An ultra-long GRB ( English Ultra-long Gamma Ray Burst ) is a gamma ray burst at a cosmic distance with a duration of more than 10,000 seconds. The separation of the ultra-long GRBs from the normal gamma ray bursts is mainly theoretically motivated, since the previous outbreak models cannot reproduce the behavior of the ultra-long GRBs.
Gamma Ray Bursts are the most energetic explosions known in space. According to the duration of the gamma radiation bursts, they are usually divided into two groups. With the short GRBs, 90 percent of the emitted gamma radiation can be detected within less than two seconds, while with the long GRBs the duration of 90 percent of the observed gamma radiation is between 2 and several 1,000 seconds. The division into long and short gamma ray bursts probably also reflects a different explosion mechanism. While the short GRBs are created at the time of the merger of two neutron stars, the long GRBS are explained using the collapsar model, according to which an ultra-relativistic jet is created in a stripped-envelope supernova in a Wolf-Rayet star and is aligned with the earth.
Almost all ultra-long gamma ray bursts show afterglow in the X-ray range for a few days to weeks. This afterglow is mainly interpreted as an interaction between the matter accelerated to almost the speed of light in the jet and the circumburst material. The interaction creates a shock front that spreads through the matter and emits electromagnetic radiation by means of braking and synchrotron radiation.
The spectrum of the ultra-long gamma ray bursts does not differ from the spectra of long GRBs. The gamma light curve also shows the great diversity in the course of radiation as it is characteristic of gamma ray bursts. However, the light curve in the X-ray range and in the optical is not correlated in contrast to normal GRBs. The ultra-long GRBs appear to be predominantly found in blue compact galaxies. This is a subgroup of the dwarf galaxies in which there is a high rate of star formation .
Possible mechanisms of formation for ultra-long gamma ray bursts
- Destruction of a white dwarf by tidal forces in the gravitational field of a black hole . Accordingly, the ultra-long GRBs would be a special form of a tidal disruption event .
- A modification of the collapsar model, according to which the supernova explosion is not taking place in a Wolf-Rayet star, but originate in a blue supergiant or Yellow hypergiants has
- This subspecies of gamma ray bursts takes place in an environment with a very low gas density. This would slow down the jet over a long period of time and the radiation could be emitted for longer. However, so far all potential forerunners of the Kollapsar model show a significant stellar wind before their supernova explosion.
- In the context of the Kollapsar model, matter relapses into the central machine of the gamma ray burst, which means that it can maintain the jet for a longer period of time.
- The ultra-long gamma ray bursts are triggered by the rare, super luminous supernovae. The super luminous supernovae emit ten times more energy in the range of electromagnetic radiation than the hypernovae .
- GRB 130925A
- GRB 121027A
- GRB 111209A
- GRB 101225A
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