4,831 research outputs found
Constraining the distribution of dark matter at the Galactic centre using the high-resolution Event Horizon Telescope
We investigate constraints on the distribution of dark matter in the
neighbourhood of the Galactic centre that may eventually be attained with the
high-resolution Event Horizon Telescope (EHT). The shadow of a black hole in
vacuum is used to generate a toy model describing how dark matter affects the
size of the shadow of the supermassive black hole located at the Galactic
centre. Observations by the EHT may constrain the properties of the dark matter
distribution in a possible density spike around the black hole. Current
uncertainties due to both the resolution of the telescope and the analysis of
stellar orbits prevent one from discerning the effect of dark matter on the
measured size of the shadow. The change in the size of the shadow induced by
dark matter can be seen as an additional uncertainty in any test of general
relativity that relies on using the angular size of the shadow to estimate the
Schwarzschild radius of the black hole.Comment: 5 pages, 4 figures. v2: a few comments and references added, one
figure replaced for clarity, results unchanged, matches version published in
A&
Remnants of galactic subhalos and their impact on indirect dark-matter searches
Dark-matter subhalos, predicted in large numbers in the cold-dark-matter scenario, should have an impact on dark-matter-particle searches. Recent results show that tidal disruption of these objects in computer simulations is overefficient due to numerical artifacts and resolution effects. Accounting for these results, we re-estimated the subhalo abundance in the Milky Way using semianalytical techniques. In particular, we showed that the boost factor for gamma rays and cosmic-ray antiprotons is increased by roughly a factor of twoJ.L. and M.S. are partly supported by the Agence Nationale pour la Recherche (ANR)
Project No. ANR-18-CE31-0006, the Origines, Constituants, et EVolution de lâUnivers (OCEVU) Labex (No. ANR-11-LABX-0060), the CNRS IN2P3-Theory/INSU-PNHE-PNCG project âGalactic Dark Matter,â and the European Unionâs Horizon 2020 Research and Innovation Program under Marie SkĆodowska-Curie Grant Agreements No. 690575 and No. 674896, in addition to recurrent funding by the Centre National de la Recherche Scientifique (CNRS) and the University of Montpellier. T.L. is supported by the European Unionâs Horizon 2020 Research and Innovation Program under the Marie SkĆodowska-Curie grant agreement No. 713366. The work of TL was also supported by the Spanish Agencia Estatal de InvestigaciĂłn through grants PGC2018-095161-B-I00, IFT Centro de Excelencia Severo Ochoa SEV-2016-0597, and Red Consolider MultiDark FPA2017-90566-RED
Configuration mixing within the energy density functional formalism: pathologies and cures
Configuration mixing calculations performed in terms of the Skyrme/Gogny
Energy Density Functional (EDF) rely on extending the Single-Reference energy
functional into non-diagonal EDF kernels. The standard way to do so, based on
an analogy with the pure Hamiltonian case and the use of the generalized Wick
theorem, is responsible for the recently observed divergences and steps in
Multi-Reference calculations. We summarize here the minimal solution to this
problem recently proposed [Lacroix et al, arXiv:0809.2041] and applied with
success to particle number restoration[Bender et al, arXiv:0809.2045]. Such a
regularization method provides suitable corrections of pathologies for EDF
depending on integer powers of the density. The specific case of fractional
powers of the density[Duguet et al, arXiv:0809.2049] is also discussed.Comment: 5 pages, Proceedings of the French-Japanese Symposium, September
2008. To be published in Int. J. of Mod. Phys.
Looking for ultralight dark matter near supermassive black holes
Measurements of the dynamical environment of supermassive black holes (SMBHs)
are becoming abundant and precise. We use such measurements to look for
ultralight dark matter (ULDM), which is predicted to form dense cores
("solitons") in the centre of galactic halos. We search for the gravitational
imprint of an ULDM soliton on stellar orbits near Sgr A* and by combining
stellar velocity measurements with Event Horizon Telescope imaging of M87*.
Finding no positive evidence, we set limits on the soliton mass for different
values of the ULDM particle mass . The constraints we derive exclude the
solitons predicted by a naive extrapolation of the soliton-halo relation, found
in DM-only numerical simulations, for (from Sgr A*) and
(from M87*). However, we present
theoretical arguments suggesting that an extrapolation of the soliton-halo
relation may not be adequate: in some regions of the parameter space, the
dynamical effect of the SMBH could cause this extrapolation to over-predict the
soliton mass by orders of magnitude.Comment: 9 pages + appendices, 5 + 2 figures. v2: some clarifications and
references added; conclusions unchanged; version published in JCAP. v3: few
typos correcte
s- and d-wave superconductivity in a two-band model
Superconductivity in strongly correlated systems is a remarkable phenomenon
that attracts a huge interest. The study of this problem is relevant for
materials as the high Tc oxides, pnictides and heavy fermions. In this work we
study a realistic model that includes the relevant physics of superconductivity
in the presence of strong Coulomb correlations. We consider a two-band model,
since most of these correlated systems have electrons from at least two
different atomic orbitals coexisting at their Fermi surface. The Coulomb
repulsion is taken into account through a local repulsive interaction. Pairing
is considered among quasi- particles in neighbouring sites and we allow for
different symmetries of the order parameter. In order to deal with the strong
local correlations, we use the well known slave boson approach that has proved
very successful for this problem. Here we are interested in obtaining the zero
temperature properties of the model, specifically its phase diagram and the
existence and nature of superconducting quantum critical points. We show that
these can arise by increasing the mixing between the two bands. Since this can
be controlled by external pressure or doping, our results have a direct
relation with experiments. We show that the superconductor-to-normal transition
can be either to a metal, a correlated metal or to an insulator. Also we
compare the relative stability of s and d-wave paired states for different
regions of parameter space and investigate the BCS- BEC crossover in the
two-band lattice model as function of the strength of the pairing interaction.Comment: 21 pages, 14 figure
Rejectionist islamism in Saudi Arabia : the story of Juhayman al-'Utaybi revisited
The storming of the Mecca mosque by Juhayman al-[ain]Utaybi and his fellow rebels in November 1979 represents one of the most spectacular events in the modern history of Saudi Arabia. Yet, it is one of the least understood. Even decades after the event, many important questions remain unanswered. Who were the rebels, and what did they want? Why and how did Juhayman's group come into existence? What happened with the rebels and their ideas after the Mecca events? This article seeks to shed light on the story and legacy of Juhayman al-[ain]Utaybi with new information gathered from extensive fieldwork in Saudi Arabia and elsewhere
Saudi Arabia Backgrounder: Who are the Islamists?
This briefing, based on dozens of interviews in the country between March and May 2004, examines the genealogy of Saudi Arabia's various Islamist groupings.ICG Middle East Report N°31
Effect of anisotropy in the underscreened Kondo lattice
We study the effect of crystal field anisotropy in the underscreened
Kondo lattice model. Starting from the two orbital Anderson lattice model and
including a local anisotropy term, we show, through Schrieffer-Wolff
transformation, that local anisotropy is equivalent to an anisotropic Kondo
interaction (). The competition and coexistence
between ferromagnetism and Kondo effect in this effective model is studied
within a generalized mean-field approximation. Several regimes are obtained,
depending on the parameters, exhibiting or not coexistence of magnetic order
and Kondo effect. Particularly, we show that a re-entrant Kondo phase at low
temperature can be obtained. We are also able to describe phases where the
Kondo temperature is smaller than the Curie temperature (). We propose
that some aspects of uranium and neptunium compounds that present coexistence
of Kondo effect and ferromagnetism, can be understood within this model.Comment: 7 pages, 3 figure
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