Diffuse continuum emission and large extended sources at MeV energies

TL;DR

Future MeV gamma-ray surveys will significantly advance our understanding of Galactic and extragalactic diffuse emission and the origin of large-scale structures. By focusing on the 10–100 MeV window and leveraging propagation models, the work shows how DGE spectra and morphology can disentangle IC versus pion-decay components and constrain LIS CR electrons. It demonstrates that MeV spectroscopy of the Fermi bubbles and Loop I can discriminate between leptonic and hadronic emission, with magnetic field strength affecting the predictions. The paper argues for next-generation MeV instruments (e.g., COSI as a stepping stone, followed by newASTROGAM/AMEGO-X) to achieve the sensitivity and angular resolution needed to map diffuse emission, resolve populations in the EGB, and probe new physics through multi-messenger connections.

Abstract

Future gamma-ray survey instruments, such as newASTROGAM and AMEGO-X, will significantly improve previous and current all-sky surveys at MeV energies. In this paper we discuss the continuum emission from the Milky Way, two prominent large extended sources, the Fermi bubbles and Loop I, and the extragalactic gamma-ray background. We highlight the importance of measurements in the MeV to GeV energy range for understanding CR production and propagation in the Galaxy, for the determination of the nature of the Fermi bubbles and Loop I, and for exploring the origin of the extragalactic gamma-ray background.

Figures (5)

• Figure 1: Measurements of the DGE in the direction of the inner Galaxy during the 2016 COSI balloon flight 2023ApJ...959...90K, by COMPTEL 1994AA...292...82S1999ApLC..39..209S, and SPI 2022AA...660A.130S2022PhRvD.106b3030B in comparison to expectations 2022ApJS..262...30P2022AA...660A.130S. The models do not include line emission visible in the data at 511 keV (e$^{+}$/e$^{-}$ annihilation) and 1.81 MeV ($^{26}$Al decay). Figure adapted from 2023ApJ...959...90K.
• Figure 2: Upper panel: Comparison of the locally observed primary CR electron + positron spectra by AMS-02 2014PhRvL.113v1102A and HESS 2008PhRvL.101z1104A and the spectrum observed by Voyager-1 in interstellar space 2016ApJ...831...18C (denoted as "LIS" in panel legend) to expectations from two different models of CR propagation. Both models were computed with Galprop2022ApJS..262...30P, assuming plain diffusion + convection of CR in the ISM in one case (black), and diffusion with re-acceleration of CR in the ISM (pink) in the other case. Lower panel: Expected DGE emission in the MeV and GeV bands from the two CR propagation models shown in the upper panel. The expected emission (black/pink solid lines) is compared to the MeV observations displayed in \ref{['fig:karwin']}, and to the measurement of the diffuse emission by Fermi LAT in 2012ApJ...750....3A (red bars, right panel). Fermi-LAT measurements have been published for a different region of the sky than the MeV measurements in the left panel. The corresponding region is indicated above the respective panel. Individual contributions from various interaction processes are shown as dashed/dotted lines (see legend).
• Figure 3: Intensity of emission of the FBs (red circles, left panel) and Loop I (red circles, right panel) at latitudes $|b| > 10^\circ$2014ApJ...793...64A_FB_Fermi. Blue solid line shows the hadronic model of the $\gamma$-ray emission. Dashed orange line, green sparse dash-dotted line, and dotted red line show the primary $\pi^0$, secondary IC, and secondary bremsstrahlung components in the hadronic scenario respectively. Dashed purple line - leptonic scenario of $\gamma$-ray emission (dominated by IC emission). Bands show the 1 sigma model uncertainty ranges for statistical plus 10% systematic uncertainties in the data. Pink diamonds, brown squares, and grey upward triangles show the extragalactic diffuse $\gamma$-ray background measured by COMPTEL2000AIPC..510..467W, EGRET2004ApJ...613..956S, and Fermi LAT 2015ApJ...799...86A respectively. Yellow sparse dashed lines show expected COSI sensitivity after 2 years of observations 2023arXiv230812362T for an extended source with the area of the high-latitude FBs $\Omega \approx 1$ sr and Loop I $\Omega \approx 3$ sr respectively. Expected AMEGO-X2022JATIS...8d4003C and newASTROGAMBerge:2025ICRC sensitivities for high-latitude FBs and Loop I after 3 years of observations are shown by blue dash-dot-dotted and cyan long-dashed lines respectively.
• Figure 4: Measurements of the extragalactic X-ray and $\gamma$-ray background from 1 keV to 820 GeV. The energy ranges of the upcoming COSI satellite and other proposed future $\gamma$-ray space missions are indicted by the arrows above the figure. Figure adapted from 2015ApJ...799...86A.
• Figure 5: Overview of the contributions of various source populations to the total EGB. The shaded areas represent the uncertainties in the contributions according to the calculations in the respective publications. The magenta line demonstrates how a potential $\gamma$-ray signal from dark matter annihilation would lead to features in the spectrum. Non-observation of such features allows to constrain the dark matter annihilation cross section. Figure taken from 2015ApJ...800L..27A.