Events at Physics |
Galaxy (1043 e+/s) has been established for three decades now but has yet to find a convincing explanation, especially after the INTEGRAL satellite (launched in 2002) produced a radiation map showing the emission to be mostly coming from the galactic bulge (Knodlseder et al. 2004, Weidenspointner et al. 2008). While the physics of the annihilation of positrons has now mostly been understood (Guessoum, Jean, Gillard 2005), the origin and propagation of those positrons remain an unsettled question, although good progress has been made lately with interesting models and constraints presented.<br>
In this seminar I first review the current knowledge of that
positron-electron annihilation radiation, with what the spectroscopic analysis and the spatial mapping tell us about the media of annihilation (mainly) and to some extent about the sources of positrons. I explain the physical processes which the positrons undergo during their long lives and highlight the role of the interstellar gas, dust, and organic molecules. I then focus on the Galactic Center (with its supermassive black hole) and its potential contribution to explaining part, if not all, of the annihilation radiation observed at least in the Galactic bulge. Several specific proposals have recently been made for the production of positrons by the GC?s SMBH, and they will be critically reviewed on the basis of:a) the positrons? propagation away from the central compact source, given their initial kinetic energy; b) the constraint from direct annihilation emission in flight produced by relativistic positrons; c) estimates of the rates of positron production and other plausibility considerations. Finally, I briefly highlight the important issue of positron propagation from their production site(s), for which the specific physical process is uncertain (collisional or diffusional) and the conditions of the region (density, temperature, and magneticfield distributions) are complex.
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I conclude with some pointers for near-future research on the problem.