Multi-wavelength signals of dark matter annihilations at the Galactic center

TL;DR

The study tackles whether WIMP annihilations near the Galactic Center can yield detectable multi-wavelength signals to constrain or reveal DM. It builds a self-consistent framework where the DM-induced emissivity in each band depends on the DM density, the annihilation spectrum, and electron/positron propagation, including the effects of strong magnetic fields and potential density spikes, and computes synchrotron, inverse Compton, and pi0-decay gamma components under benchmark profiles. The authors find that radio synchrotron and X-ray data often provide the strongest constraints, especially for cusp-like spikes, while gamma-ray limits are powerful mainly for heavy WIMPs but can be overshadowed by multi-wavelength bounds; projected GLAST/CTA gamma-ray gains appear limited by GC backgrounds, whereas wide-field radio observations offer a promising path to tighter constraints. Overall, the work clarifies the relative strengths of multi-wavelength channels for GC DM searches and informs observational strategies, highlighting that a broad-band approach, particularly with future radio surveys, is essential for advancing GC DM constraints or potential discovery.

Abstract

We perform a systematic study of the multi-wavelength signal induced by weakly interacting massive particle (WIMP) annihilations at the Galactic Center (GC). Referring to a generic WIMP dark matter (DM) scenario and depending on astrophysical inputs, we discuss spectral and angular features and sketch correlations among signals in the different energy bands. None of the components which have been associated to the GC source Sgr A*, nor the diffuse emission components from the GC region, have spectral or angular features typical of a DM source. Still, data-sets at all energy bands, namely, the radio, near infrared, X-ray and gamma-ray bands, contribute to place significant constraints on the WIMP parameter space. In general, the gamma-ray energy range is not the one with the largest signal to background ratio. In the case of large magnetic fields close to the GC, X-ray data give the tightest bounds. The emission in the radio-band, which is less model dependent, is very constraining as well. The recent detection by HESS of a GC gamma-ray source, and of a diffuse gamma-ray component, limits the possibility of a DM discovery with next generation of gamma-ray telescopes, like GLAST and CTA. We find that the most of the region in the parameter space accessible to these instruments is actually already excluded at other wave-lenghts. On the other hand, there may be still an open window to improve constraints with wide-field radio observations.

Figures (18)

• Figure 1: Multi--wavelength spectrum of Sgr A$^*$. The radio to X--ray emissions are shown in the quiescent state or at the epoch of lowest luminosity among available observations. The plotted $\gamma$--ray sources have positions compatible with Sgr A$^*$; however, due to a poor angular resolution, EGRET cannot clearly identify the source and perhaps neither the HESS telescope. See the text for details about the observations in each band.
• Figure 2: Left Panel: Timescales for different radiative losses as a function of the $e^+ -e^-$ momentum. Synchrotron losses are shown for two reference values for the magnetic field: $B=1 \,\mu G, 1\,G$. Radiative losses associated to bremsstrahlung, ionization and Coulomb scattering are shown at the GC (lower curves) and at a distance of 100 pc from the GC (upper curves). Right Panel: Distance $d_L$ travelled by an electron with an injection energy of 1 GeV before losing most of its energy; three different guesses for the diffusion coefficient are shown, in the case of equipartition and reconnection magnetic field, see Fig. \ref{['fig:Bfield']}a (same line styles).
• Figure 3: Left Panel: Models for the magnetic fields in the central region of the Galaxy as a function of the distance from the GC. Right Panel: Magnetic field as a function of the synchrotron peak energy for few values of the observed frequencies.
• Figure 4: Left Panel:$\gamma$--ray and $e^+-e^-$ spectra per annihilation for a $1$ TeV WIMP. The three annihilation channels $b -\bar{b}$, $W^+-W^-$, and $\tau^+-\tau^-$ are taken as references. Right Panel: Multiplicity between the electron and photon yields $dN_{\gamma}/dE \times (dN_e/dE)^{-1}$ for a $1$ TeV WIMP with the same annihilation modes as in the left panel.
• Figure 5: Dark matter profiles for the benchmark models B1, B2, and B3. For comparison we plot also the NFW profile and NFW profiles modified by the original prescription by Gondolo-Silk (GS) to account for the growth of the central black hole: the value of the ratio $(\sigma v)/M_{\chi}$ are the same as in the benchmark models.