26Al gamma rays from the Galaxy with INTEGRAL/SPI

Context. The presence of radioactive Al-26 at 1.8 MeV reveals an ongoing process of nucleosynthesis in the Milky Way. Diffuse emission from its decay can be measured with gamma-ray telescopes in space. The intensity, line shape, and spatial distribution of the Al-26 emission allow for studies of these nucleosynthesis sources. The line parameters trace massive-star feedback in the interstellar medium thanks to its 1 My lifetime.Aims. We aim to expand upon previous studies of the Al-26 emission in the Milky Way, using all available gamma-ray data, including single and double events collected with SPI on INTEGRAL from 2003 until 2020.Methods. We applied improved spectral response and background as evaluated from tracing spectral details over the entire mission. The exposure for the Galactic Al-26 emission was enhanced using all event types measured within SPI. We redetermined the intensity of Galactic Al-26 emission across the entire sky, through maximum likelihood fits of simulated and model-built sky distributions to SPI spectra for single and for double detector hits.Results. We found an all-sky flux of (1.84 +/- 0.03)x10(-3) ph cm(-2) s(-1) in the 1.809 MeV line from Al-26, determined via fitting to sky distributions from previous observations with COMPTEL. Significant emission from higher latitudes indicates an origin from nearby massive-star groups and superbubbles, which is also supported by a bottom-up population synthesis model. The line centroid is found at (1809.83 +/- 0.04 keV), while the line broadening from source kinematics integrated over the sky is (0.62 +/- 0.3) keV (FWHM).

Automated summary

We studied the emission of radioactive Al-26 using all available gamma-ray data, including single and double events collected from 2003 to 2020 on the INTEGRAL telescope. By improving the spectral response and background evaluation, we enhanced the exposure of the Galactic Al-26 emission. Through maximum likelihood fits of simulated and model-built sky distributions to SPI spectra for single and double detector hits, we determined the intensity and spatial distribution of the Galactic Al-26 emission.