Unveiling the Origin of the Fermi Bubbles with MeV Photon Telescopes

The Fermi Bubbles (FBs) are a pair of large-scale ellipsoidal structures extending above and below the Galactic plane almost symmetrically aligned with the Galactic center. After more than 10 yr since their discovery, their nature and origin remain unclear. Unveiling the primary emission mechanisms, whether hadronic or leptonic, is considered to be the main tool to shed light on the topic. We explore the potential key role of MeV observations of the FB, and we provide a recipe to determine the sensitivity of Compton and Compton-pair telescopes to the extended emission of the FB. We illustrate the capabilities of the Imaging Compton Telescope COMPTEL, the newly selected NASA MeV mission Compton Spectrometer and Imager, as well as the expectations for a potential future Compton-pair telescope such as the All-sky Medium Energy Gamma-ray Observatory eXplorer.