Line shape diagnostics of Galactic $\mathsf{^{26\!}}$Al

The shape of the gamma-ray line from radioactive $\element[][26\!]{Al}$, at 1808.7 keV energy in the frame of the decaying isotope, is determined by its kinematics when it decays, typically 106y 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 $(5.4\pm 1.3)$keV FWHM, and the recent RHESSI value of $(2.0\pm 0.8)$keV FWHM, suggesting either “cold”, “hot”, or “warm” $\element[][26\!]{Al}$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 $\element[][26\!]{Al}$initial velocities and expansion history. Even though the recent RHESSI measurement reduces the need to explain a broad line corresponding to 540 km s-1mean $\element[][26\!]{Al}$velocity through extreme assumptions about grain transport of $\element[][26\!]{Al}$or huge interstellar cavities, our results suggest that standard $\element[][26\!]{Al}$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 $\element[][26\!]{Al}$.