Line shape diagnostics of Galactic ²⁶Al

Abstract

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.