11,674 research outputs found
Crossovers in the Two Dimensional Ising Spin Glass with ferromagnetic next-nearest-neighbor interactions
By means of extensive computer simulations we analyze in detail the two
dimensional Ising spin glass with ferromagnetic next-nearest-neighbor
interactions. We found a crossover from ferromagnetic to ``spin glass'' like
order both from numerical simulations and analytical arguments. We also present
evidences of a second crossover from the ``spin glass'' behavior to a
paramagnetic phase for the largest volume studied.Comment: 19 pages with 9 postscript figures also available at
http://chimera.roma1.infn.it/index_papers_complex.html. Some changes in
captions of figures 1 and
Domain walls and chaos in the disordered SOS model
Domain walls, optimal droplets and disorder chaos at zero temperature are
studied numerically for the solid-on-solid model on a random substrate. It is
shown that the ensemble of random curves represented by the domain walls obeys
Schramm's left passage formula with kappa=4 whereas their fractal dimension is
d_s=1.25, and therefore is NOT described by "Stochastic-Loewner-Evolution"
(SLE). Optimal droplets with a lateral size between L and 2L have the same
fractal dimension as domain walls but an energy that saturates at a value of
order O(1) for L->infinity such that arbitrarily large excitations exist which
cost only a small amount of energy. Finally it is demonstrated that the
sensitivity of the ground state to small changes of order delta in the disorder
is subtle: beyond a cross-over length scale L_delta ~ 1/delta the correlations
of the perturbed ground state with the unperturbed ground state, rescaled by
the roughness, are suppressed and approach zero logarithmically.Comment: 23 pages, 11 figure
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
Multiwavelength Signatures of Cosmic Ray Acceleration by Young Supernova Remnants
An overview is given of multiwavelength observations of young supernova
remnants, with a focus on the observational signatures of efficient cosmic ray
acceleration. Some of the effects that may be attributed to efficient cosmic
ray acceleration are the radial magnetic fields in young supernova remnants,
magnetic field amplification as determined with X-ray imaging spectroscopy,
evidence for large post-shock compression factors, and low plasma temperatures,
as measured with high resolution optical/UV/X-ray spectroscopy. Special
emphasis is given to spectroscopy of post-shock plasma's, which offers an
opportunity to directly measure the post-shock temperature. In the presence of
efficient cosmic ray acceleration the post-shock temperatures are expected to
be lower than according to standard equations for a strong shock. For a number
of supernova remnants this seems indeed to be the case.Comment: Invited review, to appear in the proceedings of "4th Heidelberg
International Symposium on High Energy Gamma-Ray Astronomy 2008
The Comparison of the Swift Gamma-Ray Bursts With and Without Measured Redshifts
Gamma-ray bursts, detected by the Swift satellite, are separated into two
samples: the bursts with and without determined redshifts. These two samples
are compared by the standard Student t-test and F-test. We have compared the
dispersions and the mean values of the durations, peak fluxes and fluences in
order to find any differences among these two samples. No essential differences
were found.Comment: Published in the Proceedings of the 4th Heidelberg International
Symposium on High Energy Gamma-Ray Astronomy, 200
Hadronic Production of Gamma Rays and Starburst Galaxies
The Milky Way has been estabished to emit gamma rays. These gamma rays are
presumably dominated by decays of neutral pions, although inverse Compton
scatterings and bremsstrahlung also contribute. It is plausible that other
galaxies can be diffuse sources of gamma rays in a similar manner. Starburst
galaxies are particularly interesting to study as they are expected to have
much higher cosmic-ray fluxes and interstellar matter densities. The neutral
pions are created in cosmic-ray interactions with interstellar matter.
Presented here is an overview of the recent work by Karlsson and co-workers on
proton-proton interactions and the resulting secondary particle inclusive cross
sections and angular distributions. This model can be used to calculated the
component of the gamma-ray yield and spectrum from a starburst
galaxy. The yield is expected to increase significantly (30% to 50%) and the
spectrum to be harder than the incident proton spectrum.Comment: 4 pages, 4 figures, submitted for the Proceedings of the 4th
Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, July
7-11, 2008, in Heidelberg, German
Galactic and Extragalactic Magnetic Fields
The strength of the total magnetic field in our Milky Way from radio Zeeman
and synchrotron measurements is about 6 muG near the Sun and several mG in
dense clouds, pulsar wind nebulae, and filaments near the Galactic Center.
Diffuse polarized radio emission and Faraday rotation of the polarized emission
from pulsars and background sources show many small-scale magnetic features,
but the overall field structure in our Galaxy is still under debate. -- Radio
synchrotron observations of nearby galaxies reveal dynamically important
magnetic fields of 10-30 muG total strength in the spiral arms. Fields with
random orientations are concentrated in spiral arms, while ordered fields
(observed in radio polarization) are strongest in interarm regions and follow
the orientation of the adjacent gas spiral arms. Faraday rotation of the
diffuse polarized radio emission from the disks of spiral galaxies sometimes
reveals large-scale patterns which are signatures of coherent fields generated
by dynamos, but in most galaxies the field structure is more complicated. --
Strong magnetic fields are also observed in radio halos around edge-on
galaxies, out to large distances from the plane. The synchrotron scaleheight of
radio halos allows to measure the mean outflow velocity of the cosmic-ray
electrons. The ordered halo fields mostly form an X-shaped pattern, but no
large-scale pattern is seen in the Faraday rotation data. Diffuse polarized
radio emission in the outer disks and halos is an excellent tracer of galaxy
interactions and ram pressure by the intergalactic medium. -- Intracluster gas
can also be significantly magnetized and highly polarized due to shocks or
cluster mergers.Comment: 14 pages, 14 figures. To be published in "High Energy Gamma-Ray
Astronomy", eds. F.A. Aharonian, W. Hofmann, and F.M. Rieger, AIP Conf. Proc.
Updated and added references 28/11/2008; typo corrected and references
updated 07/01/2009; typos corrected 12/01/200
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
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