Start gamma
The starts gamma (in English, gamma-ray bursts , shortened in GRB) are puffs of Photon S Gamma which appear by chance in the sky. They are located at very long distances from the Ground, and are of this fact the most luminous events of the Univers, after the Big Bang.
History
The first detections of starts gamma took place in 1969 by the American military satellites of the series Vela, charged to control the application of the prohibition of the atmospheric atomic tests. It is only into 1973 that this information was made public, opening a new astronomical research field. Until the end of the years 1980, one knew only very few things of these phenomena: they are unforeseeable, their glare is very variable and their nonthermal spectra. They are French experiments PHOEBUS and American BATSE which brought first large major advance: the starts gamma are divided into two distinct groups, of the short starts and the long starts. If the duration of the first does not exceed 2 seconds (it is more typically about some tenths of a second), the seconds can be observed in the sky during a few seconds, even a few minutes. Longest are however observable only during one score of minutes, which explains the great difficulty of their observation. The second great discovery was the fact of BeppoSAX, a satellite Italo-Dutchman which observed for the first time a remanent emission with the starts gamma the February 28th 1997. It is thanks to these observations and with following (BeppoSAX observed several tens of starts gamma) that our comprehension of the phenomenon of the starts gamma progressed. Since the end of the mission of BeppoSAX, other space observatories such as Swift (American) helped of automatic observatories supplement our knowledge on the subject.
Origin
The starts gamma are related at the ultimate stages of the stellar evolution and the black holes. The disparities observed between the long starts and the short starts for a long time resulted in thinking that the star at the origin of the start gamma, the progenitor, was to be in fact of two various nature. One thinks since 1998 that the long starts (the most studied) related to died of star massive. This fact was confirmed by the observation of several starts gamma associated with supernovas of the Ib/c type in 2003. If it is not yet clearly why all the massive stars do not produce a start gamma, one is certain on the other hand that certain massive stars produce starts gamma, and that these starts are visible for us only because we are in the line of sight of a jet of matter ejected at fantastic speeds (about 99,995 percent of the Speed of light). It is the shock of this matter with the interstellar environment which produces the remanent emission. It is supposed that these jets are produced by a black hole in formation at the time of died of massive star.
The nature of the short starts was élusive for a long time. It is finally in 2005, following observations of HETE-2 that the position specifies of a short start could be obtained. Thanks to it, it was possible to show that the characteristics of the galaxies containing the starts short gamma are very different from those of the galaxies containing the starts long gamma. This privileged the assumption that the progenitor of the short starts is not a massive star but binary containing compact objects (neutron star or black hole). These binary radiates energy in the form of gravitational waves and little by little approaches. When they become too close one to the other, the compact objects amalgamate, giving rise to a black hole. It is this birth which would be announced through the Universe by a short flash of photons gamma.
The starts gamma for better including/understanding the formation of stars
The starts long gamma are directly related to stars, and it is possible to study the formation of stars starting from the study of the starts gamma. The luminosity which characterizes them indeed makes it possible to detect them to the borders of the Universe. However, a remarkable property of the light is its finished speed: the photons which we receive from the starts the most remote gamma were sent there is more 10 billion years (time that they put to join us), and shows us the Universe such as it was at this time there. We can consequently study these completed times and better include/understand how were formed the old stars, how they evolved/moved and how they influenced the contents of the Universe.
The starts gamma in details: the model of the fireball
One explains the phenomenon of the starts gamma by the model of the fireball. In this model, a progenitor will expel matter at relativistic ultra speeds. This matter is made up almost only of electrons, easier to accelerate, and which moreover radiate their energy more effectively. The energy contained in the other particles (Proton S) is to some extent trapped, and thus lost to produce radiation (it is thought however that these accelerated protons belong to the cosmic rays observed by the astrophysicists).
The fireball is not something of homogeneous. In addition to the fact that it must have a geometry (one speaks about jets of particles focused in our direction), the ejection is not continuous but is done by spasms: the “fireball” is made up of successive layers, which travel each one at a different speed. When two layers meet (fastest catching up with the other), it occurs an abrupt emission of photons gamma. It is this track race of the various layers of matter which is responsible for the prompt emission, with its great temporal variability.
The fireball, during its expansion, also will sweep the medium surrounding the progenitor of the start, a such shock wave. At the time of this interaction, the fireball is slowed down by the medium, and will start to radiate energy with all the wavelengths. This radiation is the remanent emission.
With these two mechanisms are added other components related on the dynamics of the shocked fluids (a shock wave in return is propagated for example in the internal parts of the fireball at the time of the interaction with the external medium) or to the quantum Mécanique (such as components opposite Compton ), which complicate the general study of the radiation of the fireball. One can however note that since the remanent emission is due to the environment of the progenitor, it is the study of this emission which is privileged to know the conditions reigning around stars responsible for the starts gamma.
A new model was proposed.
See too
Reference
| Random links: | Linux loader | Wagnon | Mint in box | Andre Herrero | Noordoostpolder | 886 |