Mapping a High-Energy Universe: Compton Gamma Ray Observatory

Compton Gamma

On a quest to find out the mysteries of the observable universe, we needed to redefine the word “observable”. With the development of the electromagnetic spectrum, we realized that to the human eye only a very small range of light is actually visible. This realization led the way to the need and supply of instruments and methods to observe the effect of electromagnetic radiation in the non-visible regions as well. To fulfill this, NASA set out to launch an observatory focusing on the analysis of Gamma-rays and events producing such; the Compton Gamma Ray Observatory (CGRO), named after a pioneer in Gamma-ray physics, A.H. Compton.

Compton Gamma CGRO
Figure 01: The Compton Gamma-Ray Observatory prior to deployment, still attached to space shuttle Atlantis by the robot arm, during the STS-37 mission in April 1991. (NASA/STS-37 CREW)

Onboard the Atlantis Space Shuttle’s 8th flight (STS-37), the CGRO was launched on the 5th of April, 1991 into a near-earth-orbit so as to avoid the impact caused by the Van Allen radiation belt. With a mission to see the universe from a view in a range of 20keV to 30 GeV, the CGRO had 4 main components (experiments) to aid these observations.

Compton Gamma
Figure 02: The main components of the CGRO.

BATSE: Burst And Transient Source Experiment

Gamma-Ray Bursts (GRBs) were for many a year thought of as being limited to events occurring within our Milky Way. However, with the aid of the CGRO’s BATSE system, this was proven to be otherwise. This was an experimental apparatus that focused on analyzing an all-sky signal view in the energy range of 20 keV to 600 keV. The BATSE consisted of 8 detector modules each with two NaI(TI) scintillation detectors; arranged in the form of the faces of an octahedron. By considering three different scales in time measurements, the BATSE processed data and indicated statistically significant changes in the data stream. BATSE also provided signals to the other components to switch to relevant modes in the presence of sudden bursts.

Data over the years from this experiment led to conclusive proof that GRBs were not limited to the Milky Way and that many of these events are linked to the death of massive stars.

GRBs had bedeviled astronomers for several decades before Compton was launched, and the consensus among astronomers was that they came from neutron stars within our galaxy.

Gerald Fishman – Lead for the BATSE (experiment) portion of the CGRO

OSSE: Orientation Scintillation Spectrometer Experiment

The design of this experiment was to aid the search for sources of energy in the observable universe that are radiating energy in the range of 500 keV to 20 MeV. Special adjustments in the observatory could also be made to analyze solar flares in the order of 10 MeV as well. The specialty of this energy range is that it encompasses the highest temperature thermal emissions associated with hard X-ray emissions from compact objects; as well as the non-thermal emissions from galactic and extragalactic sources. Consisting of 4 independently operating detectors, these are usually trained on the source and its background in rotation. This exchange is mainly to ensure higher accuracy in the readings obtained by the OSSE.

COMPTEL: Imaging COMPton TELescope

For many decades the processes occurring between high energy photons in the astronomical sense were an impossibility to observe. However, with substantial proof of the Compton Effect, Compton scattering became a great place to start looking in from. The theoretical principle behind this is as follows.

Figure 03: The Principle of the Compton Telescope

Let’s consider two detector arrays separated by a distance as shown in the figure above as indicated. The positions of the interactions on the detectors are used to obtain the direction vector of the Gamma-rays; the energy loss recorded from each detector (E1 and E2) can be used to obtain the scattering angle Φ.

This principle, refined and developed electronically to obtain precise measurements of intergalactic events, led to the discovery of many newly formed radioactive isotopes. Furthermore, the COMPTEL showed three main modes of operations. The Double-Scatter Telescope mode, Burst mode, and Solar modes were invaluable in providing observatory assistance to identifying Gamma-ray sources, keeping track of GRBs as well as analyzing solar flares.

EGRET: Energetic Gamma Ray Experiment Telescope

This component aboard the CGRO was in charge of radiation in the ranges of 20 MeV to 30 GeV. With a field of view extending almost 80o, the EGRET was successful in conducting the first all-sky survey of high-energy Gamma-rays. This played a significant role in the discovery of active galaxies. The identification of a new type of such active galaxies, “Blazars” was a key discovery of the EGRET. Furthermore, observations of the Magellanic Cloud galaxies have led to the confirmation that cosmic rays are in fact, galactic in nature.

The view from the EGRET, tuned to see Gamma-rays above 100 MeV. Brighter colors show a greater concentration of Gamma-rays; a great prominence seen by the central plane of the Milky Way galaxy here (running through the middle). To the right of the image, the Vela pulsar can be seen. It is one of the five Gamma-ray pulsars discovered by the EGRET. (NASA/EGRET Team)

Gamma-Ray Skies: A Future Prospect in Astronomy

With the many discoveries of the “invisible” high-energy events occurring throughout our cosmos as seen by the Compton Gamma Ray Observatory, its mission life came to an end with the malfunction of one of its onboard gyroscopes. With public safety concerns in mind, NASA decided to perform an intentional controlled de-orbit of the observatory; the first for NASA. On the 4th of June, 2000, after 9 years of service, the CGRO reached the surface of the Pacific Ocean. But soon to follow on the quest to uncover the wonders of the Gamma-ray universe were the Neil Gehrels Swift Observatory, and the Fermi Gamma-ray Space Telescope. As continues the quest to uncover more secrets of the infinite skies, and as continues mankind’s capabilities in unlocking more progressive breakthroughs in scientific truth…


04. Schonfelder, V., & Kanbach, G. (n.d.). Imaging through Compton scattering and pair creation. 207-208. Retrieved from

Image Courtesies

01. Featured Image:
02. Figure 01:
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04. Figure 03: (Schonfelder & Kanbach)
05. Figure 04:

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