Christopher Karwin (NASA Goddard Space Flight Center)
Mitchell Institute for Fundamental Physics & Astronomy
College Station, Texas 77843
The Galactic plane is a bright source of diffuse gamma-ray emission, consisting of both continuum and line components, spanning the MeV and GeV bands. Observations of the GeV emission with Fermi-LAT have led to a number of anomalies, including excess signals above model predictions towards both the inner Galaxy (i.e. the Galactic center excess) and the Andromeda galaxy. In the MeV band (~0.1 - 30 MeV), an excess towards the inner Galaxy has also been observed, having many similarities to that of the excess in the GeV band. In this talk I will discuss these anomalies. For the MeV band, I will focus on the Compton Spectrometer and Imager (COSI), a Small Explorer satellite mission recently selected by NASA and scheduled to launch in 2027. Although not one of its primary science goals, COSI will probe the Galactic diffuse continuum emission with unprecedented sensitivity between 0.2 - 5 MeV, offering a new window into some of these longstanding anomalies, and also providing an improved means to search for new physics in mid-energy gamma rays.