The final frontier for MeV astrophysics is to observe extragalactic extended emission. Normal type grand spiral galaxies, such as the Andromeda galaxy (M31), remain terra incognita in the MeV range, and therefore put a question mark on the benchmark character of the Milky Way (MW) MeV emission. Albeit huge scientific potential in the observations of other galaxies can be expected, the useful observation time of M31 with SPI is only 200 ks. M31 has an apparent extent of 3.2 deg, making it difficult to observe as a whole for ISGRI with its angular resolution of 12 arcsin. ISGRI found ten sources coincident with the extent of M31, however its diffuse emission is still unknown, making it an ideal target for SPI with its wider angular resolution of 2.7 deg.Continuum emission from a variety of processes may be within reach in a dedicated observation campaign: Extrapolating GeV observations to the MeV band, M31 could have a flux on the order of 1e-8 to 1e-2 ph/cm2/s/keV in the band 0.02-1 MeV. Our current SPI analysis translates to tight constraints about dark matter (DM), either in the form of particles or primordial black holes (PBHs): In particular our limits are already now compatible with measurements from NuSTAR, the Voyager probes, and the CMB. The accompanying annihilation of electrons with positrons would result in a narrow and blue-shifted 511 keV line with a flux on the order of 1e-5 ph/cm/2, within reach with additional exposure time. Because M31s stellar mass is ten times larger than the MW, and because strong GeV emission has been found in M31, probably related to millisecond pulsars, the 511 keV line can be expected 20 more luminous than in the MW, resulting in a flux of 1e-5 ph/cm/2, independent of any DM contributions.Here we propose to make use SPI to observe the diffuse emission of M31 in a dedicated campaign. With an observation time of 1 Ms, the current exposure would be sextupled, and hence the sensitivity at least improved by a factor of 2.5.
The INTEGRAL (International Gamma-Ray Astrophysics Laboratory) mission, launched by the European Space Agency (ESA) on October 17, 2002, was designed to study high-energy phenomena in the universe. INTEGRAL was operating until february 2025 and it was equipped with three high-energy instruments: the Imager on Board the INTEGRAL Satellite (IBIS), the Spectrometer on INTEGRAL (SPI), and the JEM-X (Joint European Monitor for X-rays). Its Optical Monitoring Camera (OMC) provided optical V-band magnitude measurements, complementing the high-energy observations.
European Space Agency, Siegert, 2025, 'INTEGRAL Observations of the Andromeda Galaxy', 1.0, European Space Agency, https://doi.org/10.57780/esa-wvdszdy
