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[FONT="]Gamma Ray Bursts: Some History [/FONT]
[FONT="]Gamma Ray Bursts (GRBs) are brief, elusive, but outrageously luminous events. Today, we'll look at the discovery of GRBs (an accident), and the first few decades of their study. The big mystery was,
How far away are they? [/FONT]
- [FONT="]Discovery of GRBs [/FONT]
- [FONT="]What could they be? [/FONT]
- [FONT="]Are they within the galaxy, or without? [/FONT]
- [FONT="]Where are they? [/FONT]
[FONT="]Discovery of GRBs [/FONT]
[FONT="]An excellent source of information is [/FONT]
[FONT="]an account by Ray Klebasabel,[/FONT][FONT="] a scientist who worked on the [/FONT]
[FONT="]Vela Project.[/FONT][FONT="] In the 1950s, the few nuclear powers -- US, USSR, Great Britain (France would join in 1960 and China in 1964) -- decided that a ban on the testing of nuclear weapons would be a good thing. But how would each country know that the other was abiding by the terms of some Test Ban Treaty? [/FONT]
[FONT="]There are several distinctive signatures by which one may detect nuclear explosions remotely: [/FONT]
- [FONT="]seismic waves [/FONT]
- [FONT="]very low frequency sound waves [/FONT]
- [FONT="]gamma rays [/FONT]
[FONT="]The US pursued each of these avenues. One of them -- gamma rays -- produced a surprise. [/FONT]
[FONT="]Gamma rays are very, very energetic electromagnetic waves; they have energies billions or trillions of times greater than optical photons. They are produced in great quantities in nuclear reactions. In order to detect bombs exploded above the surface of the Earth, the US designed a set of satellites which would orbit the Earth to look for gamma rays. The satellites were launched in pairs, so that each could watch over roughly half of the planet. The first pair entered orbit in 1963, the second pair in 1964, and the last pair in 1965. [/FONT]
[FONT="]As scientists scanned the records from the satellites, they found evidence for short bursts of gamma rays which could not be due to nuclear weapons; some, for example, were detected by satellites on opposite sides of the Earth. The detectors typically showed a sharp initial peak of gamma rays, followed by a more gradual decline over seconds or minutes. What were they? [/FONT]
[FONT="]Some bursts were detected by more than one of the Vela satellites. The difference in the arrival times of the gamma rays at each satellite could sometimes indicate a very rough direction.
[/FONT] [FONT="]If the sources of the bursts were somewhere in the Solar System, then they would (probably) be located somewhere in the
ecliptic plane: that's the plane in which the Earth and all the other planets orbit around the Sun. But, instead, the bursts sometimes came from above the plane, and sometimes from below the plane. So they probably originated somewhere far outside the Solar System ... [/FONT]
[FONT="]The GRBs were a closely guarded secret for many years. It wasn't until 1973 that the members of the Vela team were allowed to tell other scientists of their existence. [/FONT]
[FONT="]What could they be? [/FONT]
[FONT="]Sometimes, one can use the wavelength at which an object emits electromagnetic radiation to guess at some of its properties. The wavelength of peak thermal radiation shrinks as the temperature of an object increases: [/FONT]
- [FONT="]very cold material (dense interstellar clouds) emits mostly radio waves [/FONT]
- [FONT="]warm material (like humans) emits infrared [/FONT]
- [FONT="]hot material (like stars) emits visible and UV [/FONT]
- [FONT="]very hot material emits X-rays [/FONT]
[FONT="]But it's just not possible for objects to be so hot that they emit many gamma rays; they are too energetic, even for bodies with temperatures of millions of degrees. Instead, gamma rays can be produced [/FONT]
- [FONT="]in nuclear reactions [/FONT]
- [FONT="]when relativistic particles, moving very close to the speed of light, collide with low-energy photons. The particles may give their energy to the photons, turning them into very high-energy photons -- gamma rays. This process by which collisions produce high-energy photons is called the Compton effect [/FONT]
[FONT="]So, what could be producing these bursts? We know that when some stars explode, becoming very bright
supernovae for a few weeks, the material at their core undergoes nuclear reactions which can give rise to gamma rays. Perhaps supernovae were the sources. Now, there have been no supernovae in our own galaxy for several centuries; but we can see a few hundred supernovae occur in nearby galaxies. So, if the GRBs were associated with supernovae, we'd expect them to appear where nearby galaxies appear -- spread all over the sky. Unfortunately for the supernova hypothesis, there were no obvious nearby supernovae observed at the same time as these bursts ... [/FONT]
[FONT="]Another theory was that material in an accretion disk around a black hole or compact star might somehow be accelerated to relativistic speeds (some accretion disks do appear to have relativistic jets of material shooting outwards). Accretion disks are found in close binary star systems, in which a compact star can rip material from a nearby companion. [/FONT]
[FONT="]Artist's impressions of a close binary system with accretion disk:pogore e[/FONT]
[FONT="]There are many such close binary star systems in our Milky Way galaxy, concentrated in the disk. So, if GRBs are due to these binaries, we expect most of the GRBs to be located near the disk of the Milky Way[/FONT]
[FONT="]Are they within the galaxy, or without? [/FONT]
[FONT="]In the 1980s, one could divide the GRB models -- and there were hundreds of them, literally -- into two main categories: [/FONT]
- [FONT="]close binary stars somewhere within our Milky Way [/FONT]
- [FONT="]something else, far outside the Milky Way [/FONT]
[FONT="]Astronomers divided themselves into two camps: those who believed the "galactic" explanation, and those who preferred the "cosmological" explanation. In 1995, the leaders of the two camps held a debate in Washington, D.C. [/FONT]
[FONT="]Now, distinguishing between these two possibilities was, in theory, pretty simple. If we could plot the positions of a hundred or more GRBs in a projection centered on the Milky Way's disk, then we ought to see a clear difference in the distribution of positions. If due to binary stars inside the Milky Way the GRBs ought to be found mostly in the disk of our Galaxy, and probably concentrated near the bulge (which contains most of the stars):gledate vo centarot emisijata na GAMMA RAY e pointenzivna
[/FONT]
[FONT="]So, all we needed was a set of a few hundred GRBs with decent positions, and we could settle this argument. NASA designed a satellite to do that job: the
Compton Gamma R[/FONT]
[FONT="][/FONT][FONT="]ay Observatory[/FONT]
[FONT="] [/FONT]
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