Line shape diagnostics of Galactic 26

The shape of the gamma-ray line from radioactive 26Al, at 1808.7 keV energy in the frame of the decaying isotope, is determined by its kinematics when it decays, typically 106 y after its ejection into the interstellar medium from its nucleosynthesis source. Three measurements of the line width exist: HEAO-C’s 1982 value of (0 + 3) keV FWHM, the GRIS 1996 value of (5.4± 1.3) keV FWHM, and the recent RHESSI value of (2.0± 0.8) keV FWHM, suggesting either “cold”, “hot”, or “warm” 26Al in the ISM. We model the line width as expected from Galactic rotation, expanding supernova ejecta, and/or Wolf-Rayet winds, and predict a value below 1 keV (FWHM) with plausible assumptions about 26Al initial velocities and expansion history. Even though the recent RHESSI measurement reduces the need to explain a broad line corresponding to 540 km s−1 mean 26Al velocity through extreme assumptions about grain transport of 26Al or huge interstellar cavities, our results suggest that standard 26Al ejection models produce a line on the narrow side of what is observed by RHESSI and INTEGRAL. Improved INTEGRAL and RHESSI spatially-resolved line width measurements should help to disentangle the effects of Galactic rotation from the ISM trajectories of 26Al.