The Galactic $^{26}$Al emission map as revealed by INTEGRAL/SPI

TL;DR

This study uses 10 years of INTEGRAL/SPI data to produce a detailed map of galactic $^{26}$Al emission, revealing its distribution and relation to stellar sources, and estimating the $^{60}$Fe to $^{26}$Al ratio.

Contribution

It provides the first updated $^{26}$Al emission map since COMPTEL, utilizing INTEGRAL/SPI data and advanced image reconstruction, enhancing understanding of galactic nucleosynthesis.

Findings

Inner Galaxy flux is about 3.3×10⁻⁴ ph.cm⁻².s⁻¹.

Emission correlates with young stellar populations and spiral arms.

$^{60}$Fe/$^{26}$Al ratio estimated at around 0.14.

Abstract

Diffuse emission is often challenging since it is undetectable by most instruments, which are generally dedicated to point-source studies. The $^{26}$Al emission is a good illustration: the only available $^{26}$Al map to date has been released, more than fifteen years ago, thanks to the COMPTEL instrument. However, at the present time, the SPI spectrometer aboard the INTEGRAL mission offers a unique opportunity to enrich this first result. In this paper, 2 $\times$ 10$^8$ s of data accumulated between 2003 and 2013 are used to perform a dedicated analysis, aiming to deeply investigate the spatial morphology of the $^{26}$Al emission. The data are first compared with several sky maps based on observations at various wavelengths to model the $^{26}$Al distribution throughout the Galaxy. For most of the distribution models, the inner Galaxy flux is compatible with a value of 3.3$\times$ 10$^{-4}$ ph.cm$^{-2}$.s$^{-1}$ while the preferred template maps correspond to young stellar components such as core-collapse supernovae, Wolf-Rayet and massive AGB stars. To get more details about this emission, an image reconstruction is performed using an algorithm based on the maximum-entropy method. In addition to the inner Galaxy emission, several excesses suggest that some sites of emission are linked to the spiral arms structure. Lastly, an estimation of the $^{60}$Fe line flux, assuming a spatial distribution similar to $^{26}$Al line emission, results in a $^{60}$Fe\ to $^{26}$Al ratio around 0.14, which agrees with the most recent studies and with the SN explosion model predictions.