99 research outputs found
Maximal spin and energy conversion efficiency in a symbiotic system of black hole, disc and jet
postprin
Cosmic magnetic fields in large scale filaments and sheets
We consider the possibility that cosmic magnetic field, instead of being uniformly distributed, is strongly correlated with the large scale structure of the universe. Then, the observed rotational measure of extra-galactic radio sources would be caused mostly by the clumpy magnetic field in cosmological filaments/sheets rather than by a uniform magnetic field, which was often assumed in previous studies. As a model for the inhomogeneity of the cosmological magnetic field, we adopt a cosmological hydrodynamic simulation, where the field is passively included, and can approximately represent the real field distribution with an arbitrary normalization for the field strength. Then, we derive an upper limit of the magnetic field strength by comparing the observed limit of rotational measure with the rotational measure expected from the magnetic field geometry in the simulated model universe. The resulting upper limit to the magnetic field in filaments and sheets is (B) over bar(fs) less than or similar to 1 mu G which is similar to 10(3) times higher than the previously quoted values. This value is close to, but larger than, the equipartition magnetic field strength in filaments and sheets. The amplification mechanism of the magnetic field to the above strength is uncertain. The implications of such a strength of the cosmic magnetic field are discussedopen15012
Supermassive black hole spin-flip during the inspiral
During post-Newtonian evolution of a compact binary, a mass ratio Îœ different from 1 provides a second small parameter, which can lead to unexpected results. We present a statistics of supermassive black hole candidates, which enables us first to derive their mass distribution, and then to establish a logarithmically even probability in Îœ of themass ratios at their encounter. In the mass ratio range Îœ â (1/30, 1/3) of supermassive black hole mergers representing 40% of all possible cases, the combined effect of spin-orbit precession and gravitational radiation leads to a spin-flip of the dominant spin during the inspiral phase of the merger. This provides amechanism for explaining a large set of observations on X-shaped radio galaxies. In another 40% with mass ratios Îœ â (1/30, 1/1000) a spin-flip never occurs, while in the remaining 20% of mergers with mass ratios Îœ â (1/3, 1) it may occur during the plunge. We analyze the magnitude of the spin-flip angle occurring during the inspiral as a function of the mass ratio and original relative orientation of the spin and orbital angular momentum. We also derive a formula for the final spin at the end of the inspiral in this mass ratio range. © 2010 IOP Publishing Ltd
TEV GAMMA-RAYS FROM PROTON BLAZARS
Proton acceleration in nearby blazars can be diagnosed measuring their
intense TeV -ray emission. Flux predictions for 1101+384 (Mrk421) and
1219+285 (ON231), both strong EGRET sources (0.1-10 GeV), are obtained from
model spectra of unsaturated synchrotron pair cascades fitted to publicly
available multifrequency data. An experimental effort to confirm the predicted
emission in the range 1-10 TeV would be of great importance for the problems of
the origin of cosmic rays, the era of galaxy formation and the cosmological
distance scale.Comment: 10 pages of latex using Kluwer spacekap.sty, to appear in Space
Science Review
Cosmic ray diffusion near the Bohm limit in the Cassiopeia A supernova remnant
Supernova remnants (SNRs) are believed to be the primary location of the
acceleration of Galactic cosmic rays, via diffusive shock (Fermi) acceleration.
Despite considerable theoretical work the precise details are still unknown, in
part because of the difficulty in directly observing nucleons that are
accelerated to TeV energies in, and affect the structure of, the SNR shocks.
However, for the last ten years, X-ray observatories ASCA, and more recently
Chandra, XMM-Newton, and Suzaku have made it possible to image the synchrotron
emission at keV energies produced by cosmic-ray electrons accelerated in the
SNR shocks. In this article, we describe a spatially-resolved spectroscopic
analysis of Chandra observations of the Galactic SNR Cassiopeia A to map the
cutoff frequencies of electrons accelerated in the forward shock. We set upper
limits on the electron diffusion coefficient and find locations where particles
appear to be accelerated nearly as fast as theoretically possible (the Bohm
limit).Comment: 18 pages, 5 figures. Accepted for publication in Nature Physics (DOI
below), final version available week of August 28, 2006 at
http://www.nature.com/nphy
KeV Warm Dark Matter and Composite Neutrinos
Elementary keV sterile Dirac neutrinos can be a natural ingredient of the
composite neutrino scenario. For a certain class of composite neutrino
theories, these sterile neutrinos naturally have the appropriate mixing angles
to be resonantly produced warm dark matter (WDM). Alternatively, we show these
sterile neutrinos can be WDM produced by an entropy-diluted thermal freeze-out,
with the necessary entropy production arising not from an out-of-equilibrium
decay, but rather from the confinement of the composite neutrino sector,
provided there is sufficient supercooling.Comment: 12 pages, 2 figures, published versio
The Curious Adventure of the Ultrahigh Energy Cosmic Rays
These lectures discuss the mysteries involving the production and
extragalactic propagation of ultrahigh energy cosmic rays and suggested
possible solutions.Comment: Lectures given at the D. Chalonge Euroschool, Erice, Italy, November
2000, 25 pages, 7 ps figs., expanded revision with color fig.
Origin of Cosmic Magnetic Fields
We propose that the overlapping shock fronts from young supernova remnants
produce a locally unsteady, but globally steady large scale spiral shock front
in spiral galaxies, where star formation and therefore massive star explosions
correlate geometrically with spiral structure. This global shock front with its
steep gradients in temperature, pressure and associated electric fields will
produce drifts, which in turn give rise to a strong sheet-like electric
current, we propose. This sheet current then produces a large scale magnetic
field, which is regular, and connected to the overall spiral structure. This
rejuvenates the overall magnetic field continuously, and also allows to
understand that there is a regular field at all in disk galaxies. This proposal
connects the existence of magnetic fields to accretion in disks. We not yet
address all the symmetries of the magnetic field here; the picture proposed
here is not complete. X-ray observations may be able to test it already.Comment: 18 pages, no figures; to be published in Proc. Palermo Meeting Sept.
2002, Eds. N. G. Sanchez et al., The Early Universe and the Cosmic Microwave
Background: Theory and Observation
Rapid urbanisation and security: Holistic approach to enhancing security of urban spaces
Rapid urbanisation, particularly driven by rural-urban migration, can pose a wide range of security challenges in the global south and global north. The management of such a transition, in terms of the provision of social goods and quality of life raises significant challenges. Security of contemporary urban environments has become more complex due to a greater range of risk drivers, many of which can be exacerbated by the observed and portended impacts of climate change. This chapter outlines the phenomena underlying the transition to urbanisation - and the security challenges that have been exacerbated by these transitions. In doing so this work a holistic approach to security and highlights a gradual trend in the increased securitisation of issues (such as climate change) that in the past were not considered part of typical âsecurityâ dialogues. It also introduces a decision support framework that can aid a broad range of stakeholders in making decisions about the enhancement of security of urban spaces in a context of multiple threats exacerbated by these new security challenges
Follow-up modalities in focal therapy for prostate cancer: results from a Delphi consensus project
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