Neutralino annihilation into gamma-rays in the Milky Way and in external galaxies

TL;DR

This study theoretically probes gamma rays from neutralino annihilation in the Milky Way and selected external galaxies within an effective MSSM framework, separating the flux into a cosmological factor and a supersymmetric factor. It analyzes how dark matter halo shapes (cusps vs cores) and substructure, along with neutralino mass and annihilation channels, shape the predicted fluxes and their detectability by instruments like GLAST/VERITAS. The results indicate that only a very cuspy Galactic Center signal could be detectable by both satellite and ground-based telescopes, while external galaxies remain largely below reach; comparisons with CANGAROO-II and HEGRA data show partial fits requiring substantial boosts or extreme masses. A later note on HESS data suggests even heavier neutralinos and steeper halos could plausibly account for some GC observations, underscoring the sensitivity of gamma-ray predictions to halo inner structure and particle physics parameters.

Abstract

We discuss the gamma-ray signal from dark matter annihilation in our Galaxy and in external objects, namely the Large Magellanic Cloud, the Andromeda Galaxy (M31) and M87. We derive predictions for the fluxes in a low energy realization of the Minimal Supersymmetric Standard Model and compare them with current data from EGRET, CANGAROO-II and HEGRA and with the capabilities of new-generation satellite-borne experiments, like GLAST, and ground-based Cerenkov telescopes, like VERITAS. We find fluxes below the level required to explain the possible indications of a gamma-ray excess shown by CANGAROO-II (toward the Galactic Center) and HEGRA (from M87). As far as future experiments are concerned, we show that only the signal from the galactic center could be accessible to both satellite-borne experiments and to ACTs, even though this requires very steep dark matter density profiles.

Figures (15)

• Figure 1: Comparison between cuspy and cored dark matter density profiles for the Milky Way, as a function of the distance from the center of the Galaxy. All the curves are normalized to $\rho_0\equiv\rho(R_\odot)=0.3$ GeV cm$^{-3}$.
• Figure 2: The lines denote the "cosmological factor" $\Phi^{\rm cosmo}$ for the Milky Way, calculated for different dark matter profiles, for a solid angle $\Delta \Omega = 10^{-3}$ sr (upper panel) and $\Delta \Omega = 10^{-5}$ sr (lower panel). The small boxes show a zoom at small angles toward the galactic center. A constant--density central region of radius $r_{\rm cut}=10^{-8} {\rm\,kpc}$ has been used for the cuspy profiles. The points at $\psi \simeq 81^\circ$ and $\psi \simeq 119^\circ$ denote the values of $\Phi^{\rm cosmo}$ for LMC and M31, respectively. From top to bottom the points refer to different halo profiles: Moore, NFW97, M04, iso-core in the upper panel; Moore, M04, NFW97, iso-core in the lower panel.
• Figure 3: Relative strength of the line--of--sight integral with respect to different halo profiles and different inner core radii for the Milky Way. Numbers are normalized to the highest value of the $\Phi^{\rm cosmo}$ given by a M99 profile with a physical cut--off radius of $10^{-8}$ kpc and at $\psi=0$. Left panel: solid angle $\Delta \Omega = 10^{-3}$ sr. Right panel: solid angle $\Delta \Omega = 10^{-5}$ sr.
• Figure 4: Relative strength of the line--of--sight integral with respect to different halo profiles and different inner core radii for M31 (upper panels) and the LMC (lower panels). Numbers are normalized to the highest value of the $\Phi^{\rm cosmo}$ given by a M99 profile with a physical cut-off radius of $10^{-8}$ kpc and at $\psi=0$. Left panels: solid angle $\Delta \Omega = 10^{-3}$ sr. Right panels: solid angle $\Delta \Omega = 10^{-5}$ sr.
• Figure 5: The thermally--averaged annihilation cross--section divided by the square of the neutralino mass $m_\chi$ as a function of $m_\chi$ in the frame of the eMSSM. Crosses show the WMAP-preferred zone for a DM dominant neutralino. In the small box the annihilation cross section at the present epoch is shown as a function of the neutralino relic abundance.