Diffuse Galactic emission spectrum between 0.5 and 8.0 MeV

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 ∆l × ∆b = 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 ) × 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.

Updated Constraints on Primordial Black Hole Evaporation

“…in the anti-Galactic center. This is a conservative choice, since D(Ω) is at its minimum for the anti-Galactic center direction; however, this choice is actually overly conservative: measurements of the diffuse Galactic hard X-ray/soft-γ-ray emission indicate that at a Galactic latitude of b = 15 • , the diffuse emission from the Galaxy is highly suppressed [34]. to optimize the line of sight integral D(Ω), we thus choose the l = 0, b = 15 • direction, which has negligible Galactic diffuse flux contamination to set out limits from the isotropic flux; notice that unlike in Ref.…”

The Positron Puzzle

“…Hydrodynamics simulations suggest that radioactive isotopes, and therefore the starting points of e + -propagation, can reach very high distances above the Galactic plane (Krause et al, 2021). If e + s are emitted at heights of 2 kpc or (Siegert et al, 2022a;Berteaud et al, 2022), excluding the γ-ray lines at 511 keV and 1809 keV, as well as the ortho-Ps continuum. Top left: Generic GALPROP model components can roughly explain the entire spectrum with Inverse Compton (IC) emission from the CMB, infrared (IR) light, and optical (opt, scaled up by a factor of 2, suggested by Bouchet et al (2011), for example) light, and a bremsstrahlung component (scaled up by a factor of 3 to roughly match the data).…”

“…6 Diffuse MeV continuum spectrum possibilities of the Milky Way in the range l ≤ 47.5 • , b ≤ 47.5 • . The gray data points indicate the measured Milky Way spectrum between 0.03 and 8 MeV with INTEGRAL/SPI(Siegert et al, 2022a;Berteaud et al, 2022), excluding the γ-ray lines at 511 keV and 1809 keV, as well as the ortho-Ps continuum. Top left: Generic GALPROP model components can roughly explain the entire spectrum with Inverse Compton (IC) emission from the CMB, infrared (IR) light, and optical (opt, scaled up by a factor of 2, suggested byBouchet et al (2011), for example) light, and a bremsstrahlung component (scaled up by a factor of 3 to roughly match the data).…”

Astrophysical and Cosmological Probes of Dark Matter

“…Progress will therefore be critically reliant on new insights for how to exploit the data. This is happening on many fronts, including identifying new objects in which to search for dark matter signals, such as newly discovered Milky Way dwarfs [78][79][80][81][82][83], galaxy cata-Snowmass2021 Theory Frontier: Astrophysical and Cosmological Probes of Dark Matter logs [84,85], or dark substructure [86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103][104][105], as well as the development of techniques such as cross correlation with different datasets [106][107][108][109][110][111][112][113], improvements in our modeling of diffuse backgrounds [114][115][116][117], the extension to axion searches as we describe in Sec. 2.4, and the exploitation of the dark matter brightness of the ambient Milky Way [32,[118][119][120].…”