arXiv:0802.0714 [ps, pdf, other] :
Title: Upper bounds on signals due to WIMP self--annihilation: comments on the case of the synchrotron radiation from the galactic center and the WMAP haze
Authors: A. Bottino, F. Donato, N. Fornengo, S. Scopel
Comments: 3 pages
Two recent papers reconsider the possibility that the excess of microwave emission from a region within $\sim 20^0$ of the galactic center (the {\it WMAP haze}), measured by WMAP, can be due to the synchrotron emission originated by neutralino self-annihilation; on the basis of this possible occurrence, also upper bounds on the neutralino self-annihilation cross--section are suggested. In the present note, we show that, when the rescaling of the galactic WIMP density is duly taken into account for subdominant WIMPs, the upper bound derivable generically for {\it any} signal due to self-conjugate thermal WIMPs is more stringent than the one obtainable from analysis of the WMAP haze. Our upper bound disfavors the interpretation of this excess in the microwave emission from the galactic center as mainly due to neutralino self--annihilation, except possibly for neutralinos with masses $\lsim$ 100 GeV. We also comment on the fact that an experimental upper bound which can compete with our generic upper limit, derives from measurements of cosmic antiproton fluxes, for some values of the parameters of the astrophysical propagation model. |
| |
|
arXiv:0802.0234 [ps, pdf, other] :
Title: Multi-wavelength signals of dark matter annihilations at the Galactic center
Authors: Marco Regis, Piero Ullio
Comments: 25 pages, 32 figures
We perform a systematic study of the multi-wavelength signal induced by weakly interacting massive particle (WIMP) annihilations at the Galactic Center (GC). Referring to a generic WIMP dark matter (DM) scenario and depending on astrophysical inputs, we discuss spectral and angular features and sketch correlations among signals in the different energy bands. None of the components which have been associated to the GC source Sgr A*, nor the diffuse emission components from the GC region, have spectral or angular features typical of a DM source. Still, data-sets at all energy bands, namely the radio, near infrared, X-ray and gamma-ray bands, contribute to place significant constraints on the WIMP parameter space. In general, the gamma-ray energy range is not the one with the largest signal to background ratio. In case of large magnetic fields close to the GC, X-ray data give the tightest bounds. The emission in the radio-band, which is less model dependent, is very constraining as well. The recent detection by HESS of a GC gamma-ray source, and of a diffuse gamma-ray component, limits the possibility of a DM discovery with the next generation of gamma-ray telescopes, like GLAST and CTA. We find that the most of the region in the parameter space accessible to these instruments is actually already excluded at other wave-lenghts. On the other hand, there may be still an open window to improve constraints with wide-field radio observations. |
| |
|
arXiv:0802.0025 [ps, pdf, other] :
Title: Cosmological black hole spin evolution by mergers and accretion
Authors: Emanuele Berti, Marta Volonteri
Comments: 4 pages, 3 figures
Using recent results from numerical relativity simulations of black hole mergers, we revisit previous studies of cosmological black hole spin evolution. We show that mergers are very unlikely to yield large spins, unless alignment of the spins of the merging holes with the orbital angular momentum is very efficient. If iron-line measurements and LISA observations of extreme mass-ratio inspirals (EMRIs) only yield dimensionless spins j>0.9, then prolonged accretion should be responsible for spin-up, and chaotic accretion scenarios would be very unlikely. If only a fraction of the whole population of low-redshift black holes spins rapidly, spin-alignment during binary mergers (rather than prolonged accretion) could be responsible for spin-ups. |
| |
arXiv:0801.1315 [ps, pdf, other] :
Title: The Hagedorn Soup and an Emergent Cyclic Universe
Authors: Tirthabir Biswas
Comments: 15 pages, 1 figure
One of the problems of constructing a successful cyclic universe scenario is that it has to incorporate the second law of thermodynamics. This leads to Tolman's ever shrinking cycles which eventually vanish at a finite proper time in the past. In this paper we construct a theoretically consistent (ghost-free) non-singular toy model where as the cycles shrink in the past they also spend more and more time in the Hagedorn phase where all the string states are in thermal equilibrium and entropy is conserved. Thus in such a scenario the entropy asymptotes to a finite non-zero constant in the infinite past. The universe ``emerges'' from a small (string size) geodesically complete quasi-periodic space-time. This paradigm also naturally addresses some of the classic puzzles of Big Bang cosmology, such as the largeness, horizon and flatness problems. |
| |
|
Nature 451, 159-162 :
Title: An asymmetric distribution of positrons in the Galactic disk revealed by big gamma-rays
Authors: Georg Weidenspointner et al.
Gamma-ray line radiation at 511 keV is the signature of electronÐpositron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre1, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis2, 3, 4, accreting compact objects5, 6, 7, 8, and even the annihilation of exotic dark-matter particles9 have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk (approx10Ð50¡ from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electronÐpositron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 1041 positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter. |
| |
|
|
