<sup>26</sup>Al spectroscopy with SPI: The challenge to detect Galactic rotation

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²⁶Al spectroscopy with SPI: The challenge to detect Galactic rotation

Kretschmer, K. / Diehl, R. / Hartmann, D.H. / Strong, A.W.

AbstractThe shape of the γ-ray line from radioactive ²⁶Al, at 1808.7keV energy in the frame of the decaying isotope, is determined by its kinematics when it decays, typically 10⁶yr after its ejection into the interstellar medium from its nucleosynthesis source. Three prior 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 1keV (FWHM) with plausible assumptions about ²⁶Al initial velocities and expansion history. We use this model as an input to a model-fitting approach for INTEGRAL/SPI data analysis in the 1808.7keV line. Our aim is to detect the signature of Galactic rotation and to constrain the contributions of ²⁶Al from different regions of the Galaxy to the total emission, such as putting an upper limit on the contribution of local sources.