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

Abstract

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 Te∼ 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 (Te∼ 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 (Te≥ 106 K) phase is constrained to be less than ∼8 per cent. Neither a moderately hot (Te≥ 105 K) ionized medium nor a very cold (Te≤ 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.

Galactic annihilation emission from nucleosynthesis positrons

“…The annihilation emission observed with SPI in the inner Galactic regions, the 511 keV line and the continuum below down to ∼400 keV, indicates that most positrons annihilate in the warm medium of the ISM (Churazov et al 2005;Jean et al 2006). Approximately one half annihilate in the warm ionised phase, through the formation of Ps by radiative recombination.…”

The Advanced Compton Telescope mission

“…ACT will measure the radioactive !-ray and positron emitters among them across the entire Milky Way, mapping our galaxy in a broad range of nuclear line emissions (Fig. E1) from radioactive decays ( 22 Na, 26 Al, 44 Ti, 60 Fe), nuclear de-excitations ( 12 C, 16 O, 56 Fe) and e + eannihilations.…”

About cosmic gamma ray lines

“…INTEGRAL with its spectrometer SPI then allowed us to obtain a more detailed view of the positron annihilation gamma-ray emission [86,49,87,88,89,90,91]. The inner Galaxy with its bulge region was the dominant source, while the disk of the Galaxy was faint, contrasting expectations that positron sources would be related to active star forming regions where massive stars and binary interactions would conspire to create high-energy sources.…”

“…The spectral signature consists of a line at 511 keV and a characteristic continuum towards lower energies, with relative intensities depending on how much annihilation occurs through the positronium intermediate state. A partially-ionised medium with a temperature of about 7000 K was inferred as best describe the annihilation sites [87,92]. This can now be studied for different regions, as spatially resolves spectroscopy becomes a reality (Fig.…”