Diffuse Galactic emission spectrum between 0.5 and 8.0 MeV

¹ Max-Planck-Institute for extraterrestrial Physics, Giessenbachstr. 1, 85748 Garching bei München, Germany
e-mail: tho.siegert@gmail.com; tsiegert@mpe.mpg.de
² Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Campus Hubland Nord, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
³ Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, LAPTh, 74940 Annecy, France

Received: 11 November 2021
Accepted: 25 January 2022

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 yr 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 (S/N) 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 Δl ×Δb = 95° ×95° 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 ± 0.09_stat ± 0.10_syst and an integrated flux of (5.7 ± 0.8_stat ± 1.7_syst) x 10⁻⁸ 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 γ-ray sources.