What are gamma-ray bursts?
Gamma rays are high energy electromagnetic radiation. They lie at the extreme end of the electromagnetic spectrum, which runs from radio waves and microwaves at one end, through infrared, visible light and ultraviolet, to x-rays and finally gamma-rays. If we consider gamma-rays as waves, they have extremely short wavelengths and high frequencies, but they interact with matter as if they were high energy particles, and for this reason it is most convenient to deal with them in terms of a photon picture. Each gamma-ray photon has an energy of at least 500 kilo electron volts (keV).
Electro-magnetic spectrum. [Image: OU]
The story of gamma-ray bursts began in 1973, when America’s Vela satellites were looking for breaches of the nuclear test ban treaty by monitoring sources of gamma-rays on the Earth. Instead, they often detected bright sources of gamma-rays leaking in through the back of the satellites - coming from outer space! These were indeed gamma-rays bursts - the brightest sources of gamma-rays in the sky, for the few seconds each burst lasts. But what, exactly, were they?
The picture of gamma-ray bursts has become clearer over the last fifteen years or so, in the first place thanks to NASA’s Compton Gamma Ray Observatory (CGRO). Launched in 1991, one particular instrument on board this satellite is the Burst And Transient Source Experiment (BATSE) which detected roughly one gamma ray burst per day over the course of the eight years that the satellite was operational.
A map showing the location of several thousand gamma ray bursts recorded by BATSE. [Image: NASA]
Where do gamma-ray bursts originate?
In order to begin determining what gamma-rays bursts are, the first question to answer, is "where do the gamma ray bursts originate?"
Broadly speaking we can suggest four possibilities:
- They could be around the solar system;
- or within our Galaxy;
- or around our Galaxy;
- or at ‘cosmological’ distances.
In fact we can immediately rule out two of these possibilities.
If gamma-ray bursts were within our Galaxy, they should lie where the stars do - that is, concentrated in the plane of the Galaxy’s disc. In that case, there should be a concentration of gamma-ray bursts along the middle of the map shown above, just as the stars are concentrated along the Milky Way in the sky. They are not concentrated there, so gamma-ray bursts are not within our Galaxy.
Further, if gamma ray bursts were spread around the outside of our Galaxy - in the Galaxy’s halo for instance - then we might expect them to similarly surround other galaxies. In that case we would expect to see a concentration of gamma-ray bursts surrounding our nearest neighbour in space, the Andromeda galaxy. The map shows no such concentration of gamma-ray bursts around the Andromeda galaxy, so we can infer gamma-ray bursts are not situated around our Galaxy either.
How can we tell where they are?
In order to determine where gamma-ray bursts actually are, the only information we have to go on comes from the electromagnetic radiation (photons) they emit. For each of these photons we can measure: when they arrived, where on the sky they came from, and what their energy was. In fact, the most complete information comes from the spectrum of radiation and to establish the nature of gamma ray bursts, multi-wavelength observations are needed.
The first thing to do then is to get an accurate location on the sky for the gamma-ray burst, to enable other telescopes, (ones which are working at visible, X-ray or radio wavelengths, for example), to be pointed at the same object.
And this proves to be quite difficult. The problem is that gamma-ray telescopes do not form images like conventional telescopes - gamma-rays would simply pass through conventional lenses or mirrors. Instead gamma-ray telescopes work by forming shadow images, using so-called coded aperture masks, which work on a similar principle to that involved in pin-hole cameras.
However, even this technique can only provide a location for a gamma-ray burst which is accurate to, at best, roughly half a degree. That's about the size of the full Moon on the sky - and a patch of sky the size of the full Moon may contain millions of stars! How can we tell which one is the source of the gamma-ray burst?
Part of the solution was provided in the last decade by two other satellite observatories - the Italian/Dutch Beppo SAX and NASA’s Rossi XTE. These two x-ray astronomy satellites were able to detect the fading x-ray afterglow of a gamma ray burst and locate its position on the sky much more accurately than BATSE on board CGRO alone. This enabled follow up observations at other wavelengths to be made rapidly and accurately.
GRB 990123: A case study
In order to show how the nature of gamma-ray bursts has finally been discovered, let's briefly look at the example of one particular gamma-ray burst.
GRB990123 was detected by BATSE on board CGRO at 09h 46m 56.12s on January 23rd, 1999. At the time, it was the brightest gamma-ray burst seen.
BATSE quickly localised its position to within an area shown by the large circle:
A triangulation provided by the small gamma-ray detector on board the Ulysses interplanetary spacecraft (out near Jupiter before swinging round to orbit over the Sun’s poles) produced the large arc also shown in the diagram. The gamma-ray burst must be somewhere in the region that the circle and the arc overlap.
The two instruments on Beppo SAX quickly swung round to the same part of the sky and caught the fading x-ray afterglow, to narrow down the position of the gamma-ray burst to the small circles shown on the inset. Finally the Palomar 60-inch telescope pointed at this small patch of sky and caught an optical image of a transient source that hadn’t been there on images of the same part of the sky taken 5 years earlier, also shown below. The x-ray and optical counterparts to the gamma-ray burst had been found!
When the huge Keck telescope took an optical spectrum of this counterpart the following night, it saw the usual spectral lines that are expected from stars and galaxies. But the spectral lines were all hugely shifted from the ‘normal’ positions to much longer wavelengths, that is, towards the red. In other words, the optical transient had an enormous redshift. If the redshift is interpreted in terms of a Doppler shift (the earth-bound analogy where shifted wavelengths, such as sound are caused by motion of an object away to or from an observer/listener), then it implies that the galaxy in which the gamma-ray burst occurred is travelling away from us at a speed of 74% of the speed of light.
As Hubble had shown in the 1920s, the further away galaxies are, the faster they are travelling, and this is interpreted as an overall expansion of the entire Universe. So, the redshift of 1.61 corresponds to a galaxy at an immense distance away from us. The light from the gamma-ray burst had been travelling for virtually the entire age of the Universe in order to reach us here on Earth on January 23rd, 1999.
So here is an answer to the puzzle: Gamma-ray bursts do indeed lie at vast cosmological distances away. When we see one, we are seeing an event which occurred billions of years ago, close to the edge of the observable Universe.
Over 100 x-ray afterglows to gamma-ray bursts have now been detected, and many optical transients have been seen. Of these, many have had their redshifts measured, from which it has been determined that all of them lie at vast distances away. Knowing how much energy we detect from a gamma-ray burst, and knowing how far away it is, we can calculate the total energy that must be released in a single burst event. The answer is about 1 billion billion billion billion billion joules (1045 J) - an immense and unimaginable amount of energy. The implied rate of occurrence of these things is about one every million years in each galaxy.
The latest weapon in the armoury of telescopes investigating gamma-ray bursts is the SWIFT satellite, shown here in an artist’s impression. Launched in November 2004, this is currently observing several gamma-ray bursts every week. Its gamma-ray telescope monitors the sky to pick up the bursts and then the satellite rapidly swings around to point its on-board X-ray and optical telescopes to catch the X-ray and optical afterglows as the burst fades from sight. Already SWIFT has made several important discoveries which help to unveil the mysteries of these objects.
As for what gamma-ray bursts actually are, there are currently two broad models, each of which may apply to some of the gamma-ray bursts.
The shorter ones, which are also probably the highest energy ones, may be due to the merger of two neutron stars as part of a binary star system. As two neutron stars orbit each other they will gradually lose energy and spiral together. Eventually they will crash into each other, and a gamma-ray burst may be the result.
The longer, and lower energy, gamma-ray bursts may be due to an event called a hypernova. Back in the early Universe, it is suggested that some very high mass stars formed - hundreds of times the mass of the Sun. Such ultra-massive stars would have had very short lives, and the suggestion is that when they die they don’t just explode as supernova - examples of which are seen routinely in other galaxies today - but instead they explode as a hypernova, and that could give rise to a gamma-ray burst.
Gamma-ray bursts remain something of an enigma, but the detective story is nearing its conclusion. We now know where they are, and we think we know what they are. It’s just lucky for us that they don’t seem to be happening near by!