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Astroman

Primer - Gamma Ray Bursts, Part One:History

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Astroman    5

Gamma ray bursts were discovered accidentally by satellites designed to detect nuclear test detonations by earthly cold-war opponents.  A satellite system called Vela was deployed in 1969 as a group of 4 to detect detonations anywhere on Earth in real time.  Detections were made on a daily basis, but locations were from outer space, not Earth.

And the mystery began.  Several smaller missions were launched to try to study these events.  Balloon-borne instrument packages were designed, built and flown by Dr. Jerry Fishman, now of Marshall Space Flight Center in Huntsville, Alabama.  Much was discovered, but the basic technology of HOW to detect and localize the bursts was proven in the balloon experiments.  GRB’s were the hot topic in the scientific community, so ASCA, Skylab, Solar Max and others did significant research too.  In fact, essentially every satellite launched since 1978 has had some form of gamma ray detector on board.  Most are tested and calibrated in flight by measuring known gamma ray sources and GRB’s.

NASA answered the call with the Compton Gamma Ray Observatory, launched by the Shuttle Columbia in 1991.  The second of “The Great Observatories”, (HST being the first), it held 4 instruments to detect, record and study GRB’s.  It was the first satellite dedicated to the study and at 17 tons, the largest payload up to that time.  During its lifetime, it detected 8000 events.  The instruments on board were the BATSE, (Burst And Transient Source Experiment), OSSE, (Oriented Scintillation Spectrometer Experiment) COMPTEL, (Imaging Compton Telescope) and EGRET, (Energetic Gamma Ray Experiment Telescope).

BATSE was the brainchild of Dr. Fishman, (whom I’ve met on several occasions and found to be a great guy, very passionate about his work and besides his obvious intelligence, a regular guy), from the same balloon experiments.  Placed on each corner of the spacecraft, these 8 instruments detected the bursts as they occurred and the direction, allowing a rough localization.  Its energy range was between 20KeV and 1200KeV.  (eV=Electron Volts)

OSSE held 4 aimable detectors.  This design allows the detector to compare the gamma ray source with background radiation for accurate energy measurements in the notoriously noisy range.  Energy sensitivity was 50KeV to 10 MeV.

COMPTEL is the main directional detector.  Based on the “Compton Effect”, for which the observatory’s namesake won a Nobel Prize, 2 layers of detectors are used to determine the energy and arriving direction of gamma rays.  Its range is from 1 MeV to 30 MeV.

EGRET is the highest energy detector on board.  It’s also the most sensitive detector at these energies.  Incoming gamma rays trigger a cascade of electron-positron pairs.  These are measured for direction and counted for energy level.  Its range is 20MeV to 30 BeV.

CGRO was deorbited in June, 2000, ostensibly for “safety reasons”.  It had a gyro that had failed and politicos pulled the plug.  (It was actually used as a pawn to persuade Russia to bring down the unsafe and dangerous MIR.  But don’t tell them I know, I’ll have to kill you.  Jerry Fishman still can't talk about it without getting snorked.)

Other GRB missions include HETE I and II, Chandra X-Ray Observatory, Multiple Mirror X-ray telescope, SWIFT and some others I forget at the moment.

HETE I and II were, ahem, less than spectacular successes.  Actually, HETE I crashed into the see at launch, and HETE II has decided that the Sun is a source of extra-galactic GRB’s.  It forgets that there are known sources of gamma rays, such as Cygnus X-1, the Crab Nebula and various other known soft gamma repeaters.  It does sometimes provide good localizations, but they are rare.

The SWIFT mission is the current bonanza operation.  I get localizations at least once per week, and sometimes in bunches.  It is the first mission to automatically slew the spacecraft toward an incoming GRB and take images, sometimes in real time.  Its band is in the ultraviolet and soft x-ray range, (10eV- 10 KeV).

Questions so far?

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