Gamma-ray Burst FAQ (original) (raw)

1. What are Gamma-ray bursts, and what are gamma rays?

Gamma ray bursts (GRBs for short) are intense and short (approximately 0.1-100 seconds long) bursts of gamma-ray radiation that occur all over the sky approximately once per day at very large distances from Earth. Gamma rays are very energetic photons (E>10^5 eV), which represent the most extreme portion of the electromagnetic spectrum (ranging from radio waves at the lowest energies through visible optical light at higher energies, to gamma rays at the highest energies).

2.How are Gamma-ray bursts named?

The naming system for gamma ray bursts is very simple: "GRB yymmdd". For exmaple, a gamma ray burst which occured on July 4, 1999 is called GRB 990704. If there is more than one gamma ray burst on the same day, the letter a, b, c, etc. are added to the name (for example, the second gamma ray burst on July 4, 1999 is called GRB 990704b).

3. Where do Gamma-ray bursts occur?

Up until the 1990s and the launch of the Compton Gamma Ray Observatory (CGRO; see next question) there was a heated debate in the astronomical community about the source of, and distance to gamma ray bursts. One group claimed that gamma ray bursts occur in our own galaxy (the Milky Way), while others claimed that they occur in very distant galaxies. The main reason put forward by the group which claimed a local origin was the extreme energy release that is necessary to explain the observed emission from gamma ray bursts (see question 10). However, from the information gathered by CGRO, and later confirmation from observations of gamma-ray burst afterglows (see below), it was determined unambiguously that gamma-ray bursts take place in very distant galaxies (several billion light years away). The most distant Gamma-ray burst detected so far occured 13 billion light years away. This means that the gamma ray emission from gamma ray bursts that we observe now has been emitted billions of years ago, when the Universe was much younger.

4. How often do Gamma-ray bursts occur?

Based on almost 30 years of observing gamma ray bursts, we now think that on average there is one gamma ray burst per day somewhere in the Universe. However, recent developments in the study of gamma ray bursts indicates that the true number of these events may be 500 times larger. This means that we only see one out of every 500 gamma ray bursts.

5. How are gamma-ray bursts detected?

Gamma ray bursts are detected by satellites orbiting the Earth and travelling through the Solar system. They can only be detected from space because the Earth's atmosphere absorbs gamma rays and therefore we cannot observe them from the ground. The first gamma ray bursts were detected by the Vela satellites, which were launched in the 1960s to ensure compliance with the Nuclear Test Ban Treaty. Since then several thousand gamma ray bursts have been detected by satellites such as the Compton Gamma Ray Observatory (CGRO) and the Interplanetary Network (IPN).

6. What is the distribution of Gamma ray bursts on the sky?

The distribution of several thousand bursts which were detected primarily by the Burst And Transient Source Experiment (BATSE) on CGRO is uniform across the sky. This means that there is no prefered direction from which we detect more gamma ray bursts. This distribution was the first indication that gamma ray bursts occur in bery distant galaxies and not in our own galaxy.

7. How much energy is released in gamma ray bursts?

Gamma ray bursts release extremely large amount of energy - approximately 10^52 ergs (or 10^45 joules), with the most extreme bursts releasing up to 10^54 ergs. This is the equivalent of turning a star like the Sun into pure energy (using Einstein's famous equation E=mc^2). This is also the amount of energy released by 1000 stars like the Sun over their entire lifetime! In practice, over the few seconds that a gamma ray burst occurs, it releases almost the same amount of energy as the entire Universe! This exteremly large energy release is the reason that astronomers initially believed that gamma ray bursts come from our own galaxy (see question 6). For those of us who live with rolling blackouts (i.e. Californians), the energy from a gamma ray burst (if it was converted to electricity) could supply the entire world's energy needs for a billion billion billion (that's 1 with 27 zeroes after it) years!

8. Is there more than one type of gamma ray burst?

The study of several thousand bursts has shown that there are two main classes of gamma ray bursts: those shorter than 2 seconds, and those longer than 2 seconds. In addition, it was found that the short bursts release more of their energy in very energetic gamma rays relative to the longer bursts. Therefore the terminology that is used to describe the two classes is "short and hard" and "long and soft". All of all bursts that have been studied in detail so far are "long and soft". Therefore, most of the details that are described in this FAQ page pertain only to the "long and soft" class.

9. What is the source (progenitor) of gamma-ray bursts?

In the first years of gamma ray burst research there were more proposed sources (or progenitors) for gamma ray bursts than the actual number of gamma ray bursts detected! However, ever since it was determined that gamma ray bursts occur at very large distances (and therefore release huge amounts of energy) the list of proposed progenitors shrunk into two main classes: very massive stars, and binary (2 star) systems composed of neutron stars or black holes. It is now thought that the "long and soft" bursts come from massive stars, while the "short and hard" bursts come from binary systems. Recently, observations of GRB 011121 (Bloom et al. 2002; Price et al. 2002) revealed a SN explosion which accompanied the GRB, and a circumburst environment typical of what is usually found around massive stars (see more on this intersting burst below). These results support the idea that the "long and soft" bursts are the end product of massive stars.

10. How are massive stars thought to produce gamma ray bursts?

Astronomers now think that the iron cores of some very massive stars (at least 30 times more massive than the Sun) can collapse into black holes several million years after they form. The energy released in the formation of the black hole emerges out of the collapsed star in the form of a gamma ray burst. Gamma ray burst astronomers call this the "collapsar" model. Other names are "hypernova" or "failed supernova" models. These names hint that there may be a connection between gamma ray bursts and supernovae (see below).

11. How are binary systems thought to produce gamma ray bursts?

It is known that most stars in the Universe reside in multiple systems of 2 (binary), 3 and even 4 stars. Some of the binary systems have two stars more massive than about ten times the mass of the Sun, and eventually after these stars die they leave behind neutron stars or black holes. Over time the two objects in the system spiral in toward each other and eventually they merge into a single black hole. As in the case of the "collapsar" model (see question 9) the formation of the black hole results in large amounts of energy release. The name astronomers use for this scenario is "coalescence".

12. Is there a relation between the progenitor of the gamma ray burst and the type of gamma ray burst?

It is now thought that the "long and soft" gamma ray burts come from the collapse of massive stars, while the "short and hard" bursts come from the merger of binary systems. This result comes from computer simulations which show that the merger of neutron star or black hole binaries occurs much faster than the collapse of the iron core of a massive star.

13. Are gamma ray bursts related to black holes?

Yes. The two main models of gamma ray bursts (see questions 9 and 10) both theorize that gamma ray bursts arise during the formation of a new black hole.

14. Are gamma ray bursts related to super-massive black holes in Quasars and Active Galactic Nuclei?

Probably not. There is evidence that some gamma ray bursts occur very close to the centers of galaxies where super-massive black holes may reside. However, most gamma ray bursts occur far enough from the centers to exclude an association with super-massive black holes. In addition, no gamma ray burst has ever been observed to repeat as would be expected if they come from super-massive black holes which are active for tens of millions of years.

15. How is the energy from the newly-formed black hole turned into gamma rays?

The energy from the newly formed black hole, along with some material from the collapsed star, is ejected outward in several shells with different speeds over a period of a few seconds. As a faster shell catches up to a slower one, the two shells collide and this collision produces gamma rays. The merged shell then continues moving out until the next shell catches up and collides with it, releasing more gamma rays. This process is called "internal shocks".

16. What are afterglows?

An afterglow is the emission that follows a gamma ray burst in other parts of the spectrum, ranging from radio waves to X-rays, and lasting from a few days to several years. The afterglows fade away over time in a well-understood manner. The discovery of the first afterglows in 1997 was made possible by the Italian-Dutch satellite BeppoSAX. This discovery (and the detection of approximately 50 afterglows over the past 4 years) has revolutionized the field of gamma ray burst astronomy.

17. How are gamma ray burst afterglows detected?

The first step in the detection of afterglows is always the detection of a new gamma ray burst by a satellite such as the IPN, BeppoSAX, and HETE-II (see qeustion 4). The information from the satellite is quickly sent down to Earth and is distributed to gamma ray burst astronomers by email, pagers, and cellular phones. When astronomers get the information, they observe the part of the sky where the gamma ray burst occured, and look for an object which fades quickly. Different kinds of telescopes are used including radio and optical telescopes all over the world. When the afterglow is found, its exact position on the sky is sent around to all astronomers who subscribe to the GRB Coordinates Network (GCN). For the next few weeks to years astronomers continue to monitor the fading afterglow.

18. Do we see an afterglow from every gamma ray burst?

In principal every gamma ray burst is followed by an afterglow. However, we do not always see these afterglows for several reasons. First, prior to 1997 and the launch of the BeppoSAX satellite (see question 15) it was impossible to find the position of gamma ray burst accurately enough to detect the afterglow. Second, even after 1997 the afterglows from some gamma ray bursts are too faint to detect from Earth. Third, some of the gamma ray bursts occur during the day so we cannot use optical telescopes to look for them (in this case we can still find the afterglow with radio telescopes). Finally, some gamma ray bursts occur in galaxies that contain a lot of dust. This dust can completely block the optical light from the afterglow.

19. How many afterglows have been detected since the first one in 1997?

Approximately 50 afterglows have been detected with X-ray telescopes (see question 15) so far in almost 4 years of observations. However, of these 50 afterglows only approximately 40% have also been detected with optical or radio telescopes. So, on average an afterglow is detected once a month.

20. How are afterglows formed?

As we have seen previously (question 14) the gamma ray burst is formed when shells of energy and matter ejected by the newly-formed hole collide and merge ("internal shocks"). After the shells merge into a single shell, this shell continues to move away from the black hole and, like a snowplow, it gathers up material from interstellar space (even though it is assumed that space is empty, in reality it is full of protons and electrons). As the shell sweeps up more and more material it slows down and releases energy. This is the energy that we observe as the afterglow. This process is called an "extrenal shock".

21. Why do afterglows fade over time?

As was explained in the previous question, the expanding shell from the gamma ray burst gathers up material, and imparts energy to this material. Initially, the shell has more energy and it accelerates the material that it gathers up into high energies. As the shell loses energy it gives less and less energy to the material and therefore the emission becomes weaker. Eventually, the material loses so much energy that the light from the afterglow becomes too faint to be seen from Earth even with the largest telescopes.

22. What telescopes are used to detect afterglows?

Gamma ray burst astronomers use many telescopes around the world to look for and monitor gamma ray bursts. These telescopes inculde the Hubble Space Telescope and the Chnadra X-ray Observatory in space, the Keck telescopes in Hawaii, the Very Large Array radio telescope in New Mexico (featured in the movie Contact starring Jodie Foster), the Very Large Telescope in South America, and many other smaller telescopes.

23. What do we learn from observing afterglows?

Observations of the afterglows all across the spectrum tell us many things about gamma ray bursts. First, observations of the first afterglow in 1997 confirmed that gamma ray bursts occur in very distant galaxies (see question 2). Second, from observations of the afterglow we can determine how much energy was released in the gamma ray burst. Third, we can determine how much material was present in the vicinity of the gamma ray burst. Finally, we can find information on the physics of the "external shocks" (see question 16).

24. What is scintillation and how is it related to afterglows and their sizes?

Scintillation is the reason that stars twinkle and planets don't. Why is there a difference? Simply it is because stars are very far away and therefore appear to be very small, while planets are closer to us and therefore appear to be bigger. Now, afterglows show the same twinkling since they are very far away and so appear very small. But instead of twinkling of their optical light (as in the case of stars), the twinkling of afterglows is seen in radio waves. As opposed to stars, however, afterglows become bigger with time since they expand outward (see question 19) and therefore the scintillation (twinkling) stops after a few weeks. This effect has been observed in several afterglows and it tells us how big afterglows really are.

25. How big are gamma ray burst afterglows?

The twinkling of radio waves from afterglows (see question 23) has shown that afterglows start very small (about the size of the Earth's orbit around the Sun), and then expand and become larger than the Solar system.

26. Are gamma ray bursts related to supernovae?

There are now two pieces of evidence which show that soon before the gamma ray burst happens a supernova also forms. Supernovae are explosions that accompany the death of massive stars, but they are different from gamma ray bursts in the way they release energy. What evidence is there for this connection? First, observations of some afterglows with optical telescopes show a re-brightening of these afterglows a few weeks after they are first observed (as was mentioned previously, afterglows usually just fade away until they become to faint to observe). This re-brightening cannot come from the afterglow, but it is naturally explained in terms of a supernova that happened just before the gamma ray burst. A second piece of evidence comes from the detection of a supernova (called 1998bw) in the same part of the sky and at the same time as a gamma ray burst. Recently, we have found the best evidence for the association of GRBs and supernovae based on optical and radio observations of GRB 011121, which showed a clear re-brightening of the optical light and a direct signature of the environment that is usually found around massive stars (which are the progenitors of both GRBs and supernovae).

27. Where can I find more information on gamma ray bursts?

There are many popular level and scientific articles about gamma ray bursts. For popular level discussion of gamma ray bursts try looking for articles in Scientific American. Scientific articles can be found on the astro-ph preprint server and ADS.

28. What if I have questions which are not answered on this webpage?

Feel free to email me (Edo Berger) at ejb@astro.caltech.edu

page created and maintained by Edo Berger, last updated 24 July, 2001