The PAMELA and ATIC Excesses From a Nearby Clump of Neutralino Dark Matter

Abstract

In this letter, we suggest that a nearby clump of 600-1000 GeV neutralinos may be responsible for the excesses recently observed in the cosmic ray positron and electron spectra by the PAMELA and ATIC experiments. Although neutralino dark matter annihilating throughout the halo of the Milky Way is predicted to produce a softer spectrum than is observed, and violate constraints from cosmic ray antiproton measurements, a large nearby (within 1-2 kiloparsecs of the Solar System) clump of annihilating neutralinos can lead to a spectrum which is consistent with PAMELA and ATIC, while also producing an acceptable antiproton flux. Furthermore, the presence of a large dark matter clump can potentially accommodate the very large annihilation rate required to produce the PAMELA and ATIC signals.

Figures (2)

• Figure 1: The electron plus positron spectrum from a clump of 800 GeV neutralinos annihilating to $W^+W^-$. In the top frame, the clump is assumed to be stationary and at distances of 0.1, 1, 2 or 4 kpc from the Solar System. For comparison, we show with arbitrary normalization the spectral shape prior to propagation (dotted line) and the result for a smooth NFW halo profile (dashed line). In the lower frame, results are shown for a clump moving at 400 km/s relative to the Solar System. The thick solid line denote the case in which the clump has recently reached the Solar System. Other line types describe the case in which a clump which has passed through the Solar System and is now 1 or 2 kpc away (dotted, top-to-bottom), a clump which is approaching the Solar System and is currently 1 or 2 kpc away (dashed, top-to-bottom), and a clump which passed with a closest approach of 1 kpc away from the Solar System and is now $\sqrt{2}$ or $2\sqrt{2}$ kpc away (thin solid). In each case shown, an annihilation rate of $2 \times 10^{35}$ s$^{-1}$ was used.
• Figure 2: The positron fraction (top) and the electron plus positron spectrum (bottom) from a nearby clump of annihilating neutralino dark matter. The solid lines denote the result for a very nearby clump of 600 GeV neutralinos, while the dashed and dotted lines correspond to 800 GeV neutralinos in a clump 1 and 1.2 kpc away, respectively. To normalize the curves, we used approximate annihilation rates of $7 \times 10^{35}$, $1.7 \times 10^{37}$, and $2.2 \times 10^{37}$ per second for the $D=0,1$, and $1.2$ cases, respectively. Each case provides a good fit to both the PAMELA and ATIC data. For comparison, we show in the top frame as a dot-dashed line the astrophysical expectation from cosmic ray secondary production sec.