Astrophysical Limits on Massive Dark Matter

TL;DR

Problem: place astrophysical limits on massive dark matter through indirect detection signals near the Galactic Centre. Approach: model the DM density spike around the central black hole, compute the $e^\pm$ spectrum from annihilation (via hadronic cascades) using MLLA, and model their propagation in the Galactic magnetic field under equipartition to predict the radio emission. Findings: the predicted Sgr A* radio flux can be compatible with a wide range of DM masses from TeV up to $m_X \gtrsim 10^8$ GeV and cross sections up to the unitarity bound, yielding exclusion regions in the $(m_X,\sigma v)$ plane; synchrotron constraints can be stronger than gamma-ray limits in some parameter ranges. Significance: demonstrates that radio observations provide complementary DM constraints extending to very heavy candidates (including WIMPzillas), with future gamma-ray and neutrino measurements offering cross-checks.

Abstract

Annihilations of weakly interacting dark matter particles provide an important signature for the possibility of indirect detection of dark matter in galaxy halos. These self-annihilations can be greatly enhanced in the vicinity of a massive black hole. We show that the massive black hole present at the centre of our galaxy accretes dark matter particles, creating a region of very high particle density. Consequently the annihilation rate is considerably increased, with a large number of $e^+e^-$ pairs being produced either directly or by successive decays of mesons. We evaluate the synchrotron emission (and self-absorption) associated with the propagation of these particles through the galactic magnetic field, and are able to constrain the allowed values of masses and cross sections of dark matter particles.

Figures (4)

• Figure 1: Values of $Y_e$ as a function of particle mass.
• Figure 2: Left panel: $A_{\nu}$ as a function of frequency for $m_X=1$TeV. The two upper curves correspond to the cross section $\sigma v\approx 10^{-28}/m_X^2(\rm GeV)\,\rm cm^3s^{-1},$ close to the unitarity limit; the two lower curves correspond to $\sigma v\approx 10^{-38}/m_X^2(\rm GeV)\,\rm cm^3s^{-1},$ a cross section more typical for wimps. Results for two values of the density profile are shown in each case: $\gamma=1$ (solid curves) and $\gamma=1.5$ (dashed curves). Right panel: $A_{\nu}$ as a function of the particle mass for $\nu$=408MHz, $\sigma v=10^{-10}/m_X^2$ (in physical units) and two values of $\gamma$.
• Figure 3: Comparison of Sgr A* (see Narayan et al. (1998)) observed spectrum with expected fluxes. The values of particle mass and cross section were choosen to fit the experimental data normalisation.
• Figure 4: Exclusion plot based on the comparison between predicted flux and radio observations of the galactic centre.The 3 solid curves indicate, for different values of the density profile power law index, the lower edge of the excluded regions. The dashed line shows, for comparison, the unitarity bound, $\sigma v\simeq1/m_X^2$. The shaded region is the portion of the parameter space occupied by cosmologically interesting neutralinos (i.e. those leading to $0.025<\Omega_X h^2<1$; see, e.g. Bergstrom, Ullio & Buckley (1997)).