10,138 research outputs found
Particle acceleration close to the supermassive black hole horizon: the case of M87
The radio galaxy M87 has recently been found to be a rapidly variable TeV
emitting source. We analyze the implications of the observed TeV
characteristics and show that it proves challenging to account for them within
conventional acceleration and emission models. We discuss a new pulsar-type
scenario for the origin of variable, very high energy (VHE) emission close to
the central supermassive black hole and show that magneto-centrifugally
accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk
photons to the TeV regime, leading to VHE characteristics close to the observed
ones. This suggests, conversely, that VHE observations of highly under-luminous
AGNs could provide an important diagnostic tool for probing the conditions
prevalent in the inner accretion disk of these sources.Comment: 5 pages, one figure (typos corrected); based on presentation at "High
Energy Phenomena in Relativistic Outflows", Dublin, Sept. 2007; accepted for
publication in International Journal of Modern Physics
Implications of the VHE Gamma-Ray Detection of the Quasar 3C279
The MAGIC collaboration recently reported the detection of the quasar 3C279
at > 100 GeV gamma-ray energies. Here we present simultaneous optical (BVRI)
and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the
implications of the snap-shot spectral energy distribution for jet models of
blazars. A one-zone synchrotron-self-Compton origin of the entire SED,
including the VHE gamma-ray emission can be ruled out. The VHE emission could,
in principle, be interpreted as Compton upscattering of external radiation
(e.g., from the broad-line regions). However, such an interpretation would
require either an unusually low magnetic field of B ~ 0.03 G or an
unrealistically high Doppler factor of Gamma ~ 140. In addition, such a model
fails to reproduce the observed X-ray flux. This as well as the lack of
correlated variability in the optical with the VHE gamma-ray emission and the
substantial gamma-gamma opacity of the BLR radiation field to VHE gamma-rays
suggests a multi-zone model. In particular, an SSC model with an emission
region far outside the BLR reproduces the simultaneous X-ray -- VHE gamma-ray
spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing
the observed SED of 3C279 reasonably well. However, the hadronic model requires
a rather extreme jet power of L_j ~ 10^{49} erg s^{-1}, compared to a
requirement of L_j ~ 2 X 10^{47} erg s^{-1} for a multi-zone leptonic model.Comment: Accepted for pulication. Several clarifications and additions to the
manuscript to match the accepted versio
Particle Acceleration in Mildly Relativistic Shearing Flows: the Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets
The origin of the extended X-ray emission in the large-scale jets of active
galactic nuclei (AGNs) poses challenges to conventional models of acceleration
and emission. Although the electron synchrotron radiation is considered the
most feasible radiation mechanism, the formation of the continuous large-scale
X-ray structure remains an open issue. As astrophysical jets are expected to
exhibit some turbulence and shearing motion, we here investigate the potential
of shearing flows to facilitate an extended acceleration of particles and
evaluate its impact on the resultant particle distribution. Our treatment
incorporates systematic shear and stochastic second-order Fermi effects. We
show that for typical parameters applicable to large-scale AGN jets, stochastic
second-order Fermi acceleration, which always accompanies shear particle
acceleration, can play an important role in facilitating the whole process of
particle energization. We study the time-dependent evolution of the resultant
particle distribution in the presence of second-order Fermi acceleration, shear
acceleration, and synchrotron losses using a simple Fokker--Planck approach and
provide illustrations for the possible emergence of a complex (multicomponent)
particle energy distribution with different spectral branches. We present
examples for typical parameters applicable to large-scale AGN jets, indicating
the relevance of the underlying processes for understanding the extended X-ray
emission and the origin of ultrahigh-energy cosmic rays.Comment: 26 pages, 8 figures; to appear in Ap
Gamma rays from compact binary system
Some of the very high energy (VHE) gamma-ray sources detected with the modern
generation of Cherenkov telescopes have been identified with previously known
X-ray binary systems. These detections demonstrate the richness of non-thermal
phenomena in compact galactic objects containing relativistic outflows or winds
produced near black holes and neutron stars. Recently, the well-known
microquasar Cygnus X-3 seems to be associated with a gamma-ray source detected
with AGILE. Here I summarise the main observational results on gamma-ray
emission from X-ray binaries, as well as some of the proposed scenarios to
explain the production of VHE gamma-rays.Comment: 12 pages, 7 figures. Invited talk at the "Fourth Heidelberg
International Symposium on High Energy Gamma-Ray Astronomy 2008
Reply to Comment on "Triviality of the Ground State Structure in Ising Spin Glasses"
We reply to the comment of Marinari and Parisi [cond-mat/0002457 v2] on our
paper [Phys. Rev. Lett. 83, 5126 (1999) and cond-mat/9906323]. We show that the
data in the comment are affected by strong finite-size corrections. Therefore
the original conclusion of our paper still stands.Comment: Reply to comment cond-mat/0002457 on cond-mat/9906323. Final version
with minor change
Superconductor-to-Normal Phase Transition in a Vortex Glass Model: Numerical Evidence for a New Percolation Universality Class
The three-dimensional strongly screened vortex-glass model is studied
numerically using methods from combinatorial optimization. We focus on the
effect of disorder strength on the ground state and found the existence of a
disorder-driven normal-to-superconducting phase transition. The transition
turns out to be a geometrical phase transition with percolating vortex loops in
the ground state configuration. We determine the critical exponents and provide
evidence for a new universality class of correlated percolation.Comment: 11 pages LaTeX using IOPART.cls, 11 eps-figures include
Dislocations in the ground state of the solid-on-solid model on a disordered substrate
We investigate the effects of topological defects (dislocations) to the
ground state of the solid-on-solid (SOS) model on a simple cubic disordered
substrate utilizing the min-cost-flow algorithm from combinatorial
optimization. The dislocations are found to destabilize and destroy the elastic
phase, particularly when the defects are placed only in partially optimized
positions. For multi defect pairs their density decreases exponentially with
the vortex core energy. Their mean distance has a maximum depending on the
vortex core energy and system size, which gives a fractal dimension of . The maximal mean distances correspond to special vortex core
energies for which the scaling behavior of the density of dislocations change
from a pure exponential decay to a stretched one. Furthermore, an extra
introduced vortex pair is screened due to the disorder-induced defects and its
energy is linear in the vortex core energy.Comment: 6 pages RevTeX, eps figures include
Morphology and hardness ratio exploitation under limited statistics
Gamma-ray astronomy has produced for several years now sky maps for low
photon statistics, non-negligible background and comparatively poor angular
resolution. Quantifying the significance of spatial features remains difficult.
Besides, spectrum extraction requires regions with large statistics while maps
in energy bands allow only qualitative interpretation. The two main competing
mechanisms in the VHE domain are the Inverse-Compton emission from accelerated
electrons radiating through synchrotron in the X-ray domain and the
interactions between accelerated hadrons and the surrounding medium, leading to
the production and subsequent decay of Pi0 mesons. The spectrum of the VHE
emission from leptons is predicted to steepen with increasing distance from the
acceleration zone, owing to synchrotron losses (i.e. cooled population). It
would remain approximately constant for hadrons. Ideally, spectro-imaging
analysis would have the same spatial scale in the TeV and X-ray domains, to
distinguish the local emission mechanisms. More realistically, we investigate
here the possibility of improving upon the currently published HESS results by
using more sophisticated tools.Comment: 4 pages, 6 figures, Proceeding for a poster at the GAMMA08 Heidelberg
Symposiu
Low-dimensional chaos induced by frustration in a non-monotonic system
We report a novel mechanism for the occurrence of chaos at the macroscopic
level induced by the frustration of interaction, namely frustration-induced
chaos, in a non-monotonic sequential associative memory model. We succeed in
deriving exact macroscopic dynamical equations from the microscopic dynamics in
the case of the thermodynamic limit and prove that two order parameters
dominate this large-degree-of-freedom system. Two-parameter bifurcation
diagrams are obtained from the order-parameter equations. Then we analytically
show that the chaos is low-dimensional at the macroscopic level when the system
has some degree of frustration, but that the chaos definitely does not occur
without the frustration.Comment: 2 figure
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