A "Baedecker" for the Dark Matter Annihilation Signal

TL;DR

This work provides a realistic assessment of gamma-ray signals from neutralino dark matter annihilation toward the Galactic Center and nearby dwarf spheroidals under both cored and cusped density profiles. It combines particle-physics modeling with astrophysical halo modeling to compute the flux via $\Phi_\gamma(\psi)=\frac{N_\gamma\langle\sigma v\rangle}{4\pi m_χ^2}\times\frac{1}{\Delta\Omega}\int_{\Delta\Omega} d\Omega\int_{los} \rho^2[r(s)] ds$ and the line-of-sight integral $\langle J\rangle_{\Delta\Omega}=\frac{1}{\Delta\Omega}\int_{\Delta\Omega} J(\psi)d\Omega$, then evaluates backgrounds and detector capabilities for ACTs and GLAST. It finds the Galactic Center flux is highly sensitive to the inner-profile, dropping by about two orders of magnitude when adopting a cored profile, thus diminishing GC detectability; monochromatic lines are unlikely to be observed with near-future instruments. However, continuum emission from the dwarfs (and the Milky Way at mid-latitudes) could be detectable, particularly if the dSphs have cusped halos (e.g., Sagittarius or Canis Major), while GLAST offers robust constraints through wide-field observations independent of the exact inner halo shape. The results emphasize the critical role of inner-halo structure in predicting signals and guiding observational strategies.

Abstract

We provide a ``Baedecker'' or travel guide to the directions on the sky where the dark matter annihilation signal may be expected. We calculate the flux of high energy gamma-rays from annihilation of neutralino dark matter in the centre of the Milky Way and the three nearest dwarf spheroidals (Sagittarius, Draco and Canis Major), using realistic models of the dark matter distribution. Other investigators have used cusped dark halo profiles (such as the Navarro-Frenk-White) to claim a significant signal. This ignores the substantial astrophysical evidence that the Milky Way is not dark-matter dominated in the inner regions. We show that the annihilation signal from the Galactic Centre falls by two orders of magnitude on substituting a cored dark matter density profile for a cusped one. The present and future generation of high energy gamma-ray detectors, whether atmospheric Cerenkov telescopes or space missions like GLAST, lack the sensitivity to detect any of the monochromatic gamma-ray annihilation lines. The continuum gamma-ray signal above 1 GeV and above 50 GeV may however be detectable either from the dwarf spheroidals or from the Milky Way itself. If the density profiles of the dwarf spheroidals are cusped, then the best prospects are for detecting Sagittarius and Canis Major. However, if the dwarf spheroidals have milder, cored profiles, then the annihilation signal is not detectable. For GLAST, an attractive strategy is to exploit the wide field of view and observe the Milky Way at medium latitudes, as suggested by Stoehr et al. This is reasonably robust against changes in the density profile.

Figures (2)

• Figure 1: Exclusion limits for the discrete line $\chi \chi \rightarrow \gamma \gamma$. For all the experiments, only the most favorable cases are shown. The green, red and blue points correspond to mSUGRA models with $\Omega_{\rm CDM} h^2$ in the range 0.005--0.2, as discussed in Section IIIA. The red points satisfy the more stringent WMAP constraints $0.09 < \Omega_{\rm CDM} h^2 < 0.13$. The exclusion limits for $\chi \chi \rightarrow Z\gamma$ are very similar and not shown here.
• Figure 2: Exclusion limits for continuum $\gamma$-ray emission above 1 GeV (top) and 50 GeV (bottom). Only the most favorable cases are shown. For $E_\gamma > 1$ GeV, only curves for GLAST are drawn, as ACTs are insensitive at such low energies. Above 50 GeV, curves are shown for both GLAST and second generation ACTs. The green, red and blue points correspond to mSUGRA models with $\Omega_{\rm CDM} h^2$ in the range 0.005--0.2, as discussed in Section IIIA. The red points satisfy the more stringent WMAP constraints $0.09 < \Omega_{\rm CDM} h^2 < 0.13$.