doscope, a NaI(Tl) calorimeter known as TASC (Total. Absorption Shower Counter), and a plastic scintillator an- ticoincidence dome to reject charged particles.
Performance of the EGRET Astronomical Gamma Ray Telescope P. L. Nolan , D. L. Bertsch , C. E. Fichtel , R. C. Hartman , R. Hofstadter , E. B. Hughes , S. D. Hunter , G. Kanbach , D. A. Kni en , Y. C. Lin , J. R. Mattox , H. A. Mayer-Hasselwander , P. F. Michelson , C. von Montigny , K. Pinkau , H. Rothermel , E. Schneid , M. Sommer , P. Sreekumar , D. J. Thompson a
b
a;h
b
b
c
c
b;g
b;f
c
b
Stanford University, NASA Goddard Space Flight Center, Max Planck Institut fur Extraterrestrische Physik, Hampden-Sydney College, Grumman Aerospace, Computer Sciences Corporation, USRA deceased a
c
a
a
e
a;h
d
c
c
b
c
b
d
e
f
g
h
Abstract On April 5, 1991, the Space Shuttle Atlantis carried the Compton Gamma Ray Observatory (CGRO) into orbit, deploying the satellite on April 7. The EGRET instrument was activated on April 15, and the rst month of operations was devoted to veri cation of the instrument performance. Measurements made during that month and in the subsequent sky survey phase have veri ed that the instrument time resolution, angular resolution, and gamma ray detection eciency are all within nominal limits. EGRET picture
I. Introduction NASA's Compton Gamma Ray Observatory, launched on April 5, 1991, is a large (16000 kg) satellite designed to study cosmic gamma rays over a broad energy range (30 keV to 30 GeV). It is currently engaged in a 15-month survey of the entire sky, and then it will return to targets of particular interest. Of the four scienti c instruments on the CGRO, the Energetic Gamma Ray Experiment Telescope (EGRET), studies the highest-energy photons ( 20 MeV) [1]. As shown schematically in Figure 1, EGRET includes a multilayer spark chamber, a plastic scintillator trigger hodoscope, a NaI(Tl) calorimeter known as TASC (Total Absorption Shower Counter), and a plastic scintillator anticoincidence dome to reject charged particles. A gamma ray will pass through the plastic veto dome and then interact in one of the thin tantalum foils between the layers of the upper spark chamber. The secondary electron and positron may trigger the hodoscope, which consists of a 4 4 array of plastic scintillator tiles in the middle of the spark chamber volume and a similar array at the bottom. A coincidence signal from combinations of tiles selected by preset electronics or command, together with a time of ight signature indicating downward-moving par>
This work was supported by NASA contract NAS5-27557 and grant NAG5-1605
Figure 1: Schematic view of EGRET detector ticles, initiates the spark chamber high voltage pulse and later the readout of the spark chamber and energy data [2]. The recorded spark chamber X and Y projected views, energy information, gamma ray arrival time, and auxiliary information are then transmitted to the ground as one \event."
Figure 2: Average light curve of the Crab Pulsar for photons with energy greater than 100 MeV, as measured on April 21 to May 6, 1991. Each histogram bin corresponds to 0.33 milliseconds.
II. Event Detection and Recognition Eciency Not all events are caused by gamma rays. The data analysis system on the ground must select the events which are unmistakably recognized as pairs produced by gamma rays, and extract from these the arrival direction and energy of each gamma ray. A small fraction of gamma-ray events will be rejected by this procedure. The recognition of gamma ray events is a dicult problem in pattern recognition. The SAGE (Structure and Analysis of Gamma Events) analysis code is the product of a long development beginning with balloon-borne instruments and the SAS-2 satellite in the 1970's. It contains the distilled experience of several physicists who have built spark chambers and studied thousands of event \pictures." The capabilities of SAGE contribute to the detection eciency of EGRET just as do any of the instrument's physical parameters. The behavior of SAGE has been tested extensively on real gamma ray data from a calibration of EGRET at SLAC in 1986 [3, 4], in which there was a nearly pure and monoenergetic gamma ray beam. The automatic classi cation was reviewed by EGRET scientists and trained analysts. SAGE's parameters were adjusted and additional rules were added to handle the patterns observed in the spark chamber pictures. SAGE was able to recognize and analyze 88% of 200 MeV events at normal incidence. The automatic analysis missed less than 6% of the events that could be recognized by human analysts. In ight, the volume of data precludes human analysis of all the events. SAGE accepts or rejects the obvious cases and reserves the questionable events for the analysts. In addition, a randomly selected sample of 1% of all events is
scanned to maintain a check on the performance of SAGE. Some new data patterns were found in the rst month of
ight data which required further additions to the SAGE algorithm. A sample of events from early June showed the following behavior: SAGE rejected 80.5% of all events and accepted 7.5%. The remaining 12.0% were submitted to analysts, who accepted 2.5% and rejected 9.5%. Thus a total of 10% of all events are accepted as good gammarays. Of the rejected events, the majority are rejected because the primary particles appear to be coming through the back or sides of EGRET, or because there are too few or too many sparks to distinguish tracks. The ability of SAGE to recognize gamma-ray events depends somewhat on the completeness and cleanness of the spark chamber pictures. A spark chamber layer may not have a spark detectable by the readout electronics in either the X or Y view, or both; in addition, spurious \sparks" may appear. When there are many such occurrences, it becomes dicult to recognize charged-particle tracks. The spark chamber eciency may be adjusted by changing the spark high voltage and the threshold of the sense ampli ers. However, there is a tradeo between eciency and lateral spreading of sparks, which reduces the accuracy with which tracks can be located. Spark chamber performance degrades in the long run because of gas contaminants that are formed by the sparks. Since launch the spark chamber performance has degraded as expected, while the gamma ray detection eciency has declined only slightly. Between June and August the fraction of events requiring review rose from 12.0% to 15.6%. The degradation is being monitored continuously, and we project that the chamber gas will not need to be replaced before December, 1991. The tanks contain enough gas for ve re lls in ight. This leads to an estimate of about four years for EGRET's useful lifetime.
III. Time, Energy, and Direction Measurement EGRET's ability to measure the arrival time, energy, and direction of gamma rays is best illustrated by examining some preliminary scienti c results. In all respects the performance is consistent with what was expected before launch. In order to follow the pulsations of the fastest pulsars, EGRET records the time of each event with a precision of 8 microseconds. The spacecraft clock is claimed to have an absolute accuracy of 50 microseconds, after several in ight adjustments. Figure 2 illustrates that the necessary accuracy has been achieved. This Figure shows the average light curve of the Crab Pulsar for photons with energy 100 MeV. The sharpness of the main peak con rms that the spacecraft timing is stable to less than a millisecond over the whole two-week exposure. EGRET's sensitivity and energy-measuring capability >
ness of Geminga's spectrum is demonstrated by its relative weakness in the low-energy band. The gure illustrates that EGRET's point spread function becomes sharper with increasing gammaray energy. There are two causes for this e ect: (1) When the electron-positron pair is produced, a random (and unmeasured) amount of momentum is transferred to a nearby nucleus. This fraction becomes relatively smaller as the photon energy increases. (2) The electron and positron undergo multiple scatterings in the conversion foils before their directions can be measured. The scattering angles decrease for higher energy. The measured point spread functions in ight are consistent with measurements made on the ground. The alignment of the spark chamber system can be checked by observing a known point source. The estimated position of the Crab Pulsar is in error by only 3.3 arc minutes, better than the predicted accuracy of 5 arc minutes. Such accurate position determinations can be obtained only for bright sources with hard spectra, well away from regions of high di use ux.
IV. Low-Energy Mode
Figure 3: Energy spectrum of the quasar 3C279 as measured on June 15-28, 1991. are illustrated in Figure 3, which shows the di erential energy spectrum of the quasar 3C279, observed on June 15{28 [5]. This quasar, with a redshift of 0.538 (distance 5 109 light years), is the most distant source of highenergy gamma rays detected so far. The ux of 2 8 10?6cm?2s?1 ( 100 MeV) is about 70% of the Crab's. Had it been this bright in the 1970's when the COS-B satellite was operating, it would have been detected easily. The spectrum is consistent with a power law with exponent ?2 020 07 over the energy range 50 MeV to 10 GeV. Only two photons were detected in the 5{10 GeV range, but the background is negligible in that band. Figure 4 is a pair of images of the Galactic anticenter region in two di erent energy bands, 50{100 and 500{1000 MeV. The two prominent peaks in the map are strong point sources, the Crab Pulsar and Geminga. The hard>
:
E >
:
:
In addition to the spark chamber system, EGRET has an independent low-energy mode. This system consists of the NaI calorimeter used with the spark chamber, a pair of pulse-height analyzers (PHA) covering the 1{140 MeV range, and four rate counters with time resolution 2.048 seconds for four energy thresholds. In normal operation the PHAs produce a 256-channel spectrum every 32.768 seconds. This is useful for studying solar ares or other long-lived phenomena. When the BATSE instrument on CGRO detects a gamma ray burst, EGRET accumulates four low-energy spectra, each for a time interval selectable from 0.125 to 15.785 seconds. The TASC calorimeter is buried inside the CGRO spacecraft, so any photons which reach it must pass through a considerable amount of material. The distortions in the spectrum caused by this can be undone only by detailed modeling of all the masses along the particular line of sight. A good model of the spacecraft mass distribution has recently become available, so it will soon be possible to deconvolve the incident spectra of bursts and ares. EGRET's ability to observe cosmic gamma ray bursts is illustrated in Figure 5, which shows the observations of the May 3 burst [6]. The upper panel shows the counting rates above the thresholds in the low-energy analyzer. There is clear evidence for emission above 20 MeV. The next panel shows the counting rate in the plastic anticoincidence dome, which responds mainly to hard X-rays. Finally, at the bottom are shown the arrival times of gamma rays detected in the spark chamber.
Figure 5: The cosmic gamma ray burst of 3 May, 1991. The upper panel shows the counting rate in the large NaI calorimeter with four di erent energy thresholds. The second panel shows the counting rate in the plastic anticoincidence dome, which responds mainly to hard X-rays. At the bottom are the arrival times of gamma rays detected in the spark chamber.
References
[1] G. Kanbach et al., \The Project EGRET (Energetic Gamma Ray Experiment Telescope) on NASA's Gamma-Ray Observatory GRO," Space Sci. Rev., 49, pp. 69{84, 1988. [2] S. D. Hunter, \Calibration and Adjustment of the EGRET Coincidence/Time-of-Flight System," Nuc. Inst. Meth., A307, p. 520, 1991. [3] D. J. Thompson et al., \Calibration of the EGRET Instrument for the Gamma Ray Observatory," in preparation, 1991. [4] D. J. Thompson et al., \Calibration of the EGRET High-Energy Gamma-Ray Telescope in the Range 20{10,000 MeV with a Tunable Beam of QuasiFigure 4: Map of gamma ray emission from the region Monoenergetic Gamma Rays at SLAC," IEEE Trans. around the Galactic anticenter in two energy bands. The Nucl. Sci. , NS-34 , pp. 36{40, 1987. two prominent peaks are the strong point sources Geminga and the Crab Pulsar. [5] R. C. Hartman et al., \Detection of High Energy Gamma Radiation from Quasar 3C279 by the EGRET Telescope on the Gamma Ray Observatory," Ap. J., 385, in press, January 1992. [6] E. Schneid et al., \EGRET Detection of High Energy Gamma Rays from the 1991 May 3 Gamma Ray Burst," submitted to Astron. Astrophys., 1991.
