Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL

The electron-positron annihilation spectrum observed by SPI/INTEGRAL during deep Galactic Centre region exposure is reported. The line energy (510.954+/-0.075 keV) is consistent with the unshifted annihilation line. The width of the annihilation line is 2.37 +/- 0.25 keV (full width at half-maximum), while the strength of the ortho-positronium continuum suggests that the dominant fraction of positrons (94 +/- 6 per cent) form positronium before annihilation. Compared to the previous missions, these deep INTEGRAL observations provide the most stringent constraints on the line energy and width.

Under the assumption of an annihilation in a single-phase medium, these spectral parameters can be explained by a warm T_e~ 7000 to 4 × 10⁴ K gas with degree of ionization larger than a few 10^-2. One of the widespread phases of the interstellar medium (ISM) - warm (T_e~ 8000 K) and weakly ionized (degree of ionization ~0.1) - satisfies these criteria. Other single-phase solutions are also formally allowed by the data (e.g. cold, but substantially ionized ISM), but such solutions are believed to be astrophysically unimportant.

The observed spectrum can also be explained by annihilation in a multiphase ISM. The fraction of positrons annihilating in a very hot (T_e>= 10⁶ K) phase is constrained to be less than ~8 per cent. Neither a moderately hot (T_e>= 10⁵ K) ionized medium nor a very cold (T_e<= 10³ K) neutral medium can make a dominant contribution to the observed annihilation spectrum. However, a combination of cold/neutral, warm/neutral and warm/ionized phases in comparable proportions could also be consistent with the data.