Annihilating Positrons in the Galactic Centre Region
While in the early Universe particles and antiparticles were present in almost equal numbers, our today's world is (luckily) asymmetric and dominated by particles, e.g. protons and electrons. Still, some antiparticles are routinely produced in nature even now. For instance positrons - anti-particles to a usual electron - are created during the beta+ decay of radioactive isotopes, collisions of high-energy photons and decay of heavy particles. Positrons are also easily produced in the lab and are actively used in medicine and industry.
Copious production of positrons is taking place in the Galactic Centre region. INTEGRAL sees strong emission from an area of a few degrees around the dynamic centre of the Milky Way, where every second about 10 billion tons of positrons are colliding with electrons and annihilating. The observed gamma-ray glow has a unique spectrum, which consists of a narrow line at 511 keV - an unmistakable signature of cold electron-positron annihilation, and a broad continuum, indicating that most positron-electron pairs form atoms of positronium before annihilating.
The origin of the Galactic Centre positrons remains a puzzle. Possible scenarios include the beta+ decay of radioactive isotopes cooked in SNIa explosions, activity of our Galaxy's supermassive black hole and decay/annihilation of enigmatic dark matter particles. Whatever the true origin, INTEGRAL observations indicate that since their birth and until the final gamma-ray flash positrons probably live in a radiatively cooling interstellar medium. When the medium cooling time is shorter than the annihilation time, the gas cools to low temperatures before the positron has a chance to annihilate. This process results in spectral signatures resembling those observed from the Galactic Centre by INTEGRAL.
The picture shows an INTEGRAL all sky map at 511 keV in galactic co-ordinates with the INTEGRAL observed spectrum (upper left) and a predicted model spectrum of the cooling process in the ISM (lower right).
Reference and more details:
Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL revisited: annihilation in a cooling ISM?
Churazov, E., Sazonov, S., Tsygankov, S., Sunyaev, R., Varshalovich, D., 2011.
Monthly Notices of the Royal Astronomical Society 411, 1727-1743.
