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Dark Matter Annihilation
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Title: Constraint on dark matter annihilation with dark star formation using Fermi extragalactic diffuse gamma-ray background data
Authors: Qiang Yuan (IHEP, UNLV), Bin Yue (NAOC, GUCAS), Bing Zhang (UNLV), Xuelei Chen (NAOC, PKU)

It has been proposed that during the formation of the first generation stars there might be a "dark star" phase in which the power of the star comes from dark matter annihilation. The adiabatic contraction process to form the dark star would result in a highly concentrated density profile of the host halo at the same time, which may give enhanced indirect detection signals of dark matter. In this work we investigate the extragalactic \gamma-ray background from dark matter annihilation with such a dark star formation scenario, and employ the isotropic \gamma-ray data from Fermi-LAT to constrain the model parameters of dark matter. The results suffer from large uncertainties of both the formation rate of the first generation stars and the subsequent evolution effects of the host halos of the dark stars. We find, in the most optimistic case for \gamma-ray production via dark matter annihilation, the expected extragalactic \gamma-ray flux will be enhanced by 1-2 orders of magnitude. In such a case, the annihilation cross section of the supersymmetric dark matter can be constrained to the thermal production level, and the leptonic dark matter model which is proposed to explain the positron/electron excesses can be well excluded. Conversely, if the positron/electron excesses are of a dark matter annihilation origin, then the early Universe environment is such that no dark star can form.

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Title: The cosmic X-ray and gamma-ray background from dark matter annihilation
Authors: Jesus Zavala (1), Mark Vogelsberger (2), Tracy R. Slatyer (3), Abraham Loeb (2), Volker Springel (4) ((1) MPA, (2) Harvard/CfA, (3) IAS, (4) HITS)

The extragalactic background light (EBL) observed at multiple wavelengths is a promising tool to probe the nature of dark matter since it might contain a significant contribution from gamma-rays produced promptly by dark matter annihilation. Additionally, the electrons and positrons produced in the annihilation give energy to the CMB photons to populate the EBL with X-rays and gamma-rays. We here create full-sky maps of the radiation from both of these contributions using the high-resolution Millennium-II simulation. We use upper limits on the contributions of unknown sources to the EBL to constrain the intrinsic properties of dark matter using a model-independent approach that can be employed as a template to test different particle physics models (including those with a Sommerfeld enhancement). These upper limits are based on observations spanning eight orders of magnitude in energy (from soft X-rays measured by CHANDRA to gamma-rays measured by Fermi), and on expectations for the contributions from blazars and star forming galaxies. To exemplify this approach, we analyse a set of benchmark Sommerfeld-enhanced models that give the correct dark matter abundance, satisfy CMB constraints, and fit the cosmic ray spectra measured by PAMELA and Fermi without any contribution from local subhalos. We find that these models are in conflict with the EBL constraints unless the contribution of unresolved subhalos is small and the annihilation signal dominates the EBL. We conclude that provided the collisionless cold dark matter paradigm is accurate, even for conservative estimates of the contribution from unresolved subhalos and astrophysical backgrounds, the EBL is at least as sensitive a probe of these types of scenarios as the CMB. Our results disfavour an explanation of the positron excess measured by PAMELA based only on dark matter annihilation in the smooth Galactic halo.

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Title: Dark Matter as a Possible New Energy Source for Future Rocket Technology
Authors: Jia Liu
(Version v2)

Current rocket technology can not send the spaceship very far, because the amount of the chemical fuel it can take is limited. We try to use dark matter (DM) as fuel to solve this problem. In this work, we give an example of DM engine using dark matter annihilation products as propulsion. The acceleration is proportional to the velocity, which makes the velocity increase exponentially with time in non-relativistic region. The important points for the acceleration are how dense is the DM density and how large is the saturation region. The parameters of the spaceship may also have great influence on the results. We show that the (sub)halos can accelerate the spaceship to velocity  10^{- 5} c \sim 10^{- 3} c. Moreover, in case there is a central black hole in the halo, like the galactic center, the radius of the dense spike can be large enough to accelerate the spaceship close to the speed of light.

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