Searching for the 511 keV annihilation line from galactic compact objects with the IBIS gamma ray telescope

INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: giovanni.decesare@iasf-roma.inaf.it

Received: 14 January 2011
Accepted: 17 April 2011

Abstract

Context. The first detection of a gamma ray line with an energy of about 500 keV from the center of 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 was due to the 511 keV photons generated by electron-positron annihilation. Even though the physical process are known, the astrophysical origin of this radiation is still a mystery.

Aims. The spectrometer SPI aboard the INTEGRAL gamma-ray satellite has been used to produce the first all-sky map in light of the 511 keV annihilation, but no direct evidence of any 511 keV galactic compact objects has been found. Owing to its moderate angular resolution, these SPI data are still compatible with a distribution of point sources clustered in the bulge of our Galaxy. Thanks to the fine angular resolution and the large field of view, the IBIS imager on the INTEGRAL satellite gives us the unique opportunity to search for a possible 511 keV line from point sources associated to known objects, such as X-ray binaries, or supernovae, or even new ones.

Methods. We present the first deep IBIS 511 keV all-sky map, obtained by applying standard analysis to about 5 years of data. Possible 511 keV signals are also searched over hour-day-month timescales. The IBIS sensitivity at 511 keV depends on the detector quantum efficiency at this energy and on the background. Both these quantities were estimated in this work.

Results. We find no evidence of Galactic 511 keV point sources. With an exposure of 10 Ms, in the center of the Galaxy, we estimate a 1.6 × 10^-4   ph   cm^-2   s^-1 flux 2 sigma upper limit. A similar limit is given in a wide area in the Galactic center region with similar exposures. The IBIS 511 keV flux upper limits for microquasars and supernova remnants detected in the hard X domain (E > 20   keV) are also reported.

Conclusions. Our results are consistent with a diffuse e ⁺ e ⁻  annihilation scenario. If positrons are generated in compact objects, we expect that a significant fraction of them propagate in the interstellar medium before they are annihilated away from their birth places.