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We present the results of a systematic search for outbursts in the narrow positron annihilation line on various time scales (5x10^4 - 10^6 s) based on the SPI/INTEGRAL data obtained from 2003 to 2008. We show that no outbursts were detected with a statistical significance higher than ~6 sigma for any of the time scales considered over the entire period of observations. We also show that, given the large number of independent trials, all of the observed spikes could be associated with purely statistical flux fluctuations and, in part, with a small systematic prediction error of the telescopes instrumental background. Based on the exposure achieved in ~6 yr of INTEGRAL operation, we provide conservative upper limits on the rate of outbursts with a given duration and flux in different parts of the sky.
The first detection of a gamma ray line with an energy of about 500 keV from the center our Galaxy dates back to the early seventies. Thanks to the astrophysical application of high spectral resolution detectors, it was soon clear that this radiation
The first gamma-ray line originating from outside the solar system that was ever detected is the 511 keV emission from positron annihilation in the Galaxy. Despite 30 years of intense theoretical and observational investigation, the main sources of p
The signature of positron annihilation, namely the 511 keV $gamma$-ray line, was first detected coming from the direction of the Galactic center in the 1970s, but the source of Galactic positrons still remains a puzzle. The measured flux of the annih
We present a possible explanation of the recently observed 511 keV $gamma$-ray anomaly with a new ``millicharged fermion. The new fermion is light (${cal O}({rm MeV})$) but has never been observed by any collider experiments mainly because of its tin
The imaging spectrometer SPI on board ESAs INTEGRAL observatory provides us with an unprecedented view of positron annihilation in our Galaxy. The first sky maps in the 511 keV annihilation line and in the positronium continuum from SPI showed a puzz