First Catalogue of Unique Patch of Gamma-Ray Universe
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453September 16, 1999
Portsmouth, N.H. -- The science team behind the Imaging Compton Telescope (COMPTEL) aboard NASA's Compton Gamma Ray Observatory (CGRO) has produced its first catalogue of gamma-ray sources in an energy band much wider than that covered by optical telescopes and a million times more energetic. The team presents this catalogue at the 5th Compton Symposium, held this year from September 15-17 in Portsmouth, N.H.
Gamma rays occupy the highest energy range in the electromagnetic spectrum, well beyond visible light, ultraviolet and X-rays. They are produced by extreme forces of energy and by nuclear decay. The COMPTEL catalogue comprises 63 gamma-ray sources. Thirty-two of these are steady sources, such as neutron stars and black hole candidates; the remaining 31 are mysterious gamma-ray bursters, which outshine the entire universe before fading within a few seconds.
"COMPTEL has opened a new window in astronomy," said Dr. V. Schönfelder of the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany and COMPTEL principal investigator. "The COMPTEL bibliography has nearly 500 publications. The instrument has played an essential role in establishing gamma-ray astronomy as an integrated part of astronomy and astrophysics." MPE shares the task of COMPTEL data processing with the University of New Hampshire and NASA's Goddard Space Flight Center in Greenbelt, Md. Goddard, in addition, controls day-to-day operations of the Observatory.
COMPTEL, one of four instruments aboard CGRO, captures a part of the electromagnetic spectrum that has been previously unexplored, and it fills a gap between more easily detectable lower- and higher-energy gamma rays. This sliver of the spectrum, Dr. Schönfelder said, contains crucial information about light sources that are "invisible" to any other telescope.
One unique source that appears in the COMPTEL energy range is a titanium isotope, Ti-44. "The detection of decaying titanium, with a half-life of 60 years, is a powerful method to discover young, previously unknown supernova remnants in the Galaxy," Dr. Schönfelder said. Supernovae produce heavy metals as they explode. Another such metal is aluminum-26, with a half-life of 700,000 years and also visible only to COMPTEL. This, Dr. Schönfelder said, is an excellent tracer for ancient supernova remnants.
Other objects in COMPTEL's range include pulsars and active galactic nuclei (AGN), which are thought to host massive black holes of one million to one billion times the mass of the Sun. The most massive AGN often display their maximum luminosity in the COMPTEL energy range, making gamma-ray observations crucial for understanding these objects.
While the other instruments aboard CGRO have now produced several catalogues since the satellite's launching in 1991, the COMPTEL catalogue was eight years in the making. This was due to the unique physics of COMPTEL's observable energy band and the mechanics of the instrument itself. COMPTEL had greater difficulty discerning between source and background gamma ray radiation. Thus, careful modeling of background energy was needed before the COMPTEL team could isolate distinct gamma-ray sources for the catalogue.
Unlike optical light and X-rays, gamma rays cannot be captured and reflected in mirrors. The high-energy photons would pass right through such a device. COMPTEL must utilize a process called Compton scattering, where a gamma ray strikes an electron and loses energy, similar to a cue ball striking an eight ball. COMPTEL has two sets of detectors that scatter gamma rays -- that is, the detectors act like billiard balls. The detectors are aligned one below the other. The gamma ray passes through, striking an electron in one detector and then another electron in the second detector. By combining measurements of the loss of energy and the change of trajectory, the COMPTEL scientists can construct a likelihood map of the probable gamma ray source location.
The COMPTEL data analysis is performed jointly by MPE; the Space Research Organization of the Netherlands in Utrecht, Netherlands; the Institute for the Study of Earth, Oceans, and Space at the University of New Hampshire; and the Space Science Department of the Astrophysics Division of ESA/ESTEC in Noordwijk, Netherlands.
The four instruments on CGRO together cover a range of the electromagnetic spectrum millions of times wider than the patch of visible light, from red to violet. Compton Gamma Ray Observatory is the second of NASA's Great Observatories and the gamma-ray equivalent to the Hubble Space Telescope and the Chandra Observatory. Compton was launched aboard the Space Shuttle Atlantis in April, 1991, and at 17 tons, it was the largest astrophysical payload ever flown at that time.
The COMPTEL Catalogue has been submitted for publication in the Astronomy and Astrophysics Supplement Series.
The Compton Symposium, hosted by the University of New Hampshire and NASA Goddard Space Flight Center, is the fifth in a series of international symposia dedicated to research in gamma-ray astronomy with an emphasis on results from CGRO.
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Carmelle Druchniak
UNH News Bureau
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