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  Current Research Highlight :: May 2005 all highlights

Annihilation of positrons in the Galaxy

The central part of our Galaxy is a site of copious production of positrons - antiparticles of a usual electron. INTEGRAL, ESA's gamma-ray observatory, with the collaboration of scientists from the Max Planck Institute for Astrophysics, is now making the most precise measurements of the positron annihilation spectrum of the Galaxy. The observed flux implies that of order 1043 positrons per second are annihilating in the inner part of our Galaxy. An important and unsolved problem is the origin of these positrons - the hypotheses range from stellar nucleosynthesis to annihilating dark matter. INTEGRAL in particular can study the properties of the interstellar medium where positrons are annihilating.

Fig. 1: Spectrum of the e+e- annihilation radiation detected by SPI towards the GC region after 3.5 million seconds exposure. The red line shows the positron annihilation line, while blue line shows the continuum spectrum associated with the three photon ortho-positronium decay.

Fig. 2: Heavily smoothed image of the Milky Way emission in the electron-positron annihilation line. Central bright spot corresponds to the Galactic Center region.

While great efforts have to be made for the production of sizable quantities of antimatter in laboratories, some antiparticles, especially the light ones, are routinely produced in nature. For instance the so-called ß+ decay of radioactive isotopes leads to the creation of a positron — a complete analog of the usual electron, but with a positive charge. This happens for example during the nuclear decay of the aluminium isotope Al26 into magnesium Mg26. Positrons are produced in multitude inside the reactors of usual nuclear power plants. In astrophysical conditions positrons can as well be born during nuclear decay of radioactive elements, produced in supernovae and nova explosions. They can also appear in the vicinity of rapidly rotating neutron stars or black holes, be produced by interaction of cosmic rays with ordinary matter or by gamma-ray bursts. But the most fascinating is a possible link between positron production and so-called "dark matter" particles. These dark matter particles provide a very prominent contribution to the total mass of the Universe, six times larger than the contribution of the ordinary matter we are made of. Some versions of the modern theory allow such particles to annihilate with each other, albeit very rarely, and to give rise to positrons along with photons and other particles.

While there is no shortage in possible channels for positron production, it remains to be determined which one is the most important. Scientists at MPA use the INTEGRAL gamma-ray observatory to search for signatures of positron production in our Galaxy. INTEGRAL is a space project of the European Space Agency with participation of Russia and USA, focused on imaging and ultra-fine spectroscopy in the gamma-ray band of the electromagnetic spectrum. A collision of a positron and an electron may result in the annihilation of the particle and antiparticle and generation of a flash of gamma-ray photons which INTEGRAL can detect. Particularly useful for positron annihilation studies is the spectrometer SPI developped at CESR (Toulouse) and MPE (Garching). In collaboration with Russian groups large share of the INTEGRAL observing time was spent on observing the Galactic Center region which is the most powerful source of the annihilation radiation on the sky. As a result more than 200000 annihilations photons were accumulated by SPI during first year, allowing for most accurate determination of the annihilation spectrum. The analysis of various data sets obtained by INTEGRAL is now going on in many scientific groups in Europe, Russia and USA.

In most scenarios positrons are born hot, i.e. their kinetic energy is at least of the order of their rest mass. The cross section for annihilation is very small and the positrons first cool down (lose their kinetic energy) via collisions with particles of ordinary matter. Once the energy of the positron decreases below 100 eV mutual attraction of positrons and electrons starts to dominate and they form so-called positronium atoms. Positronium is analogous to the hydrogen atom with the proton replaced by a positron. Positronium is thus composed of a particle and an antiparticle and so it lives less than 10-7 seconds before annihilating in a gamma-ray flash. Positronium appears in two flavors: ortho and para-positronium, with the former accounting for about 75% of all positronium atoms and the latter making up the remaining 25%, but living only 10-10 seconds. Para-positronium annihilation produces two photons with energy equal to the rest mass of the electron or the positron — 511 keV, which can be observed by telescopes as a narrow gamma-ray line. Ortho-positronium decays into 3 photons and instead of a narrow line a broad continuum is observed. These are exactly the signatures which INTEGRAL sees coming from the central region of the Galaxy as shown in Fig.1. Based on data from the first year of INTEGRAL operations, the position of the line center is found to coincide with the rest energy of the electron or positron to an accuracy better than one part out of 10000. Both the width of the line and ratio of the fluxes in the narrow line and low energy continuum are consistent with the annihilation of positrons in a warm (T ≈ 8000K) slightly ionized gas which occupies a large volume in the disk of the Galaxy.

Thus INTEGRAL observations do show that plenty of positrons are annihilating in warm partly ionized matter in the Galaxy every second. The question of the ultimate source of these positrons is yet to be answered with ongoing INTEGRAL observations. A large fraction of positrons are annihilating in the innermost region of the Galaxy. Type I supernovae — the "standard" candidate for positrons production could be the primary source, but to account for the bulk of the Galactic positrons they have to escape from a dense supernova remnant. Production of positrons by dark matter particles therefore remains an attractive option. A crucial test would be provided by comparing the exact morphology of the annihilation line intensity distribution with the distribution of potential sources of positrons, including compact sources, supernova remnants and dark matter. INTEGRAL is now making a detailed map of annihilation radiation to finally resolve this problem.

E. Churazov, R. Sunyaev, S. Sazonov, M. Revnivtsev

Further information:

E. Churazov, R. Sunyaev, S. Sazonov, M. Revnivtsev, D. Varshalovich, Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL, 2005, MNRAS, 357, 1377

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