Gamma-Ray Bursts:

Energy Release with Human Experience
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    1 ergs = energy to make a mosiquito jump
 10^3 ergs = ball drop
10^10 ergs = hit by truck
10^15 ergs = smart bomb
10^20 ergs = H bomb
10^26 ergs = killer asteroid
10^40 ergs = Death Star 

Energy Release with Astronomical Experience
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10^33 ergs/s = Sun
10^39 ergs/s = nova
10^41 ergs/s = SN
10^45 ergs/s = galaxy

10^52 ergs/s = GRB
The Vela-5A nuclear test detection satellite was part of a program run jointly by the Advanced Research Projects of the U.S. Department of Defense and the U.S. Atomic Energy Commission, managed by the U.S. Air Force, to verfy the Atmospheric Test Ban Treaty.

It and its twin, Vela-5B, were placed 180 degrees apart in nearly circular orbits at a geocentric distance of 118,000 km on 23 May 1969; the orbital period was 112 hours. The x-ray and gamma-ray detector was located 90 degrees from the spin axis, and so covered the celestial sphere twice per satellite orbit. Data were telemetered in 1-sec count accumulations.

In 1973, the Vela satellites discovered brief bursts of gamma-rays of cosmic origin coming from random directions on the sky. Networks of satellites carrying gamma-ray burst detectors were established in the inner solar system in the 1970's and 1980's (every space probe was mounted with a small gamma-ray detector) to produce source locations for these transients using a method analogous to triangulation of ships at sea. However, inspection of the relatively small locations produced no obvious counterparts to the gamma-ray bursts at any other wavelength.

The mystery of gamma-ray bursts has been prolonged by the impracticality of building and launching a gamma-ray telescope with focusing optics that could maneuver and point to the unknown direction of a burst while it is still in progress. This was partially resolved by the launch of the Compton Gamma-Ray Observatory (CGRO) in the late 1980's.

One of CGRO's experiments is EGRET which produced an all-sky map at gamma-ray energies above 100 MeV in galactic coordinates. The diffuse emission, which appears brightest along the galactic plane, is primarily due to cosmic ray interactions with the interstellar medium. The Vela, Geminga, and Crab pulsars are clearly visible as bright knots of emission in the galactic plane in the right portion of the image.

The burst-monitoring experiment on CGRO, BATSE (Burst And Transient Source Experiment), localizes bursts by comparing the burst's different intensities as measured by its eight detectors pointing to the eight octants of the sky.

The resulting coarse localizations have uncertainties of several degrees in radius, making positive identification of the corresponding object at optical or X-ray wavelengths very difficult: literally millions of candidate objects appear within the probable gamma-ray error region, and only one (or none) is related to the gamma-ray burst.

As can be seen from the uniform distribution of burst positions in this recent figure, no preferred direction for the burst sources is apparent. If the burst sources were associated with our own Milky Way Galaxy, then a distribution concentrated toward the Galactic plane (represented by the central horizontal line in the figure -- the "galactic equator"), like that seen by CGRO's EGRET instrument. Such reasoning led some gamma-ray astrophysicists to believe that bursts are coming either from very local sources (just outside the Solar System) or the very distant reaches of the Universe. In the later case, these brief events would be, momentarily, much brighter than entire galaxies.

In the late 1990's, a promising CGRO project takes advantage of real-time telemetry acquisition and calculation of burst source directions. The coarse BATSE, and other orbiting x-ray telescopes, positions are quickly relayed the burst's approximate location to fast-slewing, ground-based cameras primed to search for an elusive optical flash. Larger telescopes too captured the burster's optical counterpart and at right is an image identifying the candidate from the 60 inch Palomar reflector. At left is a prior sky survey image of the region which astonishingly shows a discernible smudge near the same position, likely the burster's host galaxy.

The galaxy and bright burst suggest that this is the closest yet localized gamma-ray burst.

Do the powerful explosions known as gamma-ray bursts (GRBs) originate in galaxies? This subject took on new light with the release of a Hubble Space Telescope image of the sky surrounding GRB 990123. This burst was cataloged as one of the most powerful GRBs ever. The optical transient (OT) counterpart to the GRB can be seen as the bright spot just below center. Once so bright it was briefly visible with just binoculars, this OT has since become four million times dimmer and continues to fade. Now, it can be seen easily with only a large telescope. The diffuse object above is of particular interest because it appears to be the host galaxy of GRB 990123. This distant galaxy seems to have a normal brightness but an irregular shape. This discovery increases the evidence that most OTs do occur in galaxies.

The big, unsolved mystery concerning gamma-ray bursts is their source of energy. The output of a GRB exceeds the total output from over 100 billion supernova's and a billion times the output of an entire galaxy of stars. This would require a black hole the size of a cluster of galaxies (hard to miss) or the collision of a matter and anti-matter about the mass of a star (there are no anti-matter stars). We are left with the possiblity that our knowledge of physics is incomplete to explain this phenomenon.