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?
An excellent source of information is an account by Ray Klebasabel, a scientist who worked on the Vela Project. 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?
There are several distinctive signatures by which one may detect nuclear explosions remotely:
The US pursued each of these avenues. One of them -- gamma rays -- produced a surprise.
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.
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?
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.
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 ...
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.
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:
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
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 ...
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.
Artist's impressions of a close binary system with accretion disk:
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
In the 1980s, one could divide the GRB models -- and there were hundreds of them, literally -- into two main categories:
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.
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):
On the other hand, if due to some event far beyond the Milky Way, the GRBs ought to be seen in all directions, pretty much equally:
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 Ray Observatory.
GRO was launched on April 5, 1991. One of the instruments on board, the Burst And Transient Source Experiment or BATSE, immediately started detecting and locating GRBs. After about one year, BATSE had a catalog of 261 burst positions. This is their distribution:
The conclusion: GRBs aren't within our Milky Way. After eight years of operation, it's even more obvious:
So, GRBs aren't inside our own galaxy, but far beyond it. But where? Are they in nearby galaxies, or distant galaxies? Or perhaps in quasars? Or could they be far from any ordinary, visible object, floating alone in intergalactic space?
The problem is that it's hard to pin down the location of a GRB precisely. Gamma rays have such high energy that they penetrate ordinary mirrors instead of bouncing off them, so they can't be focused like optical light. It is possible to construct very large, complicated detectors on the Earth's surface which can measure a gamma-ray photon's direction to within a fraction of a degree
And if one looks inside that area, there are LOTS of stars and galaxies visible:
In fact, in a typical circle of radius 5 degrees, there are tens or hundreds of thousands of optical sources visible to big telescopes. This image (part of the Hubble Deep Field) covers about 1/1000 of a square degree -- about one-millionth the area of a 5-degree error circle!
It's impossible to match up the GRB with any particular one. Rats.
So, people thought -- Is there any way to improve the positions of GRBs? And they thought up a clever plan.
NASA and other space agencies send spacecraft to other planets in the solar system. As they fly around the solar system, the spacecraft are very, very far from each other, and very, very far from Earth. If we put little gamma-ray detectors on the spacecraft, we can use them to triangulate and pin down the position of a GRB. Here's how:
As long as we know
How well does this method work? With two spacecraft, and precise timing, we know the angle of the source relative to the line connecting the two spacecraft.
Unfortunately, that angle can arise anywhere on a big circle on the sky:
But if a third spacecraft detects the burst, then its location along the circle is fixed to two small spots, typically smaller than BATSE's error circles.
Below is one example: a GRB which occurred on August 1, 1993, was observed by Mars Observer (on its way from Earth to Mars), Ulysses (far above the plane of the solar system), and the Compton Gamma Ray Observatory (in orbit around Earth). By combining the time at which each spacecraft detected the burst, astronomers pinned down its position to the area below:
This is "only" about 1 degree high by 0.5 degrees wide. Yet it was still too big for astronomers to be able to find any optical counterpart to the gamma ray burst.
Despite a number of ingenious experiments and much hard work, as of early 1997, astronomers still had no clue to the true distance and nature of a single GRB ...
Note added Feb 14, 2001:
On February 12, 2001, the NEAR spacecraft landed on the asteroid EROS. That was quite an accomplishment, but it disabled NEAR's gamma-ray detectors. As a result, the network of spacecraft which can triangulate to pin down the location of a gamma-ray burst has shrunk from 3 to 2 -- which means the error boxes will be large. Here's a note from GRB-hunter Kevin Hurley:
TITLE: GCN GRB OBSERVATION REPORT NUMBER: 931 SUBJECT: IPN Status Report DATE: 01/02/13 19:10:33 GMT FROM: Kevin Hurley at UCBerkeley/SSL
As many of you are no doubt aware, the Near Earth Asteroid Rendezvous mission executed a controlled descent to the surface of the asteroid Eros on February 12, bringing the mission to a successful end. However, this has also deprived the 3rd Interplanetary Network of the second distant point which is required to produce small error boxes. The network now consists of Ulysses, in heliocentric orbit, BeppoSAX (GRBM), HETE-II, and RXTE, in low earth orbit, and Konus-Wind, at the L1 Lagrange point. In this configuration, the IPN can produce (as it did prior to NEAR), a) single annuli (using Ulysses and one other spacecraft), which will help reduce the sizes of the error boxes derived from HETE-II, RXTE, and the BeppoSAX WFC and NFI, and b) long, narrow error boxes (using Ulysses, Konus, and a near-Earth spacecraft). In general, our policy will be NOT to issue GCN notices for the bursts detected by the IPN in this mode unless 1) they are imaged by BeppoSAX, HETE-II, or RXTE, 2) they are very unusual in some sense, 3) they are suspected to originate from known or new soft gamma repeaters, or 4) potential users of such notices notify us of their requirements. (We do, however, intend to triangulate these events and produce a catalog of them eventually. We will also begin work immediately on a catalog of NEAR bursts.) The Mars Odyssey '01 mission, to be launched in April, has two independent gamma-ray burst detection systems. Because the mission goes farther from Earth than NEAR, and because the detection systems have better time resolution, this rejuvenated network promises to perform better than the old one. Due to financial constraints, there is still some uncertainty about the duty cycles of these systems during the 9 month cruise phase to Mars. (Although once they are in orbit, they are expected to operate continuously for one Mars year, i.e. through 2003.) We look forward to continuing to issue small error box notices, perhaps as soon as a few months from now. Kevin Hurley (on behalf of the Ulysses GRB team) Thomas Cline (on behalf of the NEAR GRB team) Scott Barthelmy (on behalf of the GCN)
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Copyright © Michael Richmond. This work is licensed under a Creative Commons License.