26Al in the inner galaxy: large-scale spectral characteristics derived with SPI/INTEGRAL

Aims. We performed a spectroscopic study of the 1809 keV gamma-ray line from 26 Al decay in the Galaxy using the SPI imaging spectrometer with its high-resolution Ge detector camera on the INTEGRAL observatory. Methods. We analyzed observations of the first two mission years, fitting spectra from all 7130 telescope pointings in narrow energy bins to models of instrumental background and the 26 Al sky. Instrumental background is estimated from independent tracers of cosmic-ray activation. The shape of the 26 Al signal is compared to the instrumental response to extract the width of the celestial line. Results. We detect the 26 Al line at ≃16σ significance. The line is broadened only slightly, if at all; we constrain the width to be below 2.8 keV (FWHM, 2cr). The average Doppler velocities of 26 Al at the time of its decay in the interstellar medium (τ ∼ 1.04 My) therefore are probably around 100 km s -1 , in agreement with expectations from Galactic rotation and interstellar turbulence. The flux (3.3 (±0.4) x 10 -4 ph cm -2 s -1 rad -1 ) and spatial distribution of the emission are found consistent with previous observations. The derived amount of 26 Al in the Galaxy is 2.8 (±0.8) M ⊙ .