The contribution of ultracompact dark matter minihalos to the isotropic radio background

Abstract

The ultracompact minihalos could be formed during the earlier epoch of the universe. The dark matter annihilation within them is very strong due to the steep density profile, $\rho \sim r^{-2.25}$. The high energy electrons and positrons from the dark matter annihilation can inverse Compton scatter (ICS) with the background photons, such as CMB photons, to acquire higher energy. On the other hand, the synchrotron radiation can also be produced when they meet the magnetic field. In this paper, we study the signals from the UCMHs due to the dark matter annihilation for the radio, X-ray and $\gamma$-ray band. We found that for the radio emission the UCMHs can provide one kind of source for the radio excess observed by ARCADE 2. But the X-ray signals due to the ICS effect or the $\gamma$-ray signals mainly due to the prompt emission from dark matter would exceed the present observations, such as Fermi, COMPTEL and CHANDRA. We found that the strongest limits on the fraction of UCMHs come from the X-ray observations and the constraints from the radio data are the weakest.Comment: 6 pages, 8 figures, Comments Welcome! Some Refs. are added, some presentation have been corrected. The conclusions remain unchanged. One important reference has been corrected. Some presentations are changed and added according to the referee's comments. Accepted for publication in PR