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.
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.
  GRB 000926 Observation at Ouda Station, Kyoto University
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
  GRB 001025A Observation at Ouda Station, Kyoto University
 GRB 001025B Observation at Ouda Station, Kyoto University
GRB 010119, Observation at Nyrola Observatory
GRB 010222 (Kyoto first detection)
GRB 030229 (Super-bright GRB!)
GRB in general (in Japanese)
GRB observing guide (in Japanese)
Supernovae in general
Return to Daisaku Nogami's page