Dark matter annihilation: The origin of cosmic gamma-ray background at 1–20 MeV

The origin of the cosmic $\gamma$-ray background at 1–20 MeV remains a mystery. We show that $\gamma$-ray emission accompanying annihilation of 20 MeV dark-matter particles explains most of the observed signal. Our model satisfies all of the current observational constraints, and naturally provides the origin of “missing” $\gamma$-ray background at 1–20 MeV and 511 keV line emission from the Galactic center. We conclude that $\gamma$-ray observations support the existence of 20 MeV dark-matter particles. Improved measurements of the $\gamma$-ray background in this energy band undoubtedly test our proposal.

Figure 1

Cosmic $\gamma$-ray background from dark-matter annihilation. The dotted, short-dashed, and long-dashed lines show $m_X=10$, 20, and 50 MeV, respectively. The curves which sharply cut off at 511 keV represent background light from line emission[18], while the others which extend to higher-energy represent the internal bremsstrahlung.

Figure 2

The total cosmic $\gamma$-ray background produced by dark-matter annihilation, AGNs [3], and Type Ia supernovae [12]. The dotted, short-dashed, and long-dashed lines show $m_X=10$, 20, and 50 MeV, respectively. The supernova contribution depends on the observed supernova rate, and we consider the best-fit rate (upper panel) as well as the $2\sigma$ upper limit (lower panel). The data points of HEAO-1 A4 MED [29], SMM [30], and COMPTEL [31] experiments are also shown.

Figure 3

The best-fit model of the cosmic $\gamma$-ray background. The model assumes (a) $m_X=20\mathrm{MeV}$, (b) the mean dark-matter clumping factor is twice as large as predicted by the NFW profile (due to either a steeper profile or the presence of substructures), and (c) line emission is solely produced via positronium formation. The dashed lines show each contribution separately.