Gamma rays from annihilations at the galactic center in a physical dark matter distribution

¹ Dip. Fisica, Univ. “Tor Vergata”, via Ricerca Scientifica 1, 00133 Roma, Italy e-mail: lapi@roma2.infn.it
² SISSA/ISAS, via Beirut 2-4, 34151 Trieste, Italy
³ INFN-Sezione di Roma2, via Ricerca Scientifica 1, 00133 Roma, Italy

Received: 25 September 2009
Accepted: 28 November 2009

Abstract

We discuss the γ-ray signal to be expected from dark matter (DM) annihilations at the Galactic center. We describe the DM distribution in the Galactic halo, based on the Jeans equation for self-gravitating, anisotropic equilibria. In solving the Jeans equation, we adopted the specific correlation between the density and the velocity dispersion expressed by the powerlaw behavior of the DM “entropy” with α ≈ 1.25-1.3. Indicated (among others) by several recent N-body simulations, this correlation is privileged by the form of the radial pressure term in the Jeans equation, and it yields a main-body profile consistent with the classic self-similar development of DM halos. In addition, we required the Jeans solutions to satisfy regular boundary conditions both at the center (finite pressure, round gravitational potential) and on the outskirts (finite overall mass). With these building blocks, we derived physical solutions, dubbed “α-profiles”. We find the one with α = 1.25, suitable for the Galaxy halo, to be intrinsically flatter at the center than the empirical NFW formula, yet steeper than the empirical Einasto profile. On scales of 10^-1 deg it yields annihilation fluxes lower by a factor 5 than the former, yet higher by a factor 10 than the latter. Such fluxes will eventually fall within the reach of the Fermi satellite. We show the effectiveness of the α-profile in relieving the astrophysical uncertainties related to the macroscopic DM distribution, and discuss its expected performance as a tool instrumental in interpreting the upcoming γ-ray data in terms of DM annihilation.