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Picture of the MonthMay 2006
The Galactic 511 keV Emission: A Magnetic Conveyor Belt for Positrons?The characteristic 511 keV line of positron (e+) annihilation is the first gamma-ray line ever detected outside the solar system; it was first detected with balloon-borne detectors in the seventies, from the direction of the Galactic center. With approximately 1.5 10 43 positrons annihilating every second, its luminosity is equivalent to a few thousand Suns. Despite decades of intense theoretical studies, the origin of those positrons still remains mysterious, since the positronic yields of the various candidate sources (supernovae of various types, pulsars, X-ray binaries, etc.) are highly uncertain. It appears, however, that supernovae of type Ia (SNIa: thermonuclear explosions of white dwarfs in binary systems) can indeed provide the required positron yield galaxy-wide. The first (and still preliminary) map of the spatial distribution of the 511 keV line was recently established by SPI/INTEGRAL, revealing an unexpected feature: a dominant emission from the Galactic bulge and a weak (at least three times weaker) emission from the Galactic disk. None of the known stellar populations in the Milky Way (old or young, active or "dead" objects) is expected to have such a dominant bulge component. For instance, the disk population of SNIa is expected to be ten times larger than the one of the bulge. This intriguing SPI/INTEGRAL result prompted some "exotic" explanations, as e.g. annihilation of light dark matter particles or a tangle of superconducting cosmic strings. However, in a recent paper (N. Prantzos, A&A 449, 869, 2006) it is pointed out that conventional sources (such as SNIa) can still be valid candidates, provided their positrons do not annihilate locally, but they travel far away from their origin; in that case, the Galactic disk should appear as an extended source of low surface brightness, the detection of which is below the current capability of SPI/INTEGRAL. Moreover, it is argued that a large fraction of the disk positrons can in fact be channeled to the bulge via the large scale magnetic field of the Galaxy and annihilate there. This could alleviate the problem, and even explain quantitatively the observations, as shown in the figure; however, this possibility depends critically on the properties of the Galactic magnetic field and on the propagation of low energy positrons in it, which are very poorly understood at present. The three images correspond to the same source, seen through detectors with increasing sensitivity to the 511 keV photons: from left to right the sensitivity is assumed to be 7 10-5 cm-2 s-1 (similar to the one of SPI/INTEGRAL for a 10 6 s exposure), 3.5 10-5 and 7 10-6, respectively. The source has three components (with positron annihilation rates L in 10 43 e+/s): a bulge (L=1.2, resulting from the transfer of half of the disk SNIa positrons), a thin disk (scaleheight 0.1 kpc and L=0.3, with e+ produced from the decay of radioactive 26Al, a product of massive stars), and a thick disk (scaleheight 1 kpc and L=1, from the other half of the disk SNIa positrons). Only the bulge is currently seen by SPI, while the thin disk will be clearly seen with a longer exposure; detection of the thick disk requires an extremely long exposure, or a more sensitive instrument. Credits: Nicolas Prantzos (IAP Paris)
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