Post by Andrei Tchentchik on Aug 25, 2020 15:34:13 GMT 2
(.#494).- Antimatter in our Galaxy '2' Continued…
Antimatter in our Galaxy '2' Continued…
Richard Taillet
Teacher, Physical Researcher.
Published 07/01/2005 - Modified 10/28/2015.
Archives
• B - Radioactive decays
Radioactive elements are created in the Galaxy every time a supernova explodes. They
then disintegrate and those of them that undergo the ß + radioactivity emit positrons. An excellent example of this phenomenon is provided by the Al26s isotope of aluminum. Its disintegration also produces a very characteristic gamma radiation of energy, which means that this element can be mapped in the sky (this is what the COMPTEL satellite, also on board CGRO, the Observatory at Gamma Compton, then more recently the INTEGRAL satellite). The figure opposite shows such a map of the distribution of Aluminum 26 in our galaxy, as observed by COMPTEL. As can be seen on this map, these positron sources are concentrated in the galactic disc. This could a priori explain the elongated component in the disc of the positron map obtained by OSSE, but if this were the case we should observe a more extensive positron distribution along the disc: the distribution of radioactive nuclei extends over almost 90 degrees on each side of the disc. To resolve the problem, it will be necessary to wait until INTEGRAL accumulates enough data to detect and measure in turn this component of disk.
• C - High energy phenomena
Positrons can also be produced by high energy phenomena. For example, collisions of cosmic rays on interstellar gas also produce positrons. But that's not all: as the positron is approximately 2000 times lighter than the antiproton, it occurs much more easily, and the energetic phenomena which take place for example on the surface of our Sun (and therefore also in the even more energetic events, such as in the vicinity of pulsars for example) are enough to produce them.
Solar flare seen in the ultraviolet by Soho, March 21, 2003. The image on the left represents a zoom, with the Earth on a scale. During these eruptions, electron-positron pairs are produced. An eruption in July 2002 thus produced 500 grams of positrons. The amount of energy released by the annihilation of these positrons would be enough to supply France with energy for several days.
D - The positrons of cosmic rays and the excess observed by HEAT
Cosmic rays also contain positrons. Their energy distribution has been measured several times, in particular by HEAT-PBAR. The result is surprising: an excess of positrons appears around an energy of a few GeV. By excess, it should be understood that the energy distribution of the positrons of cosmic rays is fairly well understood, except at energies close to a few GeV, for which the models give quantities lower than those which are observed. Different hypotheses have been proposed in the scientific community to explain this excess, but none is completely satisfactory. This question is still pending.
E - The positrons of the galactic center
The two mechanisms that we have just described do not explain the major part of the annihilations observed in the center of the Galaxy. The central point in the INTEGRAL map above corresponds to approximately 1043 annihilations / second, or the annihilation of 10 billion tonnes of positrons per second! Annihilation radiation also has some interesting properties. To begin with, the line is thin, which implies that the emission takes place in a relatively calm astrophysical environment. Then, by studying more precisely the 511 keV annihilation line, we can show that the annihilation takes place between electrons and positrons with very low relative velocities (they form a stable association called positronium). Indeed, we observe that part of the annihilations gives rise to the emission of 3 photons, which is only possible if there is formation of this positronium.
Let us now review some of the mechanisms that have been considered to explain this excess.
Supernovae can create radioactive elements
- The novæ (thermonuclear explosions taking place on the surface of white dwarfs accreting the mass of a companion) create 22Na, which decays by radioactivity ß +. One could imagine that the galactic center contains a significant quantity of these novæ. This hypothesis is not satisfactory, because this disintegration is accompanied, in the case of this particular element, by a characteristic gamma line at 275 keV. Now the galactic center does not show an excess in this wavelength ...
- The supernovae synthesize 56Co which decays by radioactivity ß +. This scenario is also problematic because only a small percentage of the positrons thus created can escape from the envelope of the supernovae and the emission of annihilation should be largely reabsorbed by these supernovae. In addition, the galactic center seems to contain little supernovae.- Hypernovæ (supernovae of a particular type, resulting from the collapse of very massive stars) could also produce the element 56sCo.
- The plasma that surrounds compact sources (like micro-quasars) must create electron-positron pairs, and one could imagine that the galactic center contains a significant number of these objects. The problem here is that the 511 keV annihilation line has never been seen in compact sources, and one can doubt the corresponding theoretical predictions.
The fact that the emission takes place by positronium formation is important. One of two things, either the positrons are created with a low speed (therefore at low energy), which excludes the violent phenomena which we have just described, or else they were created at high energy, but in this case it remains to explain why we do not observe the radiation which should accompany their slowing down!
As we can see, this observation by INTEGRAL poses an important problem. We will return at the end to a possible (but more speculative) explanation in terms of dark matter.
F I N .
Antimatter in our Galaxy '2' Continued…
Richard Taillet
Teacher, Physical Researcher.
Published 07/01/2005 - Modified 10/28/2015.
Archives
• B - Radioactive decays
Radioactive elements are created in the Galaxy every time a supernova explodes. They
then disintegrate and those of them that undergo the ß + radioactivity emit positrons. An excellent example of this phenomenon is provided by the Al26s isotope of aluminum. Its disintegration also produces a very characteristic gamma radiation of energy, which means that this element can be mapped in the sky (this is what the COMPTEL satellite, also on board CGRO, the Observatory at Gamma Compton, then more recently the INTEGRAL satellite). The figure opposite shows such a map of the distribution of Aluminum 26 in our galaxy, as observed by COMPTEL. As can be seen on this map, these positron sources are concentrated in the galactic disc. This could a priori explain the elongated component in the disc of the positron map obtained by OSSE, but if this were the case we should observe a more extensive positron distribution along the disc: the distribution of radioactive nuclei extends over almost 90 degrees on each side of the disc. To resolve the problem, it will be necessary to wait until INTEGRAL accumulates enough data to detect and measure in turn this component of disk.
• C - High energy phenomena
Positrons can also be produced by high energy phenomena. For example, collisions of cosmic rays on interstellar gas also produce positrons. But that's not all: as the positron is approximately 2000 times lighter than the antiproton, it occurs much more easily, and the energetic phenomena which take place for example on the surface of our Sun (and therefore also in the even more energetic events, such as in the vicinity of pulsars for example) are enough to produce them.
Solar flare seen in the ultraviolet by Soho, March 21, 2003. The image on the left represents a zoom, with the Earth on a scale. During these eruptions, electron-positron pairs are produced. An eruption in July 2002 thus produced 500 grams of positrons. The amount of energy released by the annihilation of these positrons would be enough to supply France with energy for several days.
D - The positrons of cosmic rays and the excess observed by HEAT
Cosmic rays also contain positrons. Their energy distribution has been measured several times, in particular by HEAT-PBAR. The result is surprising: an excess of positrons appears around an energy of a few GeV. By excess, it should be understood that the energy distribution of the positrons of cosmic rays is fairly well understood, except at energies close to a few GeV, for which the models give quantities lower than those which are observed. Different hypotheses have been proposed in the scientific community to explain this excess, but none is completely satisfactory. This question is still pending.
E - The positrons of the galactic center
The two mechanisms that we have just described do not explain the major part of the annihilations observed in the center of the Galaxy. The central point in the INTEGRAL map above corresponds to approximately 1043 annihilations / second, or the annihilation of 10 billion tonnes of positrons per second! Annihilation radiation also has some interesting properties. To begin with, the line is thin, which implies that the emission takes place in a relatively calm astrophysical environment. Then, by studying more precisely the 511 keV annihilation line, we can show that the annihilation takes place between electrons and positrons with very low relative velocities (they form a stable association called positronium). Indeed, we observe that part of the annihilations gives rise to the emission of 3 photons, which is only possible if there is formation of this positronium.
Let us now review some of the mechanisms that have been considered to explain this excess.
Supernovae can create radioactive elements
- The novæ (thermonuclear explosions taking place on the surface of white dwarfs accreting the mass of a companion) create 22Na, which decays by radioactivity ß +. One could imagine that the galactic center contains a significant quantity of these novæ. This hypothesis is not satisfactory, because this disintegration is accompanied, in the case of this particular element, by a characteristic gamma line at 275 keV. Now the galactic center does not show an excess in this wavelength ...
- The supernovae synthesize 56Co which decays by radioactivity ß +. This scenario is also problematic because only a small percentage of the positrons thus created can escape from the envelope of the supernovae and the emission of annihilation should be largely reabsorbed by these supernovae. In addition, the galactic center seems to contain little supernovae.- Hypernovæ (supernovae of a particular type, resulting from the collapse of very massive stars) could also produce the element 56sCo.
- The plasma that surrounds compact sources (like micro-quasars) must create electron-positron pairs, and one could imagine that the galactic center contains a significant number of these objects. The problem here is that the 511 keV annihilation line has never been seen in compact sources, and one can doubt the corresponding theoretical predictions.
The fact that the emission takes place by positronium formation is important. One of two things, either the positrons are created with a low speed (therefore at low energy), which excludes the violent phenomena which we have just described, or else they were created at high energy, but in this case it remains to explain why we do not observe the radiation which should accompany their slowing down!
As we can see, this observation by INTEGRAL poses an important problem. We will return at the end to a possible (but more speculative) explanation in terms of dark matter.
F I N .
