Positron annihilation emission from the inner Galaxy

The inner Galaxy is a strong source of extended diffuse gamma-ray emission from positron annihilation. The origin of this radiation is under debate. Various sources, including Low mass X-ray Binaries, novae, supernovae, cosmic rays, superconducting cosmic strings and dark matter have been proposed. Older results (Dermer & Skibo 1997) obtained with the Compton Gamma Ray Observatory (CGRO) had indicated a possible feature 'above' the Galactic Centre at positive galactic latitude – the "Annihilation Fountain". Early INTEGRAL results showed clearly that the emission was concentrated in the central region with no conspicuous structures towards high latitudes. But more recent results indicate that the distribution is lopsided, which may be correlated with the distribution of LMXB in the galaxy (Weidenspointner et al. 2008).

But not only the spatial distribution, also the spectrum of the annihilation radiation yields important information. As Churazov et al. (2005) found, the spectrum is consistent with being unshifted and the width of the line is very narrow with just 2.37±0.25 keV, the most stringent constraints up to date. The results are compatible with a single phase, warm and weakly ionized ISM.

²⁶Al emission and the supernova rate

The radioactive isotope ²⁶Al can be created in in various processes in stellar evolution. It decays radioactively to ²⁶Mg with a half-life of ~720,000 years – still short on astronomical scales – emitting gamma-ray photons at 1.809 MeV. Previous observations by the COMPTEL instrument onboard CGRO, had shown that the line emission is concentrated in the Galactic plane, especially the Inner Galaxy but also, e.g., in the Cygnus or Vela regions.

Measuring shifts in the line energy at different regions with unprecedented resolution, Diehl et al. (2006) showed that the source regions for this emission corotate with the Galaxy, consistent with a Galaxy-wide origin, rather than from foreground regions with local and peculiar ²⁶Al production. The present-day equilibrium mass of ²⁶Al was found to be 2.8±0.8 solar masses, which in turn allows to estimate the frequency of core-collapse supernovae as 1.9±1.1 events per century.

⁶⁰Fe line emission

⁶⁰Fe with a half-life of ~1.5 Myr is expected to be produced in core-collapse supernovae, similar to ²⁶Al. The detection of line emission at 1173 keV and 1333 keV was first reported by Harris et al. (2005) and confirmed by Wang et al. (2007). The flux ratio ⁶⁰Fe/²⁶Al was found to be substantially below the factor ≥0.40 predicted for massive stars by previous model calculations, but more recent predictions have come down again to 0.185±0.0625, in line with the observed value of 0.148±0.06.

⁴⁴Ti line emission from supernova remnants

Using the ISGRI imager, Renaud et al. (2006) detected both the 67.9 and 78.4 keV ⁴⁴Sc γ-ray lines in the SNR Cassiopeia A. Compared to previous observations the better energy resolution allowed to clearly separate both lines and the imaging in narrow spectral bands confirms the spectral results. Combining COMPTEL, BeppoSAX PDS and ISGRI measurements Renaud et al. derive a relatively high mass of synthesized ⁴⁴Ti (1.6+0.6-0.3 × 10^-4 solar masses), sugggesting that Cas A is peculiar in comparison to other young supernova remnants, from which so far no line emission from ⁴⁴Ti decay has been unambiguously detected.