Gamma-Ray Burst — In astronomy, Gamma-ray bursters (GRBs) are flashes of gamma rays that last from seconds to hours, the longer ones being followed by several days of X-ray afterglow.
They occur at random positions in the sky several times each day. They are now believed to result from tremendous explosions in far away galaxies, during the creation of a black hole from a dying star or two colliding neutron stars. The black hole, surrounded by a rotating disk of matter falling into it, probably ejects an energetic beam parallel to the axis of rotation.
This document provides a history of the GRB search and its current status.
The Discovery of GRBs
Cosmic gamma-ray bursts were discovered in the late 1960s by the US “Vela” nuclear test detection satellites. The Velas were launched to detect radiation emitted by weapons tests, but they picked up occasional bursts of gamma rays from unknown sources.
While the sensors on the Vela satellites had low angular resolution, in 1973 researchers at the US Los Alamos National Laboratory in New Mexico were able to use the data from the satellites to determine that the bursts came from deep space.
Gamma ray bursts can only be observed directly from space, as the atmosphere blocks gamma rays. Astronomers believed that once better gamma-ray detectors were put in orbit, they would be able to quickly pin down the locations of the GRBs. After all, that is what happened with X-ray sources. However, when such improved detectors were sent into space in the 1970s, optical searches of the regions where the bursts originated showed nothing of interest. The sensors were not accurate enough to pinpoint the location of the bursts for detailed study.
Further information on the burst sources proved hard to obtain, and led to more questions than answers. The first question posed by the GRBs was: are they local to our own Galaxy, or do they occur in the distant reaches of the Universe? The second question was: what mechanism causes the bursts? If they do occur in the distant Universe, the mechanism must be producing enormous amounts of energy.
Little progress was made on the matter through the 1980s, but in April 1991, the US National Aeronautics and Space Administration (NASA) launched the “Compton Gamma Ray Observatory” on board the space shuttle. One of the instruments on board Compton was the “Burst & Transient Source Experiment (BATSE)”, which could detect gamma-ray bursts and locate their positions in the sky with reasonable accuracy.
Within a year, BATSE determined that GRBs occur twice or three times a day, and are randomly distributed over the entire sky. If they were events occurring in our own Galaxy, they would be preferentially distributed in the plane of the Milky Way. Even if they were associated with the galactic halo, they would still be preferentially distributed towards the galactic center, 30,000 light years away, unless the halo were truly enormous. Besides, if that were the case, nearby galaxies would be expected to have similar haloes, but they did not show up as “hot spots” of faint gamma-ray bursts.
To many astronomers, this implied that the GRBs originated in the distant Universe, but that led to the problem of finding a mechanism that could generate so much energy. Other theorists were also still able to come up with “local” models for the GRBs, and BATSE couldn’t resolve the issue.
What is a GRB?
The combination of obvious brightness and implied distance of GRB 990123 was startling even to GRB hunters. At 9 billion light years, the gamma-ray energy released by the burster was equivalent of converting the entire mass of a star 1.3 times the mass of our Sun completely into gamma radiation. At visual wavelengths, if the burster had gone off in our own Galaxy 2,000 light years away, it would have shined twice as bright in our night sky as the Sun does during the day.
Astrophysicists have have been challenged to come up with convincing mechanisms to explain the sheer power of these bursts. One line of thought proposed that collisions between neutron stars, or between a neutron star and a black hole, could do the job. Another proposed that the bursts were caused by supernova explosions of very large stars, sometimes called “hypernovas”, with the explosive collapse of the star creating a black hole, rather than a neutron star as would be the case for a smaller star.
The Hubble observations that showed GRB 990123 to be associated with a young galaxy tended to discourage theorists who believed that the bursts were due to collisions between neutron stars or the like, since that implied a fairly high density of dead stars and that was inconsistent with a young galaxy. Supernovas, on the other hand, occur frequently in star-forming galaxies, since the big stars that die in supernovas have short lifetimes.
Even the supernova model had trouble accounting for the energy output. One way around the problem was to assume that the burst energy was only sent out in specific directions, rather than in all directions, much as some stars and galaxies emit directional high-energy “cosmic jets” from violent events. Another explanation for the the great brightness of the burst was that its light had been focused by a “gravitational lens”, caused by the distortion of space by a large galaxy between Earth and the GRB.
The lensing theory was supported by observations that seemed to indicate there was in fact a galaxy between the Earth and the GRB. However, the “galaxy” turned out to be a photographic flaw. This didn’t rule out gravitational lensing, but interest in the idea faded when Bradley E. Schafer of Yale pointed out that at a redshift of 1.6, the density of galaxies made the probability of lensing only about one in a thousand.
In any case, if limits needed to be imposed, the “beaming” hypothesis was much more plausible than lensing. Astrophysicists Bohdan Paczynski of Princeton and Stan Woosely of the University of California, Santa Cruz, independently suggested that a supernova might emit a narrow beam of gamma ray energy during its explosive collapse into a black hole, with the tightly focused beam giving the impression of a much more energetic event.
Exactly how the collapse would generate such a beam remains a puzzle. However, an analysis of the afterglows of 17 GRBs that was published in the fall of 2001 did place limits on the width of the beam, stating that it was probably only a few degrees across. With such a narrow beam, the energy of a GRB could be provided by a supernova only slightly more powerful than average. Incidentally, theorists say that if the Earth were hit by a gamma-ray beam from a burster only a few hundred light years away, it would incinerate the surface of our planet.
The narrowness of the beam also suggested that only one in 500 GRBs are seen from Earth, meaning they are a fairly common phenomenon in the Universe, probably occurring about once every minute. This means that astronomers might be able to observe “orphan afterglows”, exactly like those following a GRB, but not associated with a gamma-ray burst.
The brightness of GRBs varies rapidly, implying that their source objects are quite small: whatever causes the brightness variation cannot travel faster than the speed of light across the object. Very densely packed photons prevent each other from escaping, and astronomers therefore theorize that the energy initially leaves the object as a jet of matter, with gamma rays being created at a certain distance by internal shock waves.
There is some direct evidence of an association of a GRB with a supernova. A supernova synthesizes a wide range of heavy elements during its collapse, and many of these, particularly isotopes of nickel, are highly unstable and break down very quickly, releasing radiation. This means that a supernova actually gets brighter for a few days or weeks after its occurrence.
BeppoSAX targeted a GRB on 21 November 2001, and following the burst the Hubble Space Telescope tracked the evolution of “GRB 011121″ for an extended period of time. The light curve obtained matched that expected of a supernova. However, no valid spectrum was obtained of GRB 011121 that would conclusively prove a link to a supernova.
As the number of detailed observations of GRBs and the instruments and theoretical tools are refined, it seems only a matter of time before the nature of the bursters is understood.