The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (GRO) is sensitive in the energy range from about 20 MeV to about 30,000 MeV. Electron-positron pair production by incident gamma photons is utilized as the detection mechanism. The pair production occurs in tantalum foils interleaved with the layers of a digital spark chamber system; the spark chamber records the tracks of the electron and positron, allowing the reconstruction of the arrival direction of the gamma ray. If there is no signal from the charged particle anticoincidence detector which surrounds the upper part of the detector, the spark chamber array is triggered by two hodoscopes of plastic scintillators. A time of flight requirement is included to reject events moving backward through the telescope. The energy of the gamma ray is primarily determined by absorption of the energies of the electron and positron in a 20 cm deep NaI(Tl) scintillator

Bertsch, D. L.

Fichtel, C. E.

Hartman, R. C.

Hunter, S. D.

Kanbach, G.

Kniffen, D. A.

Kwok, P. W.

Lin, Y. C.

Mattox, J. R.

Mayer-Hasselwander, H. A.

English

NASA Technical Reports Server

N92-21 889 t
THE EGRET HIGH ENERGY GAMMA RAY TELESCOPE
R. C. HARTMAN 1 , D. L. BERTSCH I, C. E. FICIIqIEL I, S. D. HV_,_rER 1, G. KANBACI¢,
D. A. KNIFFEN 8, P. W. KWOK1,3, Y. C. LIN4, J. R. MATI'OX 1'5,
H. A. MAYER-HASSELWANDER 2, P. F. MICIIELSON 4, C. VON MONTIGNY 2, P. L. NOI_N 4,
K. PINKAU 2, H. ROTIIERMEL 2, E. SCtINE1D 6, M. SOMMER 2) P. SREEKUMAR 1'7, AND
D. J. THOMPSON1
1NASA/Goddard Space Flight Center
ZMax-Planck Institut fur Extraterrestrische Physik
3NAS/NRC Postdoctoral Research Associate
4Hansen Experimental Physics Laboratory, Stanford University
sAstronomy Programs, Computer Sciences Corporation
6Grumman Aerospace Corporation
7Universities Space Research Association
8Hampden-Sydney College
ABSTRACT
The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton
Gamma Ray Observatory is sensitive in the energy range from about 20 MeV to
about 30,000 MeV. Electron-positron pair production by incident gamma ray
photons is utilized as the detection mechanism. The pair production occurs in
tantalum foils interleaved with the layers of a digital spark chamber system; the
spark chamber records the tracks of the electron and positron, allowing the
reconstruction of the arrival direction of the gamma ray. If there is no signal
from the charged-particle anticoincidence detector which surrounds the upper
part of the detector, the spark chamber array is triggered by two hodoscopes of
plastic scintillators; a time-of-flight requirement is included to reject events
moving backward through the telescope. The energy of the gamma ray is
primarily determined by abso_tion of the energies of the electron and positron
m a 20 cm deep NaI(T1) scintillator. The Compton Observatory was placed in
orbit on April 6, 1991; EGRET completed activation and on-orbit testing and
calibration on May 16, 1991, at which time it began a 15-month full sky survey.
1. Introduction
Although the SAS-2 and COS-B gamma ray telescopes (Derdeyn et al., 1972; Fichtel et
al., 1975; Bignami et al., 1975; Scarsi et al., 1977) provided the first maps of the high
energy (-100 MeV) gamma ray sky, those observations covered only about 70% of the
celestial sphere. Furthermore, the small size of the instruments limited the statistical
weight of many of the observations, leaving a wide variety of important astrophysical
questions only partially explored. The EGRET telescope on the Compton Gamma Ray
Observatory was designed to provide at least an order of magnitude improvement in
sensitivity over the earlier instruments, as well as better angular and energy resolution
and a very broad energy range. EGRET was developed by a collaboration of 4
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https://ntrs.nasa.gov/search.jsp?R=19920012646 2020-03-17T13:15:59+00:00Z
institutions: NASA/Goddard Space Flight Center (Co-PI, Dr. C. Fichtel), Max-Planck
Institut fur ExtraterrestrischePhysik (Co-PI, Prof. K. Pinkau), StanfordUniversity (Co-PI,
Prof. R. Hofstadter, deceased),and the Grumman AerospaceCorporation.
2.The EGRET Instrument
Since the EGRET instrument has been described in detail previously (Kanbach et al.
1988,1989),only an outline of EGRET operation is provided here. Photonsin the ener_
range covered by EGRET (20 MeV to about 30 GeV), interact almost exclusivelyvm
electron-positronpair production; thus that processis the basisfor gamma ray detection
in EGRET. As illustrated in Figure 1, the gamma ray photons enter through a dome of
plastic scintillator. Having no electric charge,the gammarays produce no signal in the
scintillator. Chargedcosmicrayparticles, which outnumber the gammaraysby a factor of
103to104,do producesignalsin the dome. Thesesignalsareusedto veto the triggering of
the instrument by the charged particles, and therefore prevent them from saturating the
instrument telemetry. The conversion of the gamma rays to electron-positron pairs takes
place within a series of 27 tantalum foils which are interleaved with digital spark
chambers. If the pair particles satisfy the proper coincidence criteria in two layers of
plastic scintillation counters, including a correct time-of-flight between the scintillator
layers, the spark chambers are triggered to produce an image of the tracks of the electron
and positron. In addition to providing a unique means of identify gamma ray events, this
image also defines the direction from which the gamma ray arrived. Except at the lowest
energies, the ener_ of the gamma ray is determined primarily from the energy deposited
by the electron anel positron in a large array of NaI(TI) scintillator blocks (designated the
Total Absorption Shower Counter, or TASC) below the triggering scintillator array. At
energies below about 100 MeV, scattering measurements in the spark chamber and
corrections for energy losses in the tantalum foils are of increasing importance in the
energy determination. Since the efficiency of the spark chamber image formation
degrades slowly with time (believed to be due to the action of sparks on the ethane
quench gas in the neon/argon mixture), EGRET carries a gas supply sufficient for 5
refills.
In addition to providing energy information on events which trigger the spark chamber,
the TASC subsystem has independent modes of operation in which it records spectra of
all events which deposit energy between 1 and about 190 MeV in the NaI. Under
quiescent conditions, these spectra are recorded over 32.768 second time intervals; this
mode is appropriate for recording spectra of gamma rays from solar flares, and is
therefore designated the Solar Mode. When the TASC subsystem receives a signal from
the Burst and Transient Source Experiment (BATSE) indicating that a gamma ray burst
(GRB) has been detected, the TASC electronics switches to a Burst Mode in which
spectra are recorded during 4 previously commanded time inten, als. The commandable
range for each of the 4 intervals is 0.125 second to 15.875 seconds; normally the intervals
are set to 1.0, 2.0, 4.0 and 15.875 seconds. Because the threshold of the TASC spectral
measurements (-1 MeV) is considerably higher than that of the BATSE instrument (-20
keV), EGRET obtains spectral information on only 2-3% of the events for which BATSE
sends a burst trigger signal. In the Solar and Burst Modes, the TASC has essentiallv
omnidirectional response. However, because of intervening material in the spacecraft,
the spectra recorded can be strongly influenced by the direction from which the flare or
burst gamma rays arrive. A relatively detailed Mass Model of the CGRO spacecraft will
117
soon be available which can be used in generating instrument response functions for
specificburst or solar arrival directions.
3. EGRET On-Orbit Operations
Since completion of activation, EGRET operation has been essentially nominal. A
period of severalweeks following initial activation wasspent in optimizing the operating
conditions with regard to triggering requirements, anticoinciaence threshold, Earth
occultation and SouthAtlantic Anomaly (SAA) response. Sincethat time, the instrument
operation hasbeen essentiallystable. The two significant exceptionsare a slow gain drift
in the TASC, for which software corrections are being routinely made based on weekly
on-orbit calibrations, and the expectedslow degradationof the spark chamber _gasnoted
above. The primary effect of the latter is that the number of events for whicn manual
graphic analysis is required has increased by about 25%. Smaller corrections will
probably be required to the instrument sensitivity due to loss of gamma ray event
recognition efficmncy; this correction will be made on the basis of repeatedobservations
on regionsof diffuse flux, which do not changewith time.
The EGRET coincidencesystemhasan option to require a signal from the TASC abovea
specified threshold (1, 2.5, 6 or 13MeV) in order to trigger the spark chamber. During
the initial on-orbit testingof EGRET, it wasconfirmed that requiring the TASC 2.5 MeV
signal in the coincidencereduced the overall triggering rate ot the spark chamber; since
this is expectedto result in a correspondingly increasedgas lifetime, and therefore the
instrument time available to Guest Investigators, the TASC-in-coincidence mode is the
normal mode of operation. As shown below, the TASC-in-coincidence requirement
slightly raises the instrument energy threshold and slightly reduces its sensitivity,
especiallyto wide-angleevents.
Basedon the experiencefrom the first six monthsof operation, the expectedlifetime of a
single gas fill is expectedto be at least 7-8 months, and the total lifetime of the spark
chamber subsystemis therefore expectedto be at least 3.5-4years, roughly twice the 2-
year design lifetime. Thus EGRET is expected to be fully operational well into the
period during which a major share of its time is available to Guest Investigators. An
option which will be consideredfor the latter part of the EGRET lifetime (e.g., after the
lastgasrefill hasbeen made) is the possibledisablingof the spark chambertugger except
during observations of high priority targets, or for important time varlatmn
measurements.
4. EGRET PerformanceCharacteristics
Table I (from Appendix G of the NASA ResearchAnnouncement for Phase2 of the
GRO Guest Investigator Program,NRA 91-OSSA-22)showsthe scientific capabilities of
the principal (spark chambertriggered) mode of EGRET operation, basedon prelaunch
calibrations aswell aspreliminary analysisof on-orbit testingand calibration. The values
for effective area are still consideredpreliminary. Note that for parameterswhich are
significantly affected by the TASC-in-coincidence requirement, values are shown both
with and without that requirement. The parametervalueswithout the TASC requirement
are generally quite close to thosepresented in earlier instrument descriptions (Kanbach
et al. 1988,1989).
118
Table II (also from Appendix G of NRA 91-OSSA-22) shows the preliminary angular
accuracy of EGRET for various energies. For data analysis by Guest Investigators, point
spread functions will be provided for a set of standard energy intervals.
Since the GRO spacecraft is not repointed during Earth occultations, and additionally
because the photomultiplier tubes are turned off during SAA passage, the fraction of time
usable for spark chamber triggered operation is typically 40-45%. With the roughly 85%
livetime fraction typically observed, the effective livetime during a viewing period is found
to be 35-40% of real time.
5. Preliminary Scientific Results from the Initial Observations
A normal GRO observing period is 2 weeks in length. With the -0.5 sr EGRET field of
view, this allows completion of a full-sky survey in the first 16 months of operation (GRO
Phase 1). The first few observation periods were devoted to regions of high scientific
interest which could also provide various types of on-orbit calibration.
By combining the "tune-up" period with a normal 2-week observation and a solar target-
of-opportunity during which the sun was in the galactic anticenter, EGRET obtained very
early in the mission about a month of exposure to that region. Two bright sources, the
Crab pulsar and the enigmatic source known as Geminga, are seen in that region, for
which contour plots in two energy ranges are shown in Figure 2. An obvious difference in
spectrum is apparent in that figure; in the lower energy plot, the Crab pulsar is
considerably brighter than Geminga, while at the higher energies the situation is reversed.
Clearly, the Geminga spectrum is significantly harder than that of the Crab.
Figure 3 shows the phase histogram of the Crab pulsar; for comparison, that of the Vela
pulsar is shown in Figure 4. These two pulsars show much more similarity in gamma rays
than they do in the radio, optical or X-ray bands. Nevertheless, some differences are
apparent. While the first (main) pulse of each is quite sharp, the secondary pulse of the
Crab is much broader than that of Vela. Furthermore, the Vela pulsar shows significant
emission in the interpulse region, while the Crab pulsar shows little or none.
The most surprising observation by EGRET during the early portion of the GRO mission
is the discovery of strong gamma ray emission by the quasar 3C279. During tim latter half
of June 1991, 3C279 was nearly as bright as Geminga; it was not detected in the SAS-2
observations in the early 1970's (Fichtel et al. 1978), and at the times of the COS-B
observations (Mayer-Hasselwander et al. 1985) was at least a factor of 10 less intense than
during the EGRET observations (Hartman et al. 1991). Over the energy range 70 MeV
to 10,000 MeV the photon spectrum of 3C279 fits well to a power law with index -2.0. If
the object radiates gamma rays isotropically, its luminosity at the time of the EGRET
observation was about 1048 ergs/second. Furthermore, EGRET saw statistically
significant evidence for time variations on time scales of a few days.
In contrast with the increased emission from 3C279 since the 1970's, quasar 3C273 was
found by EGRET to be roughly a factor of 4 less intense than at the time of the COS-B
observations.
EGRET has observed gamma radiation to at least 100 MeV from both solar flares and
cosmic gamma ray bursts. In addition to detection in the independent TASC mode, 6
119
gamma rayswere observed in its spark chamber mode from the GRB on 3 May, 1991.
The burst direction obtained from a centroid of the directions of the 6 individual gamma
rays is 1 = 171._ 1.3 degrees, b = 5.3_+ 1.1 degrees.
EGRET has made observations of several regions of the galactic plane. Although
structure is seen in the galactic ridge, the separation of diffuse emission from that of point
sources will require detailed comparison of the gamma ray observations with models of
the gas and cosmic ray content of the galaxy.
6. Summary
The EGRET high energy gamma ray telescope on the Compton Gamma Ray
Observatory is successfully carrying out significant observations on a wide variety of
astronomical phenomena, including solar flares, gamma ray bursts, pulsars, galactic
structure and active galactic nuclei. Within 17 months after launch, it is expected to
complete a survey of the full sky, 30% of which has never been observed in high energy
gamma rays previously.
We wish to acknowledge the essential contributions to the EGRET program over many
years by Drs. Robert Hofstadter and E. Barrie Hughes until their deaths shortly before
tile GRO launch.
REFERENCES
Fichtel, C. E., Hartman, R. C., Kniffen, D. A., Thompson, D. J., Ogelman, K. B., Turner,
T., & Ozel, M. E. 1978, NASA Technical Memorandum 79650, Tabulated Data from
the SAS-2 High Energy Gamma Ray Telescope
Hartman, R. C., et al. 1992, ApJ (in press)
Kanbach, G., et al. 1988, Space Science Reviews, 49, 69
Kanbach, G., et al. 1989, Gamma Ray Observatory Science Workshop Proc., p.2-1
Mayer-Hasselwander, H., et al. 1985, Proc. 19th ICRC, La Jolla, 3, 383
120
Energy Range
Energy Resolution:
Effective Area (cm 2)
Table I - EGRET CAPABILITIES
20 MeV to about 30 GeV
(TASC not in coinc.)
35 MeV to about 30 GeV
(TASC in coinc.)
approx. 22% FWHM over central part of energy range
Energy(MeV) TASC not in coinc. TASC in coinc.
35 400 70 ??
100 1100 900
200 1400 1200
500 1600 1400
3000 1200 1200
10000 700 700
Point Source Sensitivity (E > 100 MeV) 6xi0-8 cm-2 s-I
(for optimum conditions; varies with location and spectrum)
Point Source Location 5-10 arcminutes
(for optimum conditions; varies with intensity, location and spectrum)
Field of View Roughly gaussian in shape, -40 degrees FWHM
Timing Accuracy 0.1 ms absolute
Table II Preliminary EGRET Point Spread Function (degrees)
Energy (MeV) projected rms (degrees)
35 70 4.3
70 150 2.6
150 500 1.4
5O0 2000 0.8
2000 30000 0.4
121
FIGURE CAPTIONS
Figure 1- Schematicdiagramof the EGRET telescope.
Figure 2 - Contour plot of gammaray intensity in the generaldirection of the galactic
anticenter, for two different energyintervals.
Figure 3 - Phasehistogramfor the Crab pulsar, 89bins per period (1 ms/bin), for the
energyrange 100-150MeV.
Figure 4 - Phasehistogramfor the Vela pulsar, 99bins per period (0.3ms/bin), for the
energyrange100-150MeV.
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The EGRET high energy gamma ray telescope

The EGRET high energy gamma ray telescope

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