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

TL;DR

The paper investigates the detectability of heavy WIMPs in DM density spikes around nearby SMBHs using very high energy gamma rays and the CTA. It develops a four-region spike model and uses the standard flux formalism $d\Phi/dE = \langle\sigma v\rangle/(8\pi m_x^2)\, dN_\gamma/dE \times J$, with a decomposed $J$-factor that couples particle physics to astrophysical structure via saturation in the inner core. The analysis identifies MW* and M31* as the primary targets, showing that M31* can exclude a broad range of TeV-scale WIMPs for optimistic spike configurations (e.g., $\gamma\approx 2.3$), while more realistic spikes ($\gamma\approx 1.5$) stay below halo limits. The results highlight that spike uncertainties dominate the sensitivity, suggesting targeted, long-exposure observations of M31* to maximize potential gains over halo searches and to enable spectrum-resolved tests of any potential DM signal.

Abstract

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.

Figures (5)

• Figure 1: The scheme of central section of DM density spike: the black disk denotes SMBH, the shaded region has radius 2$R_\bullet$ and effectively null DM density, then the inner spike core and main spike layer follow. Each layer contains respective $J$-factor component symbol.
• Figure 2: DM galactic halo density profiles for M31 and MW for three accepted model configurations. The distance axis origins correspond to the smallest possible spike radii $R_{sp}(b=0.2)$.
• Figure 3: The blue line shows the spectrum of J1745--290 source taken from 2021ApJ...913..115A and extrapolated to 100 TeV by power law. The green line shows the same spectrum assigned to M31 center by rescaling to M31 gamma-ray flux measured at $\approx$ 0.01 TeV. The dashed line shows approximate CTA sensitivity to a point source with relevant power-law spectrum in the chosen wide energy bins for $T=100$ h. More details are in section \ref{['sec:cta']}.
• Figure 4: The dependencies of various $J$-factor components on the main spike density slope $\gamma$ for both galaxies. MED DM halo density profiles were employed, $b=0.6$, $m_x=3$ TeV, $\langle\sigma v\rangle = 2.1\cdot10^{-26}$ cm$^3$/s (thermal relic). The blue line represents $J_{ext}$, orange line -- $J_{in}(\gamma)$, green line -- $J_{sp}(\gamma)$ and continuous line -- total $J(\gamma)$.
• Figure 5: The combined sensitivity limits on annihilating WIMP properties from current Fermi-LAT data and future CTA observations for both considered galaxies and annihilation channels. The blue shaded bands show the constraints from observations of DM galactic halo derived in 2020PhRvD.102d3012A2021JCAP...01..057A for MW and in 2019PhRvD..99l3027D2023JCAP...08..073M for M31. The bands' width reflects the uncertainties in DM halo density profile. The exclusion lines display the limits from DM density spikes around SMBHs derived here for the listed parameter configurations. The horizontal black dashed line provides the thermal relic annihilation cross section from 2020JCAP...08..011Ssv.