouts of chamber tracks, although other information, including a precise time vernier, the set of sub-telescopes that were triggered, and the TASC energy measurement are part of each event data. Since gamma rays can trigger the detector at any time, the event data are asynchronous to the packet formation. Events are therefore stored in a buffer on-board and are moved from there to the packets during their assembly by the on-board firmware. Up to 12 events of variable length may occur within any packet, and events may be continued from one packet to the next.

Production processing of the telemetry information consists of decoding the packet data, scaling all housekeeping data to engineering units, assembling the TASC spectra from the appropriate set of packets, separating and assembling events along with event related information, and, finally, analyzing each event. During the course of one day, approximately 7.5 Megabytes of housekeeping are accumulated to monitor instrument performance and to determine the sky exposure to gamma rays. The TASC will generate 1,760 spectra, and the spark chamber is triggered approximately 5 x 104 times.

The event analysis is the most critical part of the entire ground analysis system. Each event contains spark coordinate information in two orthogonal views. First, it is necessary to structure the event. This process involves determining which spark coordinates to associate with tracks from the gamma-ray interaction, and then to correlate the multiple track projections in the two views of the read-out. The usual signature of a gamma-ray interaction is an inverted "V" formed by the electron and positron that issue from the point where the gamma-ray conversion occurs.

Once the tracks are properly structured, the event is screened to assure that the appropriate signature has been found and that the interaction occurred within the spark chamber volume. A multiple-scattering analysis is then performed for each track to estimate the energy of each secondary particle. Analysis of the total energy of the event is made using the data from the TASC and correcting for energy loss in the plates between the spark chambers, and for leakage of particles out the side of the detector, using the track information and the scattering results. When both the scattering and energy calculations have been made, a procedure is followed to select the best energy values for the two principal tracks that form the vertex. A least-squares fit to the points of each track in each view is made where the number of points used is based on track straightness. A weighted bisector of the angle between the tracks is constructed for each orthogonal view. This direction, together with the pointing direction of the instrument, is used to determine the arrival direction of the gamma ray. In summary, the event processing provides arrival time, energy, and direction for each gamma ray.

The computer analysis that generates the initial structuring must be closely monitored by trained personnel and knowledgeable scientists who study the two orthogonal views of analyzed events in light of the developed rules and comparisons to calibration data. If improvements are to be made in the identification of the sparks associated with tracks, an analyst can make these changes using an interactive graphics workstation. The production processing system assigns categories of potential structuring problems which are then selectively reviewed manually. In addition, a random selection of events are also reviewed to insure that the system is meeting standards.

The most time consuming task, requiring trained scientists and data analysts, is the individual gamma-ray event studies on the graphics units for those events which have been flagged by the automatic analysis as needing more study for any of several specific reasons. Some of these include: uncertainty in the automatic program regarding whether the event comes from the wall, inability to satisfactorily match the two orthogonal views for any of several reasons, or starting point difficulties. This tedious process must be accomplished for an entire field of view before any of the results from that field of view can be determined since not only gamma-ray identification, but also direction and energy may be affected. The series of steps outlined above must be performed for all photons in the field of view for a given exposure before the sky map can be generated and the point source analysis be done.

The determination of the exposure factors is especially complex for EGRET. The telescope is divided into 96 sub-telescopes that are organized into groups that point in 9 different directions. During any orbit, the directional groups are dynamically commanded "off" and "on" by the Compton GRO computer depending on whether or not the Earth is in the field of view for that direction. The 9 sets of direction groupings are not independent. That is, an event may be able to trigger more than one mode. The direction modes therefore must be considered in combinations. A separate set of instrument performance properties of efficiency, angular resolution, and energy resolution as a function of energy must be referenced for each of 74 combinations in order to construct the exposure. If the TASC threshold or instrument triggering requirements are changed, another set of tables must be generated for use. Typically the determination of exposure will involve integrating the product of live-time, detection efficiency, and solid angle for each bin on the sky, taking into account the time varying mode changes. Any hardware anomalies that require turning off part of the telescope system will require that complete new performance tables be generated from the calibration event data by eliminating events that could not have been detected if the disabled portion of the instrument had not been active. A software system will be in place to generate exposures from the housekeeping data of the instrument, but it must be carefully monitored, especially if trigger requirements or thresholds are changed, or if any of the coincidence tubes have to be turned off.

The combination of overlapping fields of view is desirable in order to increase the statistical significance and, in some cases, to reduce the uncertainties produced by the low sensitivity for an object far from the axis. The analysis of a point source must also take into account the energy dependence of the angular resolution and the background spatial distribution, including its energy dependence.

The result of the standard production processing is a comprehensive EGRET primary database (PDB). From the PDB, a secondary set of databases are constructed which serve as the input for the scientific analysis. The calibration database contains information needed to produce spectra and skymaps and build the event databases. Two other crucial databases are a summary database which is a condensed form of the gamma ray event database and will be the primary input for many scientific analyses and an exposure history database which contains pointing and instrument mode information which is needed for determining the exposure maps.

An extensive library of programs exist for manipulating and analyzing science data. For instance, skymaps can be developed with the programs MAPGEN, INTMAP, and SKYMAP. MAPGEN uses the summary database to create or update gamma-ray skymaps for a user specified range of arrival directions, energy and time ranges in a variety of coordinate systems.. The output of MAPGEN is a FITS format file. The INTMAP module takes the maps created by MAPGEN and utilizes the timeline and exposure history files to create corresponding exposure and intensity maps, again in FITS format. These files can also serve as the input to SKYMAP which is useful for interactively displaying and plotting skymaps. SKYMAP capabilities include rescaling the map, adding contours, and displaying multiple maps. The program SPECTRAL is used for the analysis of the energy spectrum of the detected source.

If the temporal properties of a source are of interest, PULSAR and SEARCH are available to analyze the time series for an observation. PULSAR is an interactive program which will fold the data at an assumed pulsar period and generate the light curve. Proper barycentering, period derivative, and binary orbit corrections are made if needed. For searches over a range of trial frequencies, SEARCH will allow the user to apply statistical tests to the set of arrival times over a user specified range of periods. The significance as a function of frequency is the most important output of SEARCH.

Other programs and tables will be available in the near future. Theses include the numerous tables in the EGRET Phase 1 catalog, a program to give source positions and strengths, a table of estimates of the gamma-ray diffuse radiation, and a program to search for pulsed radiation within a limited range of parameters.

E. EGRET Guest Investigator Opportunities

For Cycle-6 and beyond it is especially important to read Section I of this chapter. The Guest Investigator Program provides for both collaborative and independent modes of participation. In the collaborative mode, the Principal Guest Investigator would choose to work closely with the EGRET scientific team. Note however that team resources in support of such activity are much more limited than in the earlier stages of the mission. Anyone interested such a collaboration should check the appropriate boxes in the "Proposal Type Form" in appendix D of the NRA.

In the independent mode the Guest Investigator will receive support from the Compton Science Support Center.

Some investigations such as the diffuse Galactic and extragalactic emission, the search for and analysis of high energy microsecond gamma ray bursts, and the development of source catalogs are not included in the NASA Research Announcement. The nature and especially the extent and degree of effort involved in these observations and analyses make them inappropriate for guest observers.

Like all experimenters on NASA spacecraft, Compton GRO Instrument Team and Guest Investigators are required to provide a processed and accessible version of data, along with instructions for use, to the National Space Science Data Center and/or other NASA managed public data archive after the proprietary period expires. These data are available upon request to anyone. The Compton Science Support Center provides information on data availability, assistance in retrieving the data, instrument properties and response, and some analysis software.

Guest Investigators visiting the Goddard Space Flight Center will have access to a variety of hardware and software tools available to supplement their own resources. Additionally, most of the EGRET analysis software is available from the CGRO-SSC for installation at ones home institution.

F. Feasibility Evaluation for EGRET Observations

In determining the feasibility of an EGRET observation, it should first be determined whether or not the Compton GRO all-sky survey has obtained the desired data already. The EGRET all-sky survey exposure is typically 6 x 108 cm2s for the central part of the energy range, but is not uniform. The Compton Science Support Center has an on-line facility(1) which may be used to obtain the actual and planned exposure for any specific direction. The proposer should state why additional EGRET observing time, beyond that from the all-sky survey, is being requested. Reasons for new observations could include: variability or suspected variability of the source; requirement for longer exposure or additional exposure time beyond that provided on the source by the all-sky survey; and the desirability of simultaneous observations with other instruments covering different spectral ranges.

Typical observing periods will normally be the equivalent of 2 weeks with full telemetry coverage; so a discussion should be presented showing that the proposed observation can reasonably be expected to succeed given one or more of these standard observing times. If the calculations show that the observation is unlikely to be successful, but the proposer wishes to request it nevertheless, then some justification for this request should be provided (for example: the target is close to an already planned target so no additional observing time is needed; or the science is sufficiently compelling and the target brightness sufficiently uncertain that it is worthwhile to attempt a very uncertain detection).

Variation of effective 
			area with inclination angle
Figure IV-2. Variation of effective area with inclination angle.
Click for larger view

Table IV-1. Preliminary EGRET effective area (on axis)
(TASC not in coin.)
(TASC in coin.)
35 0.3 x 103 cm2 0.07 x 103 cm2
100 1.1 x 103 0.9 x 103
200 1.5 x 103 1.4 x 103
500 1.6 x 103 1.5 x 103
3000 1.1 x 103 1.1 x 103
10,000 0.7 x 103 0.7 x 103

The value quoted above for EGRET sensitivity, of 6x10-8 photons cm-2s-1, refers in general to a 3-sigma detection of a point source located well outside the galactic plane and optimally positioned in the EGRET field-of-view observed for 2 weeks of time. Other considerations apply for other scientific objectives, and must be taken into account in determining the actual feasibility of a proposed observation. For EGRET,
fT = 0.40+/-0.05
is the observing efficiency (due to earth occultation, instrument dead time, and SAA passage) for the observation time T. (Note: the operation of the Compton Observatory without the tape recorders significantly reduces fT below the factor quoted, by a factor which depends on several variables.) It is suggested that the slight azimuthal dependence in sensitivity be ignored and that the effective area be treated as a separable function, i.e.

A(E,theta,phi) = AE(E) A(theta)

Table IV-1 gives the approximate values of AE(E) while Figure IV-2 gives data for A(theta)

The EGRET sky-survey was completed with TASC in coincidence in order to reduce spark chamber triggers and to improve instrument live time. Unless the enhanced effective area available with TASC out of coincidence can be shown to offset these disadvantages, TASC should be in coincidence.

Table IV-2. Preliminary EGRET point spread function (projected rmsvalues).
35 < E < 70 4.3
70 < E < 150 2.6
150 < E < 500 1.4
500 < E < 2000 0.7
2000 < E < 30000 0.4

For purposes of the Phase 4 proposals, the EGRET point spread function (psf) may be considered to be a gaussian, independent of angle with respect to the instrument axis, with preliminary rms values as a function of energy given in Table IV-2. These data will be superseded by more accurate functions to be made available to the guest investigator for the actual data analysis.

To determine the background flux, the COS-B and SAS-2 results can be used. For galactic latitude |b| < 20, the COS-B results(2) have much better statistics than the SAS-2 results(3), but the instrument background is much larger. At high galactic latitude, only the SAS-2 results(4) are useful because of the COS-B instrumental background. The Compton Science Support Center plans to provide a facility which the proposer might use remotely to determine the diffuse background in any direction and at any energy 30 < E < 20,000 MeV.

For the purpose of proposing spectral analysis, the full width at half maximum for the energy dispersion (dE/E) should be considered to be 20% for 200 < E < 3000 MeV, and 25% for 60 < E < 200 or 1000 < E < 30,000 MeV. The detailed EGRET energy dispersion data will be made available to the guest investigators for the actual data analysis.

G. Specification of EGRET Observations

The Compton GRO Observation Definition Form in Appendix D should be submitted by all proposers who request data from EGRET observations of specific targets Proposal Types 1 through 4). The EGRET instrument has well-defined data types available and there are relatively few instrument configurations which can be selected. This makes defining an observation reasonably simple. The quantities which must be specified include:

  1. Target name and pointing for the instrument. Note here that it is not always the case that EGRET observations will have the target centered. If multiple targets are to be observed simultaneously, a pointing for the center of the FOV should be specified so as to allow all targets to be observed. Equinox 2000 celestial and/or galactic coordinates should be used.
  2. Duration of the observation. This should be stated with reference to the discussion of feasibility covered in section F. The time to be entered is the net observing time (i.e. the time during which the target is in view, and the spacecraft is not in the SAA, and telemetry is being received on the ground, so that photons can be collected). At scheduling time, the observation duration may be adjusted to compensate for the loss in sensitivity for off-axis observations shown in Figure IV-2, and for the Compton GRO commanding cycle of days or weeks.
  3. The effective exposure time at launch, with full telemetry coverage, was actually about 0.4 of the number of days of observing due to target occultation, SAA periods, and dead time. At present the efficiency is somewhat less. For example, the net exposure time for a 2-week observing period for an on-axis target with full telemetry coverage would be 0.8 weeks.
  4. Non-standard instrument modes. Proposers should note that such modes may require special calibration data or have other difficulties and therefore require special justification.
  5. Any special requirements for correlative observations or other reasons, such as critical observing times or time separations for variable sources, or for simultaneous observations by one or more other Compton GRO instruments or other observatories.
  6. Expected data products (optional). Use the Compton GRO Data Products Definition Form. See section H for a description of some standard products. Anticipated requests for non-standard products should be included.

H. Data Products to be Provided to EGRET Guest Investigators

The type of data products received will depend upon the requirements of the Guest Investigator both in terms of the requested observation and the resources available to the investigator, however, the intensity or upper limit for the proposed source will always be provided. The data can be distributed both through network retrieval and by tape media.

The production time for most data products is expected to be 3 to 4 months with receipt by the guest investigator following soon after. A large part of this time is for the manual editing and inspection of candidate gamma-ray events.

Documentation will be available through the Compton Science Support Center. This includes descriptions and users guides for data analysis software, information on instrument configuration and specifications, and information regarding the science previously and currently being done with EGRET. Contact the Compton SSC for more information on documentation.

I. Use of EGRET During Cycle 7 and Beyond

Beginning in Cycle 6, an alternate procedure has been followed in order to extend the life of EGRET, wherein observations using only the central telescopes, or at most a subset of the total sub-telescopes, are being encouraged. For any viewing period, one of the following choices is available:

The Solid Angle for EGRET
Figure IV-3. The Solid Angle for EGRET. The solid angle is a complex three-dimensional function of energy; however, the enclosed figure combined with the efficiency curve as a function of energy gives a reasonably good understanding. In this figure, the upper curves for 100 MeV and 1 GeV give the solid angle for the standard mode with all telescopes activated. The lower curves give the solid angle for the same energies with only the central telescopes active. In the "fan" mode described in Section A, the solid angle at 1 GeV is approximately represented in one plane by the upper curve and in the orthogonal plane by the lower curve. At 100 MeV, a similar statement is approximately correct except the true curves are somewhat below the upper curve and somewhat above the lower curve. Detailed sensitivities for the standard energy internals will be provided by the EGRET team.
Click for larger view

  1. Use only the on-axis telescopes to look at a single source or a few sources close together. The approximate solid angle is shown in Figure IV-3. The gas usage is about one-third of that when all modes are activated.
  2. 2. Use the on-axis telescopes plus the two adjacent sets in the same plane. (This mode could, for example, be used to study a region of the galactic plane.) See the caption for Figure IV-3 for an indication of the solid angle. The gas usage is just over one-half of that when all modes are activated.
  3. Use some other telescope combination, such as the on-axis telescopes plus one adjacent set. (There would probably be a small analysis delay since special software runs for the chosen mode would be required.) See Bertsch et al. (1989) for a description of the direction modes.
  4. Use the full compliment of telescopes. This mode will be used sparingly and only in the case of exceptionally strong scientific justification.
  5. 5. Leave the EGRET spark chamber system turned off if the scientific return from a given viewing direction is expected to be low. All other systems remain in their present configuration. The spark chamber system which would only be activated as indicated in the last paragraph or as indicated below. The spark chamber and trigger system can be fully activated relatively quickly (approximately 2 seconds) upon receipt from BATSE of a high intensity burst in the field of view.

Observations requiring the use of EGRET spark chamber system during the remainder of the mission will be allocated on the basis of the scientific merit of a proposal (as determined by peer review), also taking into account the limited resource of the remaining gas supply. It is anticipated that the rate of usage realized will be about 1/3 of the usage rates over the mission prior to Cycle 6.

Some reserve spark chamber system capability will be kept as late in the mission as possible for the observation of a great solar flare or an object of special interest to obtain the greatest possible time differential.


Bertsch, D.L., et al. 1989, Proc. Gamma-Ray Observatory Science Workshop, Ed. W.N. Johnson (Greenbelt, NASA), 2-52

Thompson, D.J., et al. 1993, ApJ Supp. 86, 629-656

1. This facility, and various other utilities and information are available on the CGRO WWW site at:

2. Mayer-Hasselwander, H., et al., 1982, Astron. Astrophys, 105,167.

3. Hartman, R.C., et al., 1979, Ap.J., 230, 597.

4. Fichtel, C.E., et al., 1977, Ap.J., 217.

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