$^{26}$Al gamma rays from the Galaxy with INTEGRAL/SPI

TL;DR

This study uses extensive INTEGRAL/SPI gamma-ray data to precisely measure the distribution, intensity, and kinematic properties of $^{26}$Al emission in the Milky Way, revealing insights into ongoing nucleosynthesis and massive-star feedback.

Contribution

It provides an improved measurement of Galactic $^{26}$Al emission by analyzing all available data with enhanced spectral response and background modeling, refining previous results.

Findings

Galactic $^{26}$Al flux: (1.84±0.03)×10⁻³ ph cm⁻² s⁻¹

Line centroid at 1809.83±0.04 keV, indicating source kinematics

Detection of high-latitude emission linked to nearby massive-star groups

Abstract

The presence of radioactive $^{26}$Al at 1.8 MeV reflects ongoing 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 $^{26}$Al emission allow a study of these nucleosynthesis sources. The line parameters trace massive-star feedback in the interstellar medium due to its 1~My lifetime. We aim to deepen previous studies of the $^{26}$Al emission in the Milky Way, using all gamma-ray data including single and double events as collected with SPI on INTEGRAL from 2003 until 2020. We apply improved spectral response and background as evaluated from tracing spectral details over the entire mission. The exposure for Galactic $^{26}$Al emission is enhanced using all event types measured within SPI. We re-determine the intensity of Galactic $^{26}$Al 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. We find an all-sky flux of (1.84$\pm$0.03$)\times$10$^{-3}$~ph~cm$^{-2}$s$^{-1}$ in the 1.809~MeV line from $^{26}$Al, determined as fitted to sky distributions from previous observations with COMPTEL. Significant emission from higher latitudes indicate an origin from nearby massive-star groups and superbubbles, also supported by a bottom-up population synthesis model. The line centroid is found at (1809.83$\pm$0.04~keV, and line broadening from source kinematics integrated over the sky is (0.62$\pm0.3$)~keV (FWHM).