17-Year Fermi-LAT Observations of the Andromeda Galaxy: Updated Constraints on WIMP Properties

The Andromeda galaxy (M31) is the nearest massive spiral galaxy and is expected to reside in an extended dark matter (DM) halo, making it a useful target for indirect dark matter searches with γ- rays. In this study, we perform a template-based Fermi-LAT search for emission from annihilating and decaying dark matter in M31 using Pass 8 data over 17 years. The analysis considers both the bb¯ and τ+τ− final states for DM masses between 10GeV and 10TeV. We compare three representative M31 halo models, denoted MIN, MED, and MAX, that bracket uncertainties in the smooth halo profile and in the substructure contribution, and we quote the MED model as our reference. No DM signal is claimed; instead, we derive 95% confidence-level bounds: upper limits on ⟨σv⟩ and lower limits on the decay lifetime. For the bb¯ channel, the annihilation limits span 6.5×10−26–3.4×10−23cm3s−1 and the decay-lifetime lower limits span 1.8×1026–5.6×1026s; for the τ+τ− channel the corresponding ranges are 9.6×10−26–3.3×10−22cm3s−1 and 7.6×1025–2.5×1027s. We compare our results with existing experimental constraints. The results are complementary to high-mass M31 searches with HAWC and illustrate that the spatial morphology of the halo, not only the total astrophysical factor, is an important systematic in M31 DM constraints.