FERMI GAMMA-RAY HAZE VIA DARK MATTER AND MILLISECOND PULSARS

We study possible astrophysical and dark matter (DM) explanations for the Fermi gamma-ray haze in the Milky Way halo. As representatives of various DM models, we consider DM particles annihilating into W+W-, b (b) over bar, and e(+)e(-). In the first two cases, the prompt gamma-ray emission from DM annihilations is significant or even dominant at E > 10 GeV, while inverse Compton scattering (ICS) from annihilating DM products is insignificant. For the e(+)e(-) annihilation mode, we require a boost factor of order 100 to get significant contribution to the gamma-ray haze from ICS photons. Possible astrophysical sources of high-energy particles at high latitudes include type Ia supernovae (SNe) and millisecond pulsars (MSPs). Based on our current understanding of Ia SNe rates, they do not contribute significantly to gamma-ray flux in the halo of the Milky Way. As the MSP population in the stellar halo of the Milky Way is not well constrained, MSPs may be a viable source of gamma-rays at high latitudes provided that there are similar to(2-6) x 10(4) of MSPs in the Milky Way stellar halo. In this case, pulsed gamma-ray emission from MSPs can contribute to gamma rays around few GeV, while the ICS photons from MSP electrons and positrons may be significant at all energies in the gamma-ray haze. The plausibility of such a population of MSPs is discussed. Consistency with the Wilkinson Microwave Anisotropy Probe (WMAP) microwave haze requires that either a significant fraction of MSP spin-down energy is converted into e(+)e(-) flux or the DM annihilates predominantly into leptons with a boost factor of order 100.