Neutralino Annihilation at the Galactic Center Revisited

TL;DR

This study reassesses neutralino dark matter annihilation signals at the Galactic Center by implementing a physically motivated accretion-flow model around the central black hole, with equipartition-strength magnetic fields and radiative transfer effects. By solving the electron transport equation with injection from neutralino annihilation, advection, adiabatic compression, synchrotron losses, and synchrotron self-absorption, the authors show that a central dark-matter spike yields radio emission far exceeding observations of Sgr A*, while a pure NFW profile remains compatible. They emphasize that prior analyses underestimated the impact of BH-enviroment physics, such as advection and the correct magnetic field, and they assess the role of SSC a posteriori. The work thus tightens constraints on the inner Galactic DM distribution and neutralino parameter space, highlighting the critical role of BH accretion physics in indirect DM searches near galactic centers.

Abstract

The annihilation of neutralino dark matter in the Galactic Center (GC) may result in radio signals that can be used to detect or constrain the dark matter halo density profile or dark matter particle properties. At the Galactic Center, the accretion flow onto the central Black Hole (BH) sustains strong magnetic fields that can induce synchrotron emission by electrons and positrons generated in neutralino annihilations during advection onto the BH. Here we reanalyze the radiative processes relevant for the neutralino annihilation signal at the GC, with realistic assumptions about the accretion flow and its magnetic properties. We find that neglecting these effects, as done in previous papers, leads to the incorrent electron and photon spectra. We find that the magnetic fields associated with the flow are significantly stronger than previously estimated. We derive the appropriate equilibrium distribution of electrons and positron and the resulting radiation, considering adiabatic compression in the accretion flow, inverse Compton scattering off synchrotron photons (synchrotron self-Compton scattering), and synchrotron self-absorption of the emitted radiation. We derive the signal for a Navarro-Frenk-White (NFW) dark matter halo profile and a NFW profile with a dark matter spike due to the central BH. We find that the observed radio emission from the GC is inconsistent with the scenario in which a spiky distribution of neutralinos is present. We discuss several important differences between our calculations and those previously presented in the literature.

Figures (8)

• Figure 1: Comparison between the time scales for adiabatic compression (solid lines) and synchrotron losses (dashed lines) for $\gamma=10$ and $\gamma=1000$.
• Figure 2: Electron density per unit energy as a function of energy (solid lines) at $r=10^{3}R_g$ (upper curve) and $r=10^{4}R_g$ (lower curve). Superimposed (dashed curves), we plot the function $n(E,r)$ obtained without the effect of advection. The left panel illustrates the spike case, while the right panel applies to the NFW case.
• Figure 3: Comparison between the energy density of the synchrotron emitted photons $U_{ph}$ (solid line) and the energy density associated to the magnetic field $B^2/8\pi$ (dashed line).
• Figure 4: Optical depth as a function of the photon frequency in two cases: 1) advection plus synchrotron losses (solid line); 2) synchrotron losses only (dashed line). The calculation was carried out for a line of sight tangent to the event horizon of the black hole.
• Figure 5: Emitted luminosity in two cases: 1) advection and synchrotron losses (solid line); 2) only synchrotron losses (dashed line). The computation is performed with $m_{\chi}=100$ GeV and $\langle \sigma v \rangle_{ann}=10^{-27}$ cm$^3$/s. The left panel represents the Spike case while right panel the NFW case.