Detection prospects for heavy WIMP dark matter near supermassive black holes, particularly in M31

This work analyzes the detection prospects for weakly interacting massive particles (WIMPs) in dark matter (DM) density spikes around nearby supermassive black holes (SMBHs) by observations in very high energy gamma-ray band. Such spikes are unique targets, which provide a possibility to discover the basic thermal s-wave annihilating WIMP with any mass up to the theoretical unitarity limit ~ 100 TeV. All relevant SMBHs were checked, and only MW* and M31* were identified as worthwhile objects. Cherenkov Telescope Array (CTA) sensitivity to heavy WIMPs in M31* was estimated. It was obtained that CTA will be able to probe a major part of TeV-scale WIMP parameter space in case of optimistic spike density configuration in M31*. In certain scenarios, M31* may yield even stronger constraints than MW*. Relevant systematic uncertainties were explored.

💡 Research Summary

This paper investigates the detection prospects for heavy Weakly Interacting Massive Particles (WIMPs) by searching for very-high-energy gamma rays produced by their annihilation in dense dark matter (DM) “spikes” predicted to form around supermassive black holes (SMBHs). The primary motivation is to complement existing indirect search strategies, which are most sensitive to WIMPs lighter than ~0.1 TeV, and to probe the theoretically allowed mass range up to ~100 TeV.

The study focuses on the simplest, velocity-independent (s-wave) annihilation channel. It presents a detailed formalism for calculating the gamma-ray flux from such spikes, modeling the DM density profile as a piecewise power law with distinct regions: an inner core (where density is saturated by rapid annihilation), a main spike, and the unperturbed outer galactic halo. A key feature is that the properties of the inner core, and thus the total signal, depend non-linearly on the WIMP’s annihilation cross-section and mass due to self-annihilation equilibrium.

A comprehensive comparison of all nearby SMBHs is conducted to identify the most promising observational targets. The analysis considers distance, black hole mass, and the brightness of competing astrophysical backgrounds in high-energy bands. The study concludes that only the SMBHs at the center of the Milky Way (MW*) and the Andromeda galaxy (M31*) are worthwhile targets for deep observations with next-generation instruments like the Cherenkov Telescope Array (CTA), which has a relatively narrow field of view. Notably, M31* is highlighted as a particularly promising and previously underexplored target due to its proximity, large black hole mass, and—crucially—its observed faintness in high-energy emissions, suggesting a low astrophysical background that could enhance sensitivity to a potential DM signal.

The core result is an estimation of CTA’s sensitivity to WIMP parameters from observations of M31* and MW*. Under optimistic assumptions for the spike density profile (a steep “spike”), the simulations show that CTA could probe a major portion of the TeV-scale WIMP parameter space for thermally produced s-wave annihilating DM. In certain scenarios, constraints derived from M31* may even surpass those from MW*. However, the results are highly sensitive to systematic uncertainties, primarily the slope and normalization of the DM density spike, which depend on the poorly constrained formation history of the SMBH and the initial conditions of the DM halo.

In conclusion, the paper argues that SMBH spikes, especially around low-background targets like M31*, represent a unique and complementary avenue for discovering heavy WIMPs in the CTA era. Success hinges on significant observational time dedicated to these point sources and on advancements in our theoretical understanding of spike formation and survival in galactic centers. This approach may offer a path to exploring WIMP masses that would otherwise remain inaccessible through observations of standard DM halos.