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Register a new userDetecting the dark matter via the proper motion of gamma-rays from microhalos
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Savvas M. Koushiappas
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I discuss the prospects for detecting the dark matter via the proper motion of sub-solar mass dark matter halos in the vicinity of the solar neighbourhood. Microhalos that survive tidal disruption could exhibit proper motion of order few arcminutes per year. For dark matter particles that couple to photons, such as the lightest supersymmetric or Kaluza-Klein particles, microhalos could be detected via their Gamma-ray photon emission from annihilations. A detection of proper motion of a microhalo in the Gamma-ray part of the spectrum contains not only information about the particle physics properties of the dark matter particle, but also provides an insight into hierarchical structure formation at very early times.
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Earth-mass dark matter microhalos with size of ~100 AUs are the first structures formed in the universe, if the dark matter of the Universe are made of neutralino. Here, we report the results of ultra-high-resolution simulations of the formation and evolution of these microhalos. We found that microhalos have the central density cusps of the form $rho propto r^{-1.5}$, much steeper than the cusps of larger dark halos. The central regions of these microhalos survive the encounters with stars except in very inner region of the galaxy down to the radius of a few hundreds pcs from the galactic center. The annihilation signals from nearest microhalos are observed as gamma-ray point-sources (radius less than 1), with unusually large proper motions of ~0.2 degree per year. Their surface brightnesses are ~10% of that of the galactic center. Their S/N ratios might be better if they are far from the galactic plane. Luminosities of subhalos are determined only by their mass, and they are more than one order of magnitude luminous than the estimation by Springel et al. (2008): A boost factor can be larger than 1000. Perturbations to the millisecond pulsars by gravitational attractions of nearby earth-mass microhalos can be detected by the observations of Parkes Pulsar Timing Array (PPTA).
In this paper, we explore the possibility of a linearly polarized gamma-ray signal from dark matter annihilations in the Galactic center. Considering neutral weakly interacting massive particles, a polarized gamma-ray signal can be realized by a two-component dark matter model of Majorana fermions with an anapole moment. We discuss the spin alignment of such dark matter fermions in the Galactic center and then estimate the intensity and the polarizability of the final-state electromagnetic radiation in the dark matter annihilations. For low-mass dark matter, the photon flux at sub-GeV energies may be polarized at a level detectable in current X-ray polarimeters. Depending on the mass ratio between the final-state fermion and DM, the degree of polarization at the mass threshold can reach $70%$ or even higher, providing us with a new tool for probing the nature of dark matter in future gamma-ray polarization experiments.
Lines in the energy spectrum of gamma rays are a fascinating experimental signal, which are often considered smoking gun evidence of dark matter annihilation. The current generation of gamma ray observatories are currently closing in on parameter space of great interest in the context of dark matter which is a thermal relic. We consider theories in which the dark matters primary connection to the Standard Model is via the top quark, realizing strong gamma ray lines consistent with a thermal relic through the forbidden channel mechanism proposed in the Higgs in Space Model. We consider realistic UV-completions of the Higgs in Space and related theories, and show that a rich structure of observable gamma ray lines is consistent with a thermal relic as well as constraints from dark matter searches and the LHC. Particular attention is paid to the one loop contributions to the continuum gamma rays, which can easily swamp the line signals in some cases, and have been largely overlooked in previous literature.
Among the most stringent constraints on the dark matter annihilation cross section are those derived from observations of dwarf galaxies by the Fermi Gamma-Ray Space Telescope. As current (e.g., Dark Energy Survey, DES) and future (Large Synoptic Survey Telescope, LSST) optical imaging surveys discover more of the Milky Ways ultra-faint satellite galaxies, they may increase Fermis sensitivity to dark matter annihilations. In this study, we use a semi-analytic model of the Milky Ways satellite population to predict the characteristics of the dwarfs likely to be discovered by DES and LSST, and project how these discoveries will impact Fermis sensitivity to dark matter. While we find that modest improvements are likely, the dwarf galaxies discovered by DES and LSST are unlikely to increase Fermis sensitivity by more than a factor of ~2-4. However, this outlook may be conservative, given that our model underpredicts the number of ultra-faint galaxies with large potential annihilation signals actually discovered in the Sloan Digital Sky Survey. Our simulation-based approach focusing on the Milky Way satellite population demographics complements existing empirically-based estimates.
Recently, it has been shown that electrons and positrons from dark matter (DM) annihilations provide an excellent fit to the Fermi, PAMELA, and HESS data. Using this DM model, which requires an enhancement of the annihilation cross section over its standard value to match the observations, we show that it immediately implies an observable level of gamma-ray emission for the Fermi telescope from nearby galaxy clusters such as Virgo and Fornax. We show that this DM model implies a peculiar feature from final state radiation that is a distinctive signature of DM. Using the EGRET upper limit on the gamma-ray emission from Virgo, we constrain the minimum mass of substructures within DM halos to be > 5x10^-3 M_sun -- about four orders of magnitudes larger than the expectation for cold dark matter. This limits the cutoff scale in the linear matter power spectrum to k < 35/kpc which can be explained by e.g., warm dark matter. Very near future Fermi observations will strongly constrain the minimum mass to be > 10^3 M_sun: if the true substructure cutoff is much smaller than this, the DM interpretation of the Fermi/PAMELA/HESS data must be wrong. To address the problem of astrophysical foregrounds, we performed high-resolution, cosmological simulations of galaxy clusters that include realistic cosmic ray (CR) physics. We compute the dominating gamma-ray emission signal resulting from hadronic CR interactions and find that it follows a universal spectrum and spatial distribution. If we neglect the anomalous enhancement factor and assume standard values for the cross section and minimum subhalo mass, the same model of DM predicts comparable levels of the gamma-ray emission from DM annihilations and CR interactions. This suggests that spectral subtraction techniques could be applied to detect the annihilation signal.
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