Diffuse Galactic emission spectrum between 0.5 and 8.0 MeV

TL;DR

This study provides a new, more precise measurement of the diffuse gamma-ray emission spectrum of the Milky Way between 0.5 and 8 MeV using INTEGRAL/SPI, confirming previous results and offering insights into cosmic-ray interactions.

Contribution

First analysis with INTEGRAL/SPI that surpasses past measurements in signal-to-noise ratio, refining the diffuse emission spectrum and comparing it with cosmic-ray models.

Findings

Spectrum follows a power-law with index -1.39

No evidence of an 'MeV bump' in the spectrum

Cosmic-ray models broadly agree with the observed inverse Compton emission

Abstract

The last measurement of the diffuse emission spectrum of the Milky Way in the megaelectronvolt (MeV) photon energy range was performed by CGRO/COMPTEL more than 20 years ago. We report a new analysis with the spectrometer SPI aboard INTEGRAL in the band 0.5-8.0 MeV, finally superseding the signal-to-noise ratio of the historic observations. This is possible thanks to an elaborate instrumental background model and careful considerations of the selected data, which are strongly affected by solar activity. We base our analysis on energy-dependent spatial template fitting in a region of $\Delta l \times \Delta b = 95^\circ \times 95^\circ$ around the Galactic centre. Our flux estimates are consistent with COMPTEL measurements and show no `MeV bump'. The spectrum follows a power-law shape with index $-1.39 \pm 0.09_{\rm stat} \pm 0.10_{\rm syst}$ and an integrated flux of $(5.7 \pm 0.8_{\rm stat} \pm 1.7_{\rm syst}) \times 10^{-8}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$ between 0.5 and 8.0 MeV. We find that cosmic-ray electrons and propagation models consistent with the latest Fermi/LAT, Voyager 1, and AMS-02 data are broadly in agreement with the inferred inverse Compton spectral shape. However, a mismatch of a factor of 2-3 in normalisation with respect to baseline expectations may point to enhanced target photon densities and/or electron source spectra in the inner Galaxy, slightly modified diffusion properties, or the presence of an unresolved population of MeV $\gamma$-ray sources.