Cosmological dark matter annihilations into gamma-rays - a closer look

TL;DR

The paper develops a comprehensive framework to predict the extragalactic gamma-ray flux from WIMP dark matter annihilations by summing contributions across all halos and redshifts, incorporating realistic halo mass functions, density profiles, and absorption effects. It demonstrates that the flux is strongly enhanced by small, dense halos and possible substructure, with the spectral signature featuring a distinctive redshifted line for monochromatic channels that aids in discrimination from backgrounds. The authors explore background modeling from unresolved blazars and apply the formalism to supersymmetric DM scenarios, showing that GLAST (Fermi-LAT) could detect or constrain line signals under plausible halo and particle physics assumptions, while acknowledging substantial uncertainties in structure formation and background modeling. Overall, the work highlights the viability of using cosmological DM annihilation signals as a probe of both particle physics and the small-scale distribution of dark matter, emphasizing the potential of future gamma-ray observations to reveal or constrain WIMP properties.

Abstract

We investigate the prospects of detecting weakly interacting massive particle (WIMP) dark matter by measuring the contribution to the extragalactic gamma-ray radiation induced, in any dark matter halo and at all redshifts, by WIMP pair annihilations into high-energy photons. We perform a detailed analysis of the distinctive spectral features of this signal, recently proposed in a short letter by three of the authors, with emphasis on the signature due to monochromatic gamma-ray yields: the combined effect of cosmological redshift and absorption along the line of sight produces sharp bumps, peaked at the rest frame energy of the lines and asymmetrically smeared to lower energies. The level of the flux depends both on the particle physics scenario for WIMP dark matter and on the question of how dark matter clusters. Uncertainties introduced by the latter are thoroughly discussed implementing a realistic model inspired by results of the state-of-the-art N-body simulations and semi-analytic modeling in the cold dark matter structure formation theory. We also address the question of the potential gamma-ray background originating from blazars, presenting a novel calculation. Comparing the signal with the background, we find that there are viable configurations, in the combined parameter space defined by the particle physics setup and the structure formation scenario, for which the WIMP induced extragalactic gamma-ray signal will be detectable in the new generation of gamma-ray telescopes such as GLAST.

Figures (15)

• Figure 1: Fraction of total mass provided by objects heavier than a given mass $M$ (upper curves) or within 14 decades in mass (lower histograms) at three different redshifts and for the mass function as derived in the ellipsoidal collapse model.
• Figure 2: Concentration parameter versus mass for halos of mass $M$ at $z=0$. On the left-hand panel we reproduce from Ref. Bullock the behavior found in a large sample of simulated halos, with a binning in mass in which each marker represents the peak in the distribution and the relative bar its 68% width; the trend is reproduced with the toy models proposed in Ref. Bullock itself (Bullock et al.) and in Ref. ENS (ENS). On the right-hand side, we show an extrapolation of $c_{vir}$ to the whole mass range we need to include in our analysis according to the two toy models.
• Figure 3: Average enhancement in the $\gamma$-ray flux emitted in a halo of mass $M$ at redshift $z=0$ with respect to the case in which the same amount of dark matter is smoothly distributed. On the left-hand side we show how sensitive the result is to the concentration parameter. On the right-hand side the result for three different families of dark matter density profiles is shown.
• Figure 4: Scaling of the collected $\gamma$-ray flux with the distance $d$ between the detector and the center of a halo, for three different halo profiles. The angular acceptance of the detector is assumed to be $\Delta\Omega = 10^{-3} \rm{sr}$. The plot is for a $10^{12} M_{\odot}{\ }$ halo, the arrows indicate the position on the horizontal axis for the Milky Way and Andromeda; the case for other masses is analogous.
• Figure 5: Enhancement in the diffuse $\gamma$-ray flux compared to the case when all structures in the Universe are erased. On the left-hand side the contributions of structures of given masses at $z = 0$ are shown; on the right-hand panel we show the redshift dependence, rescaled with the term $(1+z)^3/h(z)$.