²⁶Al gamma rays from the Galaxy with INTEGRAL/SPI

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany
e-mail: rod@mpe.mpg.de
² Horn & Company Financial Services GmbH, Kaistr. 20, 40221 Düsseldorf, Germany
³ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
⁴ Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK

Received: 27 September 2022
Accepted: 20 December 2022

Abstract

Context. The presence of radioactive ²⁶Al 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 ²⁶Al 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 ²⁶Al 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 ²⁶Al emission was enhanced using all event types measured within SPI. We redetermined the intensity of Galactic ²⁶Al 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)×10⁻³ ph cm⁻² s⁻¹ in the 1.809 MeV line from ²⁶Al, 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).