Gamma-Ray Burst (GRB)

[Japanese article]

A Great Mystery in Modern Astronomy: Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are the most energetic explosive phenomenon in the Universe. The nature of GRBs is still a great mystery, even after 30 years of the discovery of this phenomenon. It was in 1973, when a mysterious "flash" in gamma rays (gamma rays are electromagnetic waves having shorter wavelengths than those of X-rays) was discovered. The flashes lasting only for 0.1 second to several hundreds of seconds. This discovery was made by chance with a satellite watching for nuclear experiments. Later research revealed that these gamma-ray bursts indeed came from somewhere in the sky, i.e. somewhere in the Universe. It has been established that GRBs are not a rare phenomenon, but are detected approximately once per day. For twenty years since the discovery, it had not been even clear whether GRBs occur in the solar system or occur outside our Galaxy.
A major breakthrough occurred in the 1990s, when explosive phenomena associated with the GRBs (called afterglow) were detected in other wavelengths (X-rays, optical light and radio). Especially from the optical observations, it has been progressively established that GRBs occur in distant galaxies, or in deep Universe. The bright appearance even in that distance indicates that the explosion energy of the GRBs is extremely large. The explosion energy is even said to surpass that of supernovae. In other words, the largest measured (assuming the isotropic radiation) energy of a GRB even surpasses a rest-mass energy (: Einstein's theory of relativity) of the sun! -- which might imply that a star entirely disappears and is completely converted into energy! Of course, such energy transformation is hard to achieve within the current knowledge of physics. Fundamental questions such as "What physical mechanism is responsible for such a violent energy release" or "What is the nature of GRBs and their afterglows" have been left unanswered.

What hinders us from revealing the nature -- shortness of the phenomena and unpredictability

It is a rare case in modern astronomy that even 30-years' long discussion and intensive research has failed to reach a fundamental consensus in revealing the nature of a certain astronomical phenomenon. Then, what hinders us from revealing the nature of GRBs? The main reason is in that the GRBs are unpredictable, transient pheomena, and that the durations of events are extremely short. As already stated, the durations of gamma-ray bursts themselves are "almost an instant" (0.1-100 seconds). This "instant" is caught by an observing satellite, and the information is relayed to ground-based observers who can then undertake follow-up observations of expected afterglows. Furthermore, the durations of afterglows are also short (usually only observable for one or two days). Even with a large telescope, a single-place ground-based observation cannot clarify the entire aspects of the afterglow phenomenon. The key requirements for GRB reseach are thus prompt delivery of satellite detections and multi-longitude observations from many observatories -- there needs to be an extensive world-wide collaboration.

The latter requirement has been realized by the availability of e-mails and the web service, which enable immediate delivery and sharing of information. At present, burst occurences are delivered by the GRB Coordinates Network (GCN), operated by NASA, the USA. These messages are also relayed by our services (vsnet-grb, vsnet-grb-info). While there has been a significant advancement in rapid electronic delivery of information, there had unavoidably a delay in a few hours in precisely localizing the GRB from the satellite observation. Subsequent ground-based observations usually only observed the faint fading afterglows. In particular, there had been very little information about early-stage afterglows until 1 hour, until the realization of the High Energy Transient Experiment (HETE) 2 satellite.

High Energy Transient Experiment (HETE) 2 satellite, and the 2002 October 4 event

The HETE-2 satellite (manufactured by a collaboration between Japan, the USA, and France) is the first satellite dedicated to GRB observations and their rapid locatizations. After the launch in 2002 October and early performance verification and adjustment phases, the HETE-2 satellite began demonstrating the outstanding ability in relaying precise localizations within several minutes.

On 2002 October 4, an e-mail issued at 12:07:02 UT relayed a precise localization of the GRB (GRB021004) which occurred at 12:06:13.57 UT. The location of the burst in the sky was very favorable for observation in Japan. Several telescopes including the Kyoto University team quickly started to follow the optical afterglow, making the GRB "best observed" in the history. On the next day, NASA issued a press release, making this GRB one of the best renowned GRBs in the history.

GRB 021004 Kyoto image
(GRB 021004 recorded by the Kyoto VSNET-GRB team)
Read more about GRB021004

Modern Intrepretation of GRBs

Recent observational evidence shows that many of "typical GRBs" (long-lasting GRBs with rich gamma-ray emissions) occur at a cosmological distance, i.e. distance around ten billion light years (when the age of the universe was several million years). This was first established by the discovery of an optical afterglow of GRB 980326. The spectroscopy of this GRB afterglow yielded a redshift (z) of 0.835, indicating that this GRB is at a cosmological distance. All of other optically identified GRBs have yieled similar redshifts (distances).

The combination of this vast distance and the observed gamma-ray strength immediately yields a vast amount of released energy. If the GRBs are emitting gamma-rays equally in all directions (astronomers refer this to isotropic radiation), the calculated energy sometimes even surpasses that of the rest-mass energy of a star (as derived from Einstein's famous equation ), just like our Sun. This implies that a star suddenly disappears, and its mass is totally converted into energy! -- although no physically meaniningful mechanism is known to convert a star into energy.

The modern interpretation of this vast amount of "observed" energy is a reflection of strongly beamed radiation, on the contrary to isotropic radiation, from the GRB. The beam is supposed to have a speed (of the bulk motion) extremely close to the speed of the light. In such a highly relatisvistic condition, the emission is strongly enhanced toward the direction of the motion (relativistic beaming, a well-known effect of the theory of special relativity). Only when the beam is heading toward us (the earth), the emission is observed as a strong GRB.

The effect of beaming is also favorable in interpreting the short-time variation (less than several miliseconds) in gamma-rays. Since the speed of propagation of physical condition is limited to the speed of light, this short time in variation implies that the varying source must be small (1 milisecond corresponds to only 300 km). If the vast amount of gamma-ray energy is confined to such a small regoin, collisions between photons produce electron(e-)-positron(e+) pairs, making the medium opaque to gamma-rays. In such a condition, gamma-rays no longer escape from such a compact region (the compactness problem). This difficulty can be solved if the gamma-rays are emitted from a medium moving toward us at a speed extremely close to the speed of the light.

It is still one of the most central problems in astronphysics what kind of astronomical phenomenon is responsible for such a highly relativistic jet-like motion. In recent years, a core collapse in a massive star (either leading to a stellar collapse or a supernova explosion) has been becoming the most promising central engine to produce such a relativistic jet. This hypothesis was strengthened by tantalizing evidence of possibly associated occurrence of GRB 980425 and SN 1998bw, and "bumps" in GRB light curves which can be attributed to emergence of supernovae. The most recent emergence of the spectroscopic signature of the supernova component (SN 2003dh) in the afterglow of GRB 030329 provides the most straightforward evidence that at least some GRBs and supernovae are indeed physically associated. The exact mechanisms how stellar matter is accelerated into a highly relativistic, highly collimated jet and how it penetrates the star are still a mystery.

There have been a number of other theories to explain GRBs, including merging of binary neutron stars, although they have not been considered as a major contributor to the entire GRB population.

Hilights and history of Amateur Observation of GRBs and VSNET-GRB Activity

[11] [16] GRB 000926 Observation at Ouda Station, Kyoto University

[12] [13] [14] GRB 000926, Observation at Nyrola Observatory
This GRB afterglow was the one first positively detected by amateur astronomers. This detection by the Nyrola Observatory GRB team (Finland), following the announcement in vsnet-grb, was also described in detail in a Sky and Telescope issue. This positive detection is the memorial which led to subsequent flourishing amateur observations of GRB afterglow

[27] [30] GRB 001025A Observation at Ouda Station, Kyoto University

[29] GRB 001025B Observation at Ouda Station, Kyoto University

[45] [46] GRB 010119, Observation at Nyrola Observatory

(Light curve of GRB 010222: real-scale in time)
This afterglow (GRB 010222) was the first one detected in Japan.

Links to other separate GRB pages on VSNET

GRB 990123

GRB 010222 (Kyoto first detection)

GRB 030229 (Super-bright GRB!)

GRB in general (in Japanese)

GRB observing guide (in Japanese)

Supernovae in general

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