GRBs' counterpart optical rays are so luminous that they might be observed in the night sky with binoculars. But because their electromagnetic waves' energy levels are high enough to stimulate and oscillate the gaseous molecules that form our atmosphere, GRBs themselves do not reach the Earth's surface. Only their weaker luminous rays avoid being captured in our gaseous shell.

GRB's Remain An Elusive Quarry

This phenomenon illustrates why it's often said that doing astronomy from earth is like listening to music without the high and low frequencies. However, above the atmosphere one can view the entire spectrum, and a wide variety of satellites are currently engaged in the GRB hunt.

Gamma astronomy now presents itself as the best method -- and sometimes the only one -- for discovering and studying GRBs and other galactic and extra-galactic phenomena such as star deaths and Active Galactic Nuclei (AGNs). About one-tenth of known galaxies have active nuclei, with centers as luminous as 100 billion suns.

About one GRB per day is detected in the whole sky; they appear from random directions. Plotting positions of non-identified GRS (gamma ray sources) positions is tricky -- researchers are not sure if it's an AGN, pulsar or black hole.

GRBs themselves consist of very energetic transients in the approximately > 100 kev wavelength range corresponding to gamma rays (a kev, or kilo electron-volt, is a 1000 units of the energy used in accelerating an electron through 1 volt).

The duration of the transients has been measured from 30 to 1000 milliseconds. But updated observations would suggest a duration of 30 milliseconds to 15 minutes and even 90 minutes. The counterpart/afterglow of GRBs in X-, optical and radio waves extends from a few days (X-rays) to a few months (radio).

BATSEs, ROTSEs and EGRETS Lend a Hand

The CGRO (Compton Gamma Ray Observatory) launched from the shuttle Atlantis in 1991 had among its instruments the BATSE (Burst and Transient Source Experiment) for gamma waves, ROTSE (Robotic and Optical Transient Search Experiment) and the EGRET (Energetic Gamma Ray Experiment Telescope).

Meanwhile, the BACODINE (Batse Coordinates Distribution Network) transmitted the coordinates of GRBs in seconds to dozens of terrestrial telescopes. This network, now called GCN (Gamma-Ray Burst Coordinates Network) consists of several satellites working together with ground-based telescopes.

The CGRO ceased observing on May 26, 2000 after one of its three gyroscopes failed and raised the possibility of an uncontrolled descent. Mission controllers steered it into a re-entry into the Pacific Ocean south of Hawaii one week later.

Currently, observations are being made from satellites like the Italo-Dutch Beppo-Sax, the American Defense Meteorological Satellite Program (DMSP) and WIND satellites, and the European Space Agency's Ulysses, whose solar orbit carries it well above the plane of the Solar System.

Trying to Predict a Pattern

GRBs' distribution is isotropic, or the same from all directions in the sky. Observations have concentrated on discovering if they are homogeneous -- if the same density of GRB sources are found at different distances from the Earth.

Beppo-Sax (whose mission is mostly X-Ray research) has localized GRB sources through their X-ray afterglow and allowed ground-based studies of the afterglow spectrum and possible host galaxy. The spectra are strongly shifted towards the red as their waves are stretched by the universe's expansion motion; thus we perceive them as being "redder". The speeds of recession are calculated at > 80 percent of the speed of light, indicating an extra-galactic origin for GRBs and indicating their homogeneous distribution.

Similarly, the age of GRBs is cosmological -- three to 10 billion light-years. For example, the Keck II telescope in Hawaii obtained a distance of seven billion light-years by measuring the spectroscopic red-shift in the afterglow of a GRB crossing a cloud containing traces of iron and magnesium.

In October 2000, NASA launched the High Energy Transient Explorer 2 (HETE-2) for observing GRBs, localizing their arrival direction and alerting the astronomical community. It also carries two cameras -- one sensitive to low energy X-rays and one to optical ones.

The GLAST, the next large NASA observatory for gamma astronomy, would be initiated towards the end of 2005 and the European INTEGRAL mission would launch in 2002 but concentrate on lower energies.

An Agile Approach To the Hunt

The ASI (Italian Space Agency) has proposed a low-cost high energy measuring mission to bridge the gap between EGRET and GLAST. The AGILE (Astrorivelatore Gamma ad Immagini LEggero) would detect very high-energy photons with a new generation of instruments derived directly from particle physics.

Gamma-wave photons do not obey geometric optics; they cannot be reflected or focalized but must be detected with an instrument capable of measuring their energy and arrival direction. AGILE's telescope would have a semiconductor detector, a calorimeter and an X-ray detector to work on all the known sources of gamma astronomy: diffuse galactic emissions, pulsars and AGNs.

The universe presents us with many cosmic sources capable of accelerating very efficiently the particles that emit gamma rays. Contrary to "cosmic rays" (typically a flux of protons), which are charged and are deviated by B fields (an abbreviation for magnetic fields), photons of high energy are not deviated and so are much better for localizing the celestial source object.

Gamma-ray telescopes are similar to particle detectors used in nuclear and subnuclear physics. CGRO/EGRET has discovered about 300 point-like gamma sources, half non-identified. It has discovered GRBs up to 20 Gev.

Some Sources Remain Undiscovered

It is interesting to note some variable sources in the plane of our galaxy that could constitute a new class of gamma emitters apart from AGNs and pulsars.

Several models have been proposed to explain the phenomenon of GRBs. One is that of a black hole surrounded by a shell of baryonic matter. The shell is situated well above the dyadosphere (or region where the field allows the creation of e+/e- pairs) of the black hole.

An impulse of electromagnetic energy (pair electromagnetic pulse) coming from a black hole with electromagnetic structure can enter into collision with the baryonic matter remnant. The plasma which impacts on the shell would thus create a GRB and would transfer kinetic energy to the baryonic shell.

Other theoretical models for GRBs include:

-- Stars in collapse/explosion (the collapsar/hypernova model)

-- Binary neutron stars which approach, fuse (coalescence) and collapse

individually producing a GRB.

-- The formation of a millisecond pulsar with very strong magnetic B field

-- Magnetars (stars with very strong magnetic fields), repeating gamma burst

sources which evolve into X-ray pulsars, become inactive, and become one of our galaxy's billions of dark objects.

-- A double-action model, in which a SN (a supernova or exploding star) would become a GRB (10 years later for example). The SN becomes a neutron star. Then the supply of matter gives a swelling. Then there is collapse into a black hole and

GRB.

So in conclusion, there are many interesting hypothesis regarding gamma-ray bursts. Energetic phenomena furnish the archaeology of the universe, and exploring the diffuse extra-galactic gamma background could help reveal more of its mysteries.