NRA 99-OSS-02 "Compton Gamma Ray Observatory"

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NRA 99-OSS-02
Appendix A

SUMMARY OF THE COMPTON GAMMA RAY OBSERVATORY
(COMPTON GRO)
MISSION AND INSTRUMENTS

I. Mission Objective

The Compton Gamma Ray Observatory is studying the various astrophysical processes (e.g., nuclear reactions, electron bremsstrahlung, matter-antimatter annihilation, elementary particle production and decay, Compton scattering, synchrotron radiation, etc.) and sources that produce high-energy electromagnetic radiation. Its observations are addressing a variety of questions relevant to understanding the universe, such as: the formation of the elements; the structure and dynamics of the Galaxy; the nature of pulsars; the existence of black holes; the origin of gamma-ray bursts; energetic and explosive phenomena occurring in galactic nuclei; the origin of the cosmic diffuse background; particle acceleration in the Sun, stars, and stellar systems; processes in supernovae; and the origin and evolution of the universe itself. Its high sensitivity, broad energy coverage, high time resolution, and complementary instrumentation permit discovery of new sources and phenomena in this most energetic region of the electromagnetic spectrum observable from space. Particularly notable is the opportunity for unprecedented high-energy coverage of a complete Solar Cycle afforded by the Compton GRO extended mission phase.

The Compton GRO was conceived, designed, and developed as a Principal Investigator class observatory, but it is now a Guest Investigator facility accessible to the international astrophysics community. Four distinctly different instruments are optimized to perform simultaneous observations of specific targets or regions. They combine to cover over five decades of energy with more than a factor of 10 improvement in sensitivity, and with improvements in spectral and spatial resolution in selected energy ranges over previously flown instruments. It is important to note that three of the four instruments on Compton GRO view large regions of the sky and, therefore, are capable of making observations of several sources simultaneously.

II. Mission Plan

Orbital operations for the Compton GRO mission have been divided into four phases sequenced in time, following an initial period of approximately 40 days immediately after launch devoted to checking out the spacecraft and instruments. For a description of the mission phases refer to Appendix C, section II.

III. Operational Characteristics

The Compton Gamma Ray Observatory is a large free-flying spacecraft with a total weight of 15900 kg, of which approximately 6000 kg is scientific payload. Compton GRO was launched on the Shuttle Atlantis on April 5, 1991, and released into the planned 450-km circular orbit with an inclination of 28.5ƒ. Using onboard propulsion, the altitude is maintained above 350 km to avoid excessive drag and below ~450 km to avoid excessive trapped particle radiation in passing through the South Atlantic magnetic anomaly. Celestial pointing is being maintained with an accuracy of ~0.1ƒ. Attitude determinations are accurate to ~2' and an absolute timing accuracy of ~0.1 msec has been achieved. Experiment data is continuously recorded at a 32 kbit/sec rate during Tracking Data Relay Satellite System (TDRSS) contacts. Typically, Compton GRO operates with all four instruments observing all the time, outside regions of the South Atlantic Anomaly. Additional information regarding the data modes for each experiment, the data flow to and at each investigator site, and the hardware and software planned for processing data at each site is contained in Appendix G (available upon request).

In March 1992, the tape recorders on Compton GRO began to exhibit unacceptably high noise levels in the playback data, and they are no longer routinely used. In their place, real-time telemetry downlinks have been scheduled through the TDRSS multi-access mode whenever the Compton GRO High-Gain Antenna can be pointed at any of the TDRSS spacecraft. The TDRSS was reconfigured in 1993, and an additional ground-tracking station was added in support of Compton GRO operations. This has yielded a minimum data recovery rate of approximately 82% since its implementation, with higher rates possible for restricted spacecraft attitudes.

IV. Science Instruments

A summary of the detector characteristics for each instrument is given in Table 1. The sensitivities in Table 1 are based on 5_10⁵ seconds exposure on source. With the anticipated 82% coverage rate, this is the exposure time normally obtained in a 2-week observation. A brief description of each instrument is given below. More comprehensive descriptions of the four instruments and their data modes appear in Appendix G. At the time of this writing, all instruments are operating within their nominal performance range, although EGRET is generally operated in a reduced-field-of-view mode to conserve spark-chamber gas (see Appendix G, section IV for details).

Oriented Scintillation Spectrometer Experiment (OSSE)
This experiment utilizes four large actively-shielded and passively-collimated Sodium Iodide (NaI) Scintillation detectors, with a 3.8^ƒ x 11.4^ƒ FWHM field of view. The large area detectors provide excellent sensitivity for both gamma-ray line and continuum emissions from ~0.1 to 10 MeV. Each detector utilizes a single-axis orientation system to implement a source/offset pointing mode of operation that permits background subtraction from celestial source contributions and also allows observation of secondary targets when the primary target is occulted by the Earth. It also permits observations of selected sources, such as transient phenomena and solar flares, without impacting the planned Observatory viewing program. (Principal Investigator: Dr. James D. Kurfess, Naval Research Laboratory, Washington, DC, E-mail: kurfess@osse.nrl.navy.mil)
Imaging Compton Telescope (COMPTEL)
This instrument operates in the 0.75-30 MeV range. It employs the unique signature of a two-step absorption of the gamma ray, i.e., a Compton collision in the first detector followed by total absorption in a second detector element. This method, in combination with effective charged-particle shield detectors and time-of-flight techniques, results in efficient suppression of the substantial instrumental background. Spatial resolution in the two detectors, together with the well-defined geometry of the Compton interaction, permits the reconstruction of the sky image over a wide field of view (~ 1 steradian) with a resolution of a few degrees. In addition, the instrument has the capability of searching for polarization of the radiation. The instrument has good capabilities for searching for weak discrete sources, weak diffuse galactic features, and for spectral and spatial features in the extragalactic diffuse radiation. (Principal Investigator: Dr. Volker Sch–nfelder, Max-Planck-Institut f¸r Extraterrestrische Physik, Garching bei M¸nchen, Germany, E-mail: vos@mpe-garching.mpg.de)
Energetic Gamma-Ray Experiment Telescope (EGRET)
The EGRET is designed to cover the energy range from 20 MeV to 30 GeV. The instrument uses a multiple layer spark chamber and thin metal conversion plates to detect gamma rays by the electron-positron pair-production process. A total energy counter 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 readout of events not associated with gamma rays. (Principal Investigator: Dr. David L. Bertsch, NASA Goddard Space Flight Center, Greenbelt, MD, E-mail: dlb@gamma.gsfc.nasa.gov.
Note: There is now a limited amount of spark-chamber gas remaining, and some sub-components of the EGRET instrument have failed. The use of EGRET in Cycle 9 and beyond will be for significant targets of opportunity. About 6 weeks of ToO observing time will be allocated for Cycle 9. The actual amount of time for which EGRET will be turned on will be determined by the project based on performance monitoring of the instrument during Cycles 8 and 9. In any case, any future use of EGRET will be determined on a competitive basis resulting from the evaluation of proposals received in response to this and future Compton GRO Guest Investigator Program announcements.
Burst and Transient Source Experiment (BATSE)
The Burst and Transient Source Experiment for Compton GRO is designed to continuously monitor a large fraction of the sky for a wide range of transient gamma-ray events. The experiment consists of eight wide-field detector modules at the eight corners of the spacecraft platform. This arrangement provides maximum continuous exposure to the unobstructed sky. The instrument has 0.1 ms time resolution, strong burst location accuracy of about three degrees, and sensitivity of ~3 x 10^-8 erg/cm² for a 1 sec burst. Pulsed sources as weak as ~0.03 x Crab pulsar can be detected using onboard or on-ground folding (10⁵ sec exposure). Using the Earth occultation technique, BATSE can monitor sources as weak as ~0.1 x Crab with one-day resolution. (Principal Investigator: Dr. Gerald J. Fishman, NASA Marshall Space Flight Center, Huntsville, AL, E-mail: fishman@ssl.msfc.nasa.gov)

Table 1 Summary of Compton GRO Detector Characteristics

	OSSE	COMPTEL	EGRET	BATSE	BATSE
				LARGE AREA	SPECTROSCOPY
ENERGY RANGE (MeV)	0.06 to 10.0	0.8 to 30.0	20 to 3 x 104	0.03 to 1.9	0.015 to 110
ENERGY RESOLUTION (FWHM)	12.5% at 0.2 MeV 6.8% at 1.0 MeV 4.0% at 5.0 MeV	8.8% at 1.27 MeV 6.5% at 2.75 MeV 6.3% at 4.43 MeV	~20% 100 to 2000 MeV	32% at 0.06 MeV 27% at 0.09 MeV 20% at 0.66 MeV	8.2% at 0.09 MeV 7.2% at 0.66 MeV 5.8% at 1.17 MeV
EFFECTIVE AREA (cm²)	2013 at 0.2 MeV 1480 at 1.0 MeV 569 at 5.0 MeV	25.8 at 1.27 MeV 29.3 at 2.75 MeV 29.4 at 4.43 MeV	1200 at 100 MeV 1600 at 500 MeV 1400 at 3000 MeV	1000 ea. at 0.03 MeV 1800 ea. at 0.1 MeV 550 ea. at 0.66 MeV	100 ea. at 0.3 MeV 127 ea. at 0.2 MeV 52 ea. at 3 MeV
POSITION LOCALIZATION (STRONG SOURCE)	10 arc min square error box (special mode; 0.1 x Crab spectrum)	0.5 - 1.0 deg (90% confidence 0.2 x Crab spectrum)	5 to 10 arc min (1s radius; 0.2 x Crab spectrum)	3ƒ (strong burst)	_____
FIELD OF VIEW	3.8ƒ x 11.4ƒ	~ 64ƒ	~ 0.6 sr	4 sr	4 sr
MAXIMUM EFFECTIVE GEOMETRIC FACTOR (cm2 sr)	13	30	1050 (~ 500 MeV)	15000	5000
ESTIMATED SOURCE LINE SENSITIVITY (5 x10⁵ sec; on source, off Galactic Plane) CONTINUUM	(3-8) x 10-5 cm-2 s-1 3 x 10^-7 cm-2 s^-1 keV-1 (@1 MeV)	1.5 x 10^-5 to 6 x 10^-5 cm^-2 s^-1 1.6 x 10^-4 cm^-2 s^-1 (3 s detection, 1-30 MeV)	7 x 10^-8 cm^-2 s^-1 (> 100 MeV) 2 x 10^-8 cm^-2 s^-1 (> 1000 MeV)	3 x 10^-8 erg cm^-2 (1 sec burst)	0.4% equivalent width (5 sec integration)