103 research outputs found
On the nonthermal X-ray emission in blazar jets
We consider particle acceleration to high energy via diffusive shock
acceleration in a simple, self-consistent shock in jet model for blazars.
Electrons are assumed to be accelerated at a shock front in relativistic jets
and radiate synchrotron emission in a post-shock region. The full time, space
and momentum dependence of the electron distribution function is used for a
calculation of the nonthermal synchrotron spectra. We discuss the evolution of
the spectral index by varying the rate at which particles enter the
acceleration process. The results indicate that the synchrotron spectral index
displays a characteristic looplike behaviour with intensity (as has been
observed in several blazars), where the orientation of the loop depends on
whether the acceleration time scale is comparable to the synchrotron cooling
time scale or not. We show that our model provides a good fit to the observed
evolution of the spectral index of Mkn 421 during a flare in 1994.Comment: 6 pages, 3 figures, conference proceedin
Refractive index in holographic superconductors
With the probe limit, we investigate the behavior of the electric
permittivity and effective magnetic permeability and related optical properties
in the s-wave holographic superconductors. In particular, our result shows that
unlike the strong coupled systems which admit a gravity dual of charged black
holes in the bulk, the electric permittivity and effective magnetic
permeability are unable to conspire to bring about the negative
Depine-Lakhtakia index at low frequencies, which implies that the negative
phase velocity does not appear in the holographic superconductors under such a
situation.Comment: JHEP style, 1+15 pages, 11 figures, version to appear in JHE
The Efficiency of Coherent Radiation from Relativistic Shocks
We discuss a mechanism for intense electromagnetic wave emission at an
astrophysical relativistic shock in a magnetized collisionless plasma. At the
magnetized shock, the particle reflection by a compressed magnetic field of the
shock produces a ring-like distribution in momentum, which gives rise to plasma
instabilities. Intense and coherent high-frequency electromagnetic waves will
be emitted if the synchrotron maser instability (SMI) is excited, whereas
non-propagating magnetic fluctuations will be generated when the Weibel
instability (WI) is the dominant mode. The problem is of great astrophysical
interest because if intense radiation is emitted, the interaction with the
upstream medium induces a large-amplitude electrostatic field (or Wakefield),
which may play a role for the acceleration of ultra-high-energy cosmic rays. We
review our recent effort to measure the efficiency of the electromagnetic wave
emission using fully self-consistent, two-dimensional (2D) particle-in-cell
(PIC) simulations for pair plasmas. We found that the emission efficiency in 2D
was systematically lower than one dimensional (1D) PIC simulation results.
However, the power remains finite even when the WI is active to generate
large-amplitude magnetic fluctuations. Astrophysical implications of the
present results are briefly discussed.Comment: 13 pages, 4 figures, conference proceeding
A powerful bursting radio source towards the Galactic Centre
Transient astronomical sources are typically powered by compact objects and
usually signify highly explosive or dynamic events. While radio astronomy has
an impressive record of obtaining high time resolution observations, usually it
is achieved in quite narrow fields-of-view. Consequently, the dynamic radio sky
is poorly sampled, in contrast to the situation in the X- and gamma-ray bands
in which wide-field instruments routinely detect transient sources. Here we
report a new transient source, GCRT J1745-3009, detected in 2002 during a
moderately wide-field radio transient monitoring program of the Galactic center
(GC) region at 0.33 GHz. The characteristics of its bursts are unlike those
known for any other class of radio transient. If located in or near the GC, its
brightness temperature (~10^16 K) and the implied energy density within GCRT
J1745-3009 vastly exceeds that observed in most other classes of radio
astronomical sources, and is consistent with coherent emission processes rarely
observed. We conclude that GCRT J1745-3009 is the first member of a new class
of radio transient sources, the first of possibly many new classes to be
identified through current and upcoming radio surveys.Comment: 16 pages including 3 figures. Appears in Nature, 3 March 200
On the Numerical Evaluation of Loop Integrals With Mellin-Barnes Representations
An improved method is presented for the numerical evaluation of multi-loop
integrals in dimensional regularization. The technique is based on
Mellin-Barnes representations, which have been used earlier to develop
algorithms for the extraction of ultraviolet and infrared divergencies. The
coefficients of these singularities and the non-singular part can be integrated
numerically. However, the numerical integration often does not converge for
diagrams with massive propagators and physical branch cuts. In this work,
several steps are proposed which substantially improve the behavior of the
numerical integrals. The efficacy of the method is demonstrated by calculating
several two-loop examples, some of which have not been known before.Comment: 13 pp. LaTe
Single Cut Integration
We present an analytic technique for evaluating single cuts for one-loop
integrands, where exactly one propagator is taken to be on shell. Our method
extends the double-cut integration formalism of one-loop amplitudes to the
single-cut case. We argue that single cuts give meaningful information about
amplitudes when taken at the integrand level. We discuss applications to the
computation of tadpole coefficients.Comment: v2: corrected typo in abstrac
Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysical
processes likely to play a role in the production of non-thermal emission in
massive stars, with emphasis on massive binaries. Even though the discussion
will start in the radio domain where the non-thermal emission was first
detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the
radio to the very high energies.
First, the theoretical aspects of the non-thermal emission from early-type
stars will be addressed. The main topics that will be discussed are
respectively the physics of individual stellar winds and their interaction in
binary systems, the acceleration of relativistic electrons, the magnetic field
of massive stars, and finally the non-thermal emission processes relevant to
the case of massive stars. Second, this general qualitative discussion will be
followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several
stellar, wind and orbital parameters can be combined in order to make some
semi-quantitative predictions on the high-energy counterpart to the non-thermal
emission detected in the radio domain.
These theoretical considerations will be followed by a census of results
obtained so far, and related to this topic... (see paper for full abstract)Comment: 47 pages, 5 postscript figures, accepted for publication in Astronomy
and Astrophysics Review. Astronomy and Astrophysics Review, in pres
A low-frequency radio halo associated with a cluster of galaxies
Clusters of galaxies are the largest gravitationally bound objects in the
Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and
dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and
relativistic particles are mixed with the gas as revealed by giant radio
haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at
cluster center. Radio haloes require that the emitting electrons are
accelerated in situ (by turbulence), or are injected (as secondary particles)
by proton collisions into the intergalactic medium. They are found only in a
fraction of massive clusters that have complex dynamics, which suggests a
connection between these mechanisms and cluster mergers. Here we report a radio
halo at low frequencies associated with the merging cluster Abell 521. This
halo has an extremely steep radio spectrum, which implies a high frequency
cut-off; this makes the halo difficult to detect with observations at 1.4 GHz
(the frequency at which all other known radio haloes have been best studied).
The spectrum of the halo is inconsistent with a secondary origin of the
relativistic electrons, but instead supports turbulent acceleration, which
suggests that many radio haloes in the Universe should emit mainly at low
frequencies.Comment: 18 pages, 4 figures, Nature 455, 94
Repair, regenerative and supportive therapies of the annulus fibrosus: achievements and challenges
Lumbar discectomy is a very effective therapy for neurological decompression in patients suffering from sciatica due to hernia nuclei pulposus. However, high recurrence rates and persisting post-operative low back pain in these patients require serious attention. In the past decade, tissue engineering strategies have been developed mainly targeted to the regeneration of the nucleus pulposus (NP) of the intervertebral disc. Accompanying techniques that deal with the damaged annulus fibrous are now increasingly recognised as mandatory in order to prevent re-herniation to increase the potential of NP repair and to confine NP replacement therapies. In the current review, the requirements, achievements and challenges in this quickly emerging field of research are discussed
NLO QCD corrections to top anti-top bottom anti-bottom production at the LHC: 2. full hadronic results
We present predictions for top anti-top bottom anti-bottom production at the
LHC in next-to-leading order QCD. The precise description of this background
process is a prerequisite to observe associated top anti-top Higgs production
in the Higgs -> bottom anti-bottom decay channel and to directly measure the
top-quark Yukawa coupling at the LHC. The leading-order cross section is
extremely sensitive to scale variations. We observe that the traditional scale
choice adopted in ATLAS simulations underestimates the top anti-top bottom
anti-bottom background by a factor two and introduce a new dynamical scale that
stabilizes the perturbative predictions. We study various kinematic
distributions and observe that the corrections have little impact on their
shapes if standard cuts are applied. In the regime of highly boosted Higgs
bosons, which offers better perspectives to observe the top anti-top Higgs
signal, we find significant distortions of the kinematic distributions. The
one-loop amplitudes are computed using process-independent algebraic
manipulations of Feynman diagrams and numerical tensor reduction. We find that
this approach provides very high numerical stability and CPU efficiency.Comment: 42 pages, LaTeX, 44 postscript figure
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