2,547 research outputs found
A Persistent High-Energy Flux from the Heart of the Milky Way : Integral's view of the Galactic Center
The Ibis/Isgri imager on Integral detected for the first time a hard X-ray
source, IGR J17456-2901, located within 1' of Sgr A* over the energy range
20-100 keV. Here we present the results of a detailed analysis of ~7 Ms of
Integral observations of the GC. With an effective exposure of 4.7 Ms we have
obtained more stringent positional constraints on this HE source and
constructed its spectrum in the range 20-400 keV. Furthermore, by combining the
Isgri spectrum with the total X-ray spectrum corresponding to the same physical
region around SgrA* from XMM data, and collected during part of the Integral
observations, we constructed and present the first accurate wide band HE
spectrum for the central arcmins of the Galaxy. Our complete analysis of the
emission properties of IGR shows that it is faint but persistent with no
variability above 3 sigma contrary to what was alluded to in our first paper.
This result, in conjunction with the spectral characteristics of the X-ray
emission from this region, suggests that the source is most likely not
point-like but, rather, that it is a compact, yet diffuse, non-thermal emission
region. The centroid of IGR is estimated to be R.A.=17h45m42.5,
decl.=-28deg59'28'', offset by 1' from the radio position of Sgr A* and with a
positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x10^35
erg/sec. Very recently, Hess detected of a source of ~TeV g-rays also located
within 1' of Sgr A*. We present arguments in favor of an interpretation
according to which the photons detected by Integral and Hess arise from the
same compact region of diffuse emission near the central BH and that the
supernova remnant Sgr A East could play an important role as a contributor of
very HE g-rays to the overall spectrum from this region.Comment: 14 pages, 11 figures, Accepted for publication in Ap
Real-space renormalization group for the random-field Ising model
We present real--space renormalization group (RG) calculations of the
critical properties of the random--field Ising model on a cubic lattice in
three dimensions. We calculate the RG flows in a two--parameter truncation of
the Hamiltonian space. As predicted, the transition at finite randomness is
controlled by a zero temperature, disordered critical fixed point, and we
exhibit the universal crossover trajectory from the pure Ising critical point.
We extract scaling fields and critical exponents, and study the distribution of
barrier heights between states as a function of length scale.Comment: 12 pages, CU-MSC-757
LHC / ILC / Cosmology Interplay
There is a strong and growing interplay between particle physics and
cosmology. In this talk, I discuss some aspects of this interplay concerning
dark matter candidates put forth by theories beyond the Standard Model. In
explaining the requirements for collider tests of such dark matter candidates,
I focus in particular on the case of the lightest neutralino in the MSSM.Comment: 7 pages, contribution to the proceedings of the IX Workshop on High
Energy Physics Phenomenology (WHEPP-9), 3-14 Jan 2006, Bhubaneswar, Indi
Computer simulation of the critical behavior of 3D disordered Ising model
The critical behavior of the disordered ferromagnetic Ising model is studied
numerically by the Monte Carlo method in a wide range of variation of
concentration of nonmagnetic impurity atoms. The temperature dependences of
correlation length and magnetic susceptibility are determined for samples with
various spin concentrations and various linear sizes. The finite-size scaling
technique is used for obtaining scaling functions for these quantities, which
exhibit a universal behavior in the critical region; the critical temperatures
and static critical exponents are also determined using scaling corrections. On
the basis of variation of the scaling functions and values of critical
exponents upon a change in the concentration, the conclusion is drawn
concerning the existence of two universal classes of the critical behavior of
the diluted Ising model with different characteristics for weakly and strongly
disordered systems.Comment: 14 RevTeX pages, 6 figure
Long-lived Charginos in the Focus-point Region of the MSSM Parameter Space
We analyse the possibility to get light long-lived charginos within the
framework of the MSSM with gravity mediated SUSY breaking. We find out that
this possibility can be realized in the so-called focus-point region of
parameter space. The mass degeneracy of higgsino-like chargino and two
higgsino-like neutralinos is the necessary condition for a long lifetime. It
requires the fine-tuning of parameters, but being a single additional
constraint in the whole parameter space it can be fulfilled in the Constrained
MSSM along the border line where radiative electroweak symmetry breaking fails.
In a narrow band close to the border line the charginos are long-lived
particles. The cross-sections of their production and co-production at the LHC
via electroweak interaction reach a few tenth of pb.Comment: LaTeX, 11 pages, 11 eps figure
Neutron scattering experiments and simulations near the magnetic percolation threshold of Fe_x Zn_{1-x} F_2
The low temperature excitations in the anisotropic antiferromagnetic Fe_{1-x}
Zn_x F_2 for x=0.25 and 0.31, at and just above the magnetic percolation
threshold concentration x_p=0.25, were measured using inelastic neutron
scattering. The excitations were simulated for x=0.31 using a localized,
classical excitation model, which accounts well for the energies and relative
intensities of the excitations observed in the scattering experiments.Comment: 6 pages, 6 figure
Specific-Heat Exponent of Random-Field Systems via Ground-State Calculations
Exact ground states of three-dimensional random field Ising magnets (RFIM)
with Gaussian distribution of the disorder are calculated using
graph-theoretical algorithms. Systems for different strengths h of the random
fields and sizes up to N=96^3 are considered. By numerically differentiating
the bond-energy with respect to h a specific-heat like quantity is obtained,
which does not appear to diverge at the critical point but rather exhibits a
cusp. We also consider the effect of a small uniform magnetic field, which
allows us to calculate the T=0 susceptibility. From a finite-size scaling
analysis, we obtain the critical exponents \nu=1.32(7), \alpha=-0.63(7),
\eta=0.50(3) and find that the critical strength of the random field is
h_c=2.28(1). We discuss the significance of the result that \alpha appears to
be strongly negative.Comment: 9 pages, 9 figures, 1 table, revtex revised version, slightly
extende
Non Markovian persistence in the diluted Ising model at criticality
We investigate global persistence properties for the non-equilibrium critical
dynamics of the randomly diluted Ising model. The disorder averaged persistence
probability of the global magnetization is found to decay
algebraically with an exponent that we compute analytically in a
dimensional expansion in . Corrections to Markov process are
found to occur already at one loop order and is thus a novel
exponent characterizing this disordered critical point. Our result is
thoroughly compared with Monte Carlo simulations in , which also include a
measurement of the initial slip exponent. Taking carefully into account
corrections to scaling, is found to be a universal exponent,
independent of the dilution factor along the critical line at , and
in good agreement with our one loop calculation.Comment: 7 pages, 4 figure
Monte Carlo Simulation of a Random-Field Ising Antiferromagnet
Phase transitions in the three-dimensional diluted Ising antiferromagnet in
an applied magnetic field are analyzed numerically. It is found that random
magnetic field in a system with spin concentration below a certain threshold
induces a crossover from second-order phase transition to first-order
transition to a new phase characterized by a spin-glass ground state and
metastable energy states at finite temperatures.Comment: 10 pages, 11 figure
Monte Carlo study of the random-field Ising model
Using a cluster-flipping Monte Carlo algorithm combined with a generalization
of the histogram reweighting scheme of Ferrenberg and Swendsen, we have studied
the equilibrium properties of the thermal random-field Ising model on a cubic
lattice in three dimensions. We have equilibrated systems of LxLxL spins, with
values of L up to 32, and for these systems the cluster-flipping method appears
to a large extent to overcome the slow equilibration seen in single-spin-flip
methods. From the results of our simulations we have extracted values for the
critical exponents and the critical temperature and randomness of the model by
finite size scaling. For the exponents we find nu = 1.02 +/- 0.06, beta = 0.06
+/- 0.07, gamma = 1.9 +/- 0.2, and gammabar = 2.9 +/- 0.2.Comment: 12 pages, 6 figures, self-expanding uuencoded compressed PostScript
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