11,212 research outputs found
Soft-excess in ULX spectra: disc emission or wind absorption?
We assess the claim that Ultra-luminous X-ray sources (ULXs) host
intermediate-mass black holes (BH) by comparing the cool disc-blackbody model
with a range of other models, namelly a more complex physical model based on a
power-law component slightly modified at various energies by smeared
emission/absorption lines from highly-ionized gas. Our main conclusion is that
the presence of a soft excess, or a soft deficit, depends entirely on the
energy range to which we choose to fit the ``true'' power-law continuum; hence,
we argue that those components should not be taken as evidence for accretion
disc emission, nor used to infer BH masses. We speculate that bright ULXs could
be in a spectral state similar to (or an extension of) the steep-power-law
state of Galactic BH candidates, in which the disc is completely comptonized
and not directly detectable, and the power-law emission may be modified by the
surrounding, fast-moving, ionized gas.Comment: To appear in the proceedings of "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors: Theory vs. Observations",
Cefalu', Sicily, June 11-24, 2006 (AIP). Compilation needs specific AIP .clo,
.cls, .sty and .tex files (included along with the paper .tex file and
figures
Impact of the inelastic proton -- nucleus cross section on the prompt neutrino flux
The description of the inelastic proton -- nucleus cross section at very high
energies is still an open question. The current theoretical uncertainty has
direct impact on the predictions of the cosmic ray and neutrino physics
observables. In this paper we consider different models for the treatment of
, compare its predictions at ultrahigh cosmic ray energies
and estimate the prompt neutrino flux at the neutrino energies that have been
probed by the IceCube Observatory. We demonstrate that depending of the model
used to describe , the predictions for the prompt neutrino
flux can differ by a factor of order of three. Such result demonstrate the
importance of a precise measurement of the inelastic proton -- nucleus cross
section at high energies.Comment: 5 pages, 3 figures; v2: corrected the range of horizontal axis in
figure 1. Matches the version published in Eur. Phys. J.
Phase diagram of random lattice gases in the annealed limit
An analysis of the random lattice gas in the annealed limit is presented. The
statistical mechanics of disordered lattice systems is briefly reviewed. For
the case of the lattice gas with an arbitrary uniform interaction potential and
random short-range interactions the annealed limit is discussed in detail. By
identifying and extracting an entropy of mixing term, a correct physical
expression for the pressure is explicitly given. As an application, the
one-dimensional lattice gas with uniform long-range interactions and random
short-range interactions satisfying a bimodal annealed probability distribution
is discussed. The model is exactly solved and is shown to present interesting
behavior in the presence of competition between interactions, such as the
presence of three phase transitions at constant temperature and the occurrence
of triple and quadruple points.Comment: Final version to be published in the Journal of Chemical Physic
Chern-Simons Invariants of Closed Hyperbolic 3-Manifolds
The Chern-Simons invariants of irreducible U(n)- flat connections on compact
hyperbolic 3-manifolds of the form {\Gamma}\H^3 are derived. The explicit
formula for the Chern-Simons functional is given in terms of Selberg type zeta
functions related to the twisted eta invariants of Atiyah-Patodi-Singer.Comment: 10 pages, 2 diagram
Turbulence in collisionless plasmas : statistical analysis from numerical simulations with pressure anisotropy
In recent years, we have experienced increasing interest in the understanding of the physical properties of collisionless plasmas, mostly because of the large number of astrophysical environments (e. g. the intracluster medium (ICM)) containing magnetic fields that are strong enough to be coupled with the ionized gas and characterized by densities sufficiently low to prevent the pressure isotropization with respect to the magnetic line direction. Under these conditions, a new class of kinetic instabilities arises, such as firehose and mirror instabilities, which have been studied extensively in the literature. Their role in the turbulence evolution and cascade process in the presence of pressure anisotropy, however, is still unclear. In this work, we present the first statistical analysis of turbulence in collisionless plasmas using three-dimensional numerical simulations and solving double-isothermal magnetohydrodynamic equations with the Chew-Goldberger-Low laws closure (CGL-MHD). We study models with different initial conditions to account for the firehose and mirror instabilities and to obtain different turbulent regimes. We found that the CGL-MHD subsonic and supersonic turbulences show small differences compared to the MHD models in most cases. However, in the regimes of strong kinetic instabilities, the statistics, i.e. the probability distribution functions (PDFs) of density and velocity, are very different. In subsonic models, the instabilities cause an increase in the dispersion of density, while the dispersion of velocity is increased by a large factor in some cases. Moreover, the spectra of density and velocity show increased power at small scales explained by the high growth rate of the instabilities. Finally, we calculated the structure functions of velocity and density fluctuations in the local reference frame defined by the direction of magnetic lines. The results indicate that in some cases the instabilities significantly increase the anisotropy of fluctuations. These results, even though preliminary and restricted to very specific conditions, show that the physical properties of turbulence in collisionless plasmas, as those found in the ICM, may be very different from what has been largely believed. Implications can range from interchange of energies to cosmic ray acceleration.Publisher PDFPeer reviewe
The improved nuclear parton distributions
In this paper we propose an improvement of the EKS nuclear parton
distributions for the small x region of high energy processes, where the
perturbative high parton density effects cannot be disregarded. We analyze the
behavior of the ratios and and verify that at small x
they are strongly modified when compared to the EKS predictions. The
implications of our results for the heavy ion collisions in RHIC and LHC are
discussed.Comment: 16 pages, 2 figure
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