7,579 research outputs found
Exploring Particle Acceleration in Gamma-Ray Binaries
Binary systems can be powerful sources of non-thermal emission from radio to
gamma rays. When the latter are detected, then these objects are known as
gamma-ray binaries. In this work, we explore, in the context of gamma-ray
binaries, different acceleration processes to estimate their efficiency: Fermi
I, Fermi II, shear acceleration, the converter mechanism, and magnetic
reconnection. We find that Fermi I acceleration in a mildly relativistic shock
can provide, although marginally, the multi-10 TeV particles required to
explain observations. Shear acceleration may be a complementary mechanism,
giving particles the final boost to reach such a high energies. Fermi II
acceleration may be too slow to account for the observed very high energy
photons, but may be suitable to explain extended low-energy emission. The
converter mechanism seems to require rather high Lorentz factors but cannot be
discarded a priori. Standard relativistic shock acceleration requires a highly
turbulent, weakly magnetized downstream medium; magnetic reconnection, by
itself possibly insufficient to reach very high energies, could perhaps
facilitate such a conditions. Further theoretical developments, and a better
source characterization, are needed to pinpoint the dominant acceleration
mechanism, which need not be one and the same in all sources.Comment: 7 pages, 1 figure, proceedings of the 13th ICATPP Conference on
Astroparticle, Particle, Space Physics and Detectors for Physics Applications
(Villa Olmo, Como 3-7 October 2011
Ground-States of Two Directed Polymers
Joint ground states of two directed polymers in a random medium are
investigated. Using exact min-cost flow optimization the true two-line
ground-state is compared with the single line ground state plus its first
excited state. It is found that these two-line configurations are (for almost
all disorder configurations) distinct implying that the true two-line
ground-state is non-separable, even with 'worst-possible' initial conditions.
The effective interaction energy between the two lines scales with the system
size with the scaling exponents 0.39 and 0.21 in 2D and 3D, respectively.Comment: 19 pages RevTeX, figures include
Central engines of Gamma Ray Bursts. Magnetic mechanism in the collapsar model
In this study we explore the magnetic mechanism of hypernovae and
relativistic jets of long duration gamma ray bursts within the collapsar
scenario. This is an extension of our earlier work [1]. We track the collapse
of massive rotating stars onto a rotating central black hole using axisymmetric
general relativistic magnetohydrodynamic code that utilizes a realistic
equation of state and takes into account the cooling associated with emission
of neutrinos and the energy losses due to dissociation of nuclei. The neutrino
heating is not included. We describe solutions with different black hole
rotation, mass accretion rate, and strength of progenitor's magnetic field.
Some of them exhibits strong explosions driven by Poynting-dominated jets with
power up to . These jets originate from the black
hole and powered via the Blandford-Znajek mechanism. A provisional criterion
for explosion is derived. A number of simulation movies can be downloaded from
http://www.maths.leeds.ac.uk/~serguei/research/movies/anim.htmlComment: 4 pages, 3 figures, GAMMA08 Heidelberg Symposiu
Exploring Particle Acceleration in Gamma-Ray Binaries
Binary systems can be powerful sources of non-thermal emission from radio to gamma rays. When the latter are detected, then these objects are known as gamma-ray binaries. In this work, we explore, in the context of gamma-ray binaries, different acceleration processes to estimate their efficiency: Fermi I, Fermi II, shear acceleration, the converter mechanism, and magnetic reconnection. We find that Fermi I acceleration in a mildly relativistic shock can provide, although marginally, the multi-10 TeV particles required to explain observations. Shear acceleration may be a complementary mechanism, giving particles the final boost to reach such a high energies. Fermi II acceleration may be too slow to account for the observed very high energy photons, but may be suitable to explain extended low-energy emission. The converter mechanism seems to require rather high Lorentz factors but cannot be discarded a priori. Standard relativistic shock acceleration requires a highly turbulent, weakly magnetized downstream medium; magnetic reconnection, by itself possibly insufficient to reach very high energies, could perhaps facilitate such a conditions. Further theoretical developments, and a better source characterization, are needed to pinpoint the dominant acceleration mechanism, which need not be one and the same in all sources
Clues to unveil the emitter in LS 5039: powerful jets vs colliding winds
LS 5039 is among the most interesting VHE sources in the Galaxy. Two
scenarios have been put forward to explain the observed TeV radiation: jets vs
pulsar winds. The source has been detected during the superior conjunction of
the compact object, when very large gamma-ray opacities are expected. In
addition, electromagnetic cascades, which may make the system more transparent
to gamma-rays, are hardly efficient for realistic magnetic fields in massive
star surroundings. All this makes unlikely the standard pulsar scenario for LS
5039, in which the emitter is the region located between the star and the
compact object, where the opacities are the largest. Otherwise, a jet-like flow
can transport energy to regions where the photon-photon absorption is much
lower and the TeV radiation is not so severely absorbed.Comment: 3 pages, 3 Figures, contribution to the "Fourth Heidelberg
International Symposium on High-Energy Gamma-Ray Astronomy 2008
Chaos in the Random Field Ising Model
The sensitivity of the random field Ising model to small random perturbations
of the quenched disorder is studied via exact ground states obtained with a
maximum-flow algorithm. In one and two space dimensions we find a mild form of
chaos, meaning that the overlap of the old, unperturbed ground state and the
new one is smaller than one, but extensive. In three dimensions the
rearrangements are marginal (concentrated in the well defined domain walls).
Implications for finite temperature variations and experiments are discussed.Comment: 4 pages RevTeX, 6 eps-figures include
Non-thermal Processes in Black-Hole-Jet Magnetospheres
The environs of supermassive black holes are among the universe's most
extreme phenomena. Understanding the physical processes occurring in the
vicinity of black holes may provide the key to answer a number of fundamental
astrophysical questions including the detectability of strong gravity effects,
the formation and propagation of relativistic jets, the origin of the highest
energy gamma-rays and cosmic-rays, and the nature and evolution of the central
engine in Active Galactic Nuclei (AGN). As a step towards this direction, this
paper reviews some of the progress achieved in the field based on observations
in the very high energy domain. It particularly focuses on non-thermal particle
acceleration and emission processes that may occur in the rotating
magnetospheres originating from accreting, supermassive black hole systems.
Topics covered include direct electric field acceleration in the black hole's
magnetosphere, ultra-high energy cosmic ray production, Blandford-Znajek
mechanism, centrifugal acceleration and magnetic reconnection, along with the
relevant efficiency constraints imposed by interactions with matter, radiation
and fields. By way of application, a detailed discussion of well-known sources
(Sgr A*; Cen A; M87; NGC1399) is presented.Comment: invited review for International Journal of Modern Physics D, 49
pages, 15 figures; minor typos corrected to match published versio
Probing the central black hole in M87 with gamma-rays
Recent high-sensitivity observation of the nearby radio galaxy M87 have
provided important insights into the central engine that drives the large-scale
outflows seen in radio, optical and X-rays. This review summarizes the
observational status achieved in the high energy (HE;<100 GeV) and very high
energy (VHE; >100 GeV) gamma-ray domains, and discusses the theoretical
progress in understanding the physical origin of this emission and its relation
to the activity of the central black hole.Comment: Invited compact review to be published in Modern Physics Letters A;
19 pages, 4 figure
Elastic lines on splayed columnar defects studied numerically
We investigate by exact optimization method properties of two- and
three-dimensional systems of elastic lines in presence of splayed columnar
disorder. The ground state of many lines is separable both in 2d and 3d leading
to a random walk -like roughening in 2d and ballistic behavior in 3d.
Furthermore, we find that in the case of pure splayed columnar disorder in
contrast to point disorder there is no entanglement transition in 3d.
Entanglement can be triggered by perturbing the pure splay system with point
defects.Comment: 9 pages, 11 figures. Accepted for publication in PR
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