Image of gamma-ray bursts

Gamma-ray bursts (GRBs) are extremely violent and gigantic phenomena, often called as "the largest explosion in the universe". They emit gamma-rays (electromagnetic wave that is even more energetic than X-rays) for a typical duration of 1-100 seconds. GRBs occur randomly on the sky at a rate of about 1 event per day. We cannot see them because our eyes are not sensitive to gamma-rays, but they are detected by gamma-ray observing satellites. They were discovered in 1970's, and they were recognized as a very mysterious phenomenon for a long time. However, the knowledge of GRBs has dramatically been improved since 1997, and now we know that they are occurring at very large, cosmological distances of more than 10 billion light years. A GRB is produced when a very massive star (more massive than 20 solar masses) ends its life with a supernova explosion. The event rate of GRBs is much lower than normal supernovae, but their explosion energy is much larger, and such energetic supernovae associated with GRBs are often called hypernovae. Before explosion, the star has already lost its original outer envelopes made by hydrogen and helium. There is a compact iron (Fe) core at the star center, and it is surrounded by lighter nuclei such as silicon (Si), magnesium (Mg), neon (Ne), oxygen (O) and carbon (C). The iron core collapses by gravitational force to form a rapidly rotating black hole. Matter around the black hole falls into it with a disk-like geometry (accretion disk), and two narrowly collimated mass outflows (jets) are driven to the directions along the axis of the disk. Gamma-rays are produced when the jets penetrate and break out from the stellar envelope. The speed of jet is almost close to the light speed (more than 99.99%!). Gamma-rays are also narrowly collimated in the directions of the jets, and GRBs can be observed only when an observer is located in the jet directions.

There are two different populations of GRBs, separated by durations shorter or longer than about 2 seconds. The above picture is for the long GRB population. The origin of short GRBs has been thought to be merging of binary neutron stars or black holes, because they occur also in galaxies of old stellar populations. This has been confirmed recently by the gravitational wave detection of a binary neutron-star merger, GW 170817, which came with a short gamma-ray burst.

GRBs are very interesting astronomical objects by themselves, but there is another important role in astronomy. GRBs are extremely bright and they can be detected even from a large, cosmological distance scale. Therefore, they can be used to probe the early star/galaxy formation activity in the universe, and to know the physical state of the early universe. In fact, GRBs are now an important tool to probe the most distant universe, with a power comparable with distant galaxies and quasars. The most distant GRBs detected by the humanbeing occurred less than 1 billion years after the big bang. (The present age of the universe is 14 billion years.)

These pictures are in the public domain. You can use the above image freely for non-profitable, educational and/or scientific purposes, under a condition of indicating the credit (Kyoto University).

Produced by: Tomonori Totani (Department of Astronomy, Kyoto University when this image was produced)