8,394 research outputs found
Gamma-ray emission from the globular clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752
Globular clusters (GCs) are emerging as a new class of gamma-ray emitters,
thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now,
eight GCs are known to emit gamma-rays at energies >100~MeV. Based on the
stellar encounter rate of the GCs, we identify potential gamma-ray emitting GCs
out of all known GCs that have not been studied in details before. In this
paper, we report the discovery of a number of new gamma-ray GCs: Liller 1, NGC
6624, and NGC 6752, and evidence for gamma-ray emission from M80, NGC 6139, and
NGC 6541, in which gamma-rays were found within the GC tidal radius. With one
of the highest metallicity among all GCs in the Milky Way, the gamma-ray
luminosity of Liller 1 is found to be the highest of all known gamma-ray GCs.
In addition, we confirm a previous report of significant gamma-ray emitting
region next to NGC 6441. We briefly discuss the observed offset of gamma-rays
from some GC cores. The increasing number of known gamma-ray GCs at distances
out to ~10 kpc is important for us to understand the gamma-ray emitting
mechanism and provides an alternative probe to the underlying millisecond
pulsar populations of the GCs.Comment: 22 pages, 7 figures, 2 tables; ApJ, in pres
On the low-temperature lattice thermal transport in nanowires
We propose a theory of low temperature thermal transport in nano-wires in the
regime where a competition between phonon and flexural modes governs the
relaxation processes. Starting with the standard kinetic equations for two
different types of quasiparticles we derive a general expression for the
coefficient of thermal conductivity. The underlying physics of thermal
conductance is completely determined by the corresponding relaxation times,
which can be calculated directly for any dispersion of quasiparticles depending
on the size of a system. We show that if the considered relaxation mechanism is
dominant, then at small wire diameters the temperature dependence of thermal
conductivity experiences a crossover from to -dependence.
Quantitative analysis shows reasonable agreement with resent experimental
results.Comment: 12 pages, 3 eps figure
Identification campaign of supernova remnant candidates in the Milky Way - I: Chandra observation of G308.3-1.4
ROSAT all-sky survey (RASS) data have provided another window to search for
supernova remnants (SNRs). In reexamining this data archive, a list of
unidentified extended X-ray objects have been suggested as promising SNR
candidate. However, most of these targets have not yet been fully explored by
the state-of-art X-ray observatories. For selecting a pilot target for a
long-term identification campaign, we have observed the brightest candidate,
G308.3-1.4, with Chandra X-ray observatory. An incomplete shell-like X-ray
structure which well-correlated with the radio shell emission at 843 MHz has
been revealed. The X-ray spectrum suggests the presence of a shock-heated
plasma. All these evidences confirm G308.3-1.4 as a SNR. The brightest X-ray
point source detected in this field-of-view is also the one locates closest to
the geometrical center of G308.3-1.4, which has a soft spectrum. The intriguing
temporal variability and the identification of optical/infrared counterpart
rule out the possibility of an isolated neutron star. On the other hand, the
spectral energy distribution from Ks band to R band suggests a late-type star.
Together with a putative periodicity of \sim1.4 hrs, the interesting excesses
in V, B bands and H-alpha suggest this source as a promising candidate of a
compact binary survived in a supernova explosion (SN).Comment: 19 pages, 10 figures, 2 tables, accepted for publication in Ap
The , , and decays in the perturbative QCD approach
Two-body non-leptonic charmed decays , ,
and are analyzed in perturbative QCD approach, where
and denote the light pseudoscalar meson and vector meson, respectively.
We test the meson wave function by a fit with experimental data of
six channels. We give the branching ratios of all the charmed B decay
channels, most of which agree with experiments amazingly well. The predicted
decays can be confronted with the future experimental data. By
straightforward calculations, our pQCD approach gives the right relative strong
phase of with experiments. We also predict the percentage of
transverse polarizations in decay channels.Comment: 25 pages, 4 figure
Theory of Networked Minority Games based on Strategy Pattern Dynamics
We formulate a theory of agent-based models in which agents compete to be in
a winning group. The agents may be part of a network or not, and the winning
group may be a minority group or not. The novel feature of the present
formalism is its focus on the dynamical pattern of strategy rankings, and its
careful treatment of the strategy ties which arise during the system's temporal
evolution. We apply it to the Minority Game (MG) with connected populations.
Expressions for the mean success rate among the agents and for the mean success
rate for agents with neighbors are derived. We also use the theory to
estimate the value of connectivity above which the Binary-Agent-Resource
system with high resource level goes into the high-connectivity state.Comment: 24 pages, 3 figures, submitted to PR
First-order super-radiant phase transitions in a multi-qubit--cavity system
We predict the existence of novel first-order phase transitions in a general
class of multi-qubit-cavity systems. Apart from atomic systems, the associated
super-radiant phase transition should be observable in a variety of solid-state
experimental systems, including the technologically important case of
interacting quantum dots coupled to an optical cavity mode.Comment: To appear in Phys. Rev. Let
Cosmological Limits on the Neutrino Mass from the Lya Forest
The Lya forest in quasar spectra probes scales where massive neutrinos can
strongly suppress the growth of mass fluctuations. Using hydrodynamic
simulations with massive neutrinos, we successfully test techniques developed
to measure the mass power spectrum from the forest. A recent observational
measurement in conjunction with a conservative implementation of other
cosmological constraints places upper limits on the neutrino mass: m_nu < 5.5
eV for all values of Omega_m, and m_nu < 2.4 (Omega_m/0.17 -1) eV, if 0.2 <
Omega_m <0.5 as currently observationally favored (both 95 % C.L.).Comment: 4 pages, 2 ps figures, REVTex, submitted to Phys. Rev. Let
Cyclic Universe with Quintom matter in Loop Quantum Cosmology
In this paper, we study the possibility of model building of cyclic universe
with Quintom matter in the framework of Loop Quantum Cosmology. After a general
demonstration, we provide two examples, one with double-fluid and another
double-scalar field, to show how such a scenario is obtained. Analytical and
numerical calculations are both presented in the paper.Comment: 11 pages, 2 figure
Electron Acceleration and the Production of Nonthermal Electron Distributions in Accretion Disk Coronae
We consider electron acceleration by obliquely propagating fast mode waves in
magnetically dominated accretion disk coronae. For low coronal plasma
densities, acceleration can exceed Coulomb drag at lower energies and energize
electrons out of the thermal background, resulting in a nonthermal tail. The
extent of this tail is determined by the balance between acceleration and
radiative cooling via inverse Compton scattering and synchrotron emission, and
usually goes out to tens of MeV. This will have direct applications for
explaining the gamma-rays from several galactic black hole candidates, such as
Cyg X-1 and GRO J0422, which show 0.5--5 MeV emissions in excess over what most
thermal models predict. Detailed time evolutions of the particle distributions
and wave spectra are also presented.Comment: AASLaTeX, two figures, accepted for ApJ
Group Theoretical Properties and Band Structure of the Lame Hamiltonian
We study the group theoretical properties of the Lame equation and its
relation to su(1,1) and su(2). We compute the band structure, dispersion
relation and transfer matrix and discuss the dynamical symmetry limits.Comment: 21 pages Revtex + 6 eps + 2 jpg figure
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