98 research outputs found
Quasi-long range order in the random anisotropy Heisenberg model
The large distance behaviors of the random field and random anisotropy
Heisenberg models are studied with the functional renormalization group in
dimensions. The random anisotropy model is found to have a phase
with the infinite correlation radius at low temperatures and weak disorder. The
correlation function of the magnetization obeys a power law . The
magnetic susceptibility diverges at low fields as . In the random field model the correlation radius is found
to be finite at the arbitrarily weak disorder.Comment: 4 pages, REVTe
FIRST-based survey of Compact Steep Spectrum sources, II. MERLIN and VLA observations of Medium-sized Symmetric Objects
A new sample of candidate Compact Steep Spectrum (CSS) sources that are much
weaker than the CSS source prototypes has been selected from the VLA FIRST
catalogue. MERLIN `snapshot' observations of the sources at 5 GHz indicate that
six of them have an FR II-like morphology, but are not edge-brightened as is
normal for Medium-sized Symmetric Objects (MSOs) and FR IIs. Further
observations of these six sources with the VLA at 4.9 GHz and MERLIN at 1.7
GHz, as well as subsequent full-track observations with MERLIN at 5 GHz of what
appeared to be the two sources of greatest interest are presented. The results
are discussed with reference to the established evolutionary model of CSS
sources being young but in which not all of them evolve to become old objects
with extended radio structures. A lack of stable fuelling in some of them may
result in an early transition to a so-called coasting phase so that they fade
away instead of growing to become large-scale objects. It is possible that one
of the six sources (1542+323) could be labelled as a prematurely `dying' MSO or
a `fader'.Comment: 13 pages, matches the version printed in Astronomy & Astrophysic
Haldane Gapped Spin Chains: Exact Low Temperature Expansions of Correlation Functions
We study both the static and dynamic properties of gapped, one-dimensional,
Heisenberg, anti-ferromagnetic, spin chains at finite temperature through an
analysis of the O(3) non-linear sigma model. Exploiting the integrability of
this theory, we are able to compute an exact low temperature expansion of the
finite temperature correlators. We do so using a truncated `form-factor'
expansion and so provide evidence that this technique can be successfully
extended to finite temperature. As a direct test, we compute the static
zero-field susceptibility and obtain an exact match to the susceptibility
derived from the low temperature expansion of the exact free energy. We also
study transport properties, computing both the spin conductance and the
NMR-relaxation rate, 1/T_1. We find these quantities to show ballistic
behaviour. In particular, the computed spin conductance exhibits a non-zero
Drude weight at finite temperature and zero applied field. The physics thus
described differs from the spin diffusion reported by Takigawa et al. from
experiments on the Haldane gap material, AgVP_2S_6.Comment: 51 pages, 5 figure
Detection of diffuse TeV gamma-ray emission from the nearby starburst galaxy NGC 253
We report the TeV gamma-ray observations of the nearby normal spiral galaxy
NGC 253. At a distance of 2.5 Mpc, NGC 253 is one of the nearest
starburst galaxies. This relative closeness, coupled with the high star
formation rate in the galaxy, make it a good candidate TeV gamma-ray source.
Observations were carried out in 2000 and 2001 with the CANGAROO-II 10 m
imaging atmospheric Cerenkov telescope. TeV gamma-ray emission is detected at
the level with a flux of at energies 0.5 TeV. The data indicate that the
emission region is broader than the point spread function of our telescope.Comment: 4 pages, double colomn, 3 figures, aa.cl
Quasi-long-range order in the random anisotropy Heisenberg model: functional renormalization group in 4-\epsilon dimensions
The large distance behaviors of the random field and random anisotropy O(N)
models are studied with the functional renormalization group in 4-\epsilon
dimensions. The random anisotropy Heisenberg (N=3) model is found to have a
phase with the infinite correlation radius at low temperatures and weak
disorder. The correlation function of the magnetization obeys a power law <
m(x) m(y) >\sim |x-y|^{-0.62\epsilon}. The magnetic susceptibility diverges at
low fields as \chi \sim H^{-1+0.15\epsilon}. In the random field O(N) model the
correlation radius is found to be finite at the arbitrarily weak disorder for
any N>3. The random field case is studied with a new simple method, based on a
rigorous inequality. This approach allows one to avoid the integration of the
functional renormalization group equations.Comment: 12 pages, RevTeX; a minor change in the list of reference
Evidence of TeV gamma-ray emission from the nearby starburst galaxy NGC 253
TeV gamma-rays were recently detected from the nearby normal spiral galaxy
NGC 253 (Itoh et al., 2002). Observations to detect the Cherenkov light images
initiated by gamma-rays from the direction of NGC 253 were carried out in 2000
and 2001 over a total period of 150 hours. The orientation of images in
gamma-ray--like events is not consistent with emission from a point source, and
the emission region corresponds to a size greater than 10 kpc in radius. Here,
detailed descriptions of the analysis procedures and techniques are given.Comment: 16 pages, 27 figures, aa.cl
Weak reaction freeze-out constraints on primordial magnetic fields
We explore constraints on the strength of the primordial magnetic field based
upon the weak reaction freeze-out in the early universe. We find that limits on
the strength of the magnetic field found in other works are recovered simply by
examining the temperature at which the rate of weak reactions drops below the
rate of universal expansion ( H). The temperature for which the
ratio at freeze-out leads to acceptable helium production implies limits
on the magnetic field. This simplifies the application of magnetic fields to
other cosmological variants of the standard big-bang. As an illustration we
also consider effects of neutrino degeneracy on the allowed limits to the
primordial magnetic field.Comment: Submitted to Phys. Rev. D., 6 pages, 2 figure
Nonthermal Emission from Star-Forming Galaxies
The detections of high-energy gamma-ray emission from the nearby starburst
galaxies M82 & NGC253, and other local group galaxies, broaden our knowledge of
star-driven nonthermal processes and phenomena in non-AGN star-forming
galaxies. We review basic aspects of the related processes and their modeling
in starburst galaxies. Since these processes involve both energetic electrons
and protons accelerated by SN shocks, their respective radiative yields can be
used to explore the SN-particle-radiation connection. Specifically, the
relation between SN activity, energetic particles, and their radiative yields,
is assessed through respective measures of the particle energy density in
several star-forming galaxies. The deduced energy densities range from O(0.1)
eV/cm^3 in very quiet environments to O(100) eV/cm^3 in regions with very high
star-formation rates.Comment: 17 pages, 5 figures, to be published in Astrophysics and Space
Science Proceeding
A search for steep spectrum radio relics and halos with the GMRT
Context: Diffuse radio emission, in the form of radio halos and relics,
traces regions in clusters with shocks or turbulence, probably produced by
cluster mergers. Some models of diffuse radio emission in clusters indicate
that virtually all clusters should contain diffuse radio sources with a steep
spectrum. External accretion shocks associated with filamentary structures of
galaxies could also accelerate electrons to relativistic energies and hence
produce diffuse synchrotron emitting regions. Here we report on Giant Metrewave
Radio Telescope (GMRT) observations of a sample of steep spectrum sources from
the 74 MHz VLSS survey. These sources are diffuse and not associated with
nearby galaxies.
Aims: The main aim of the observations is to search for diffuse radio
emission associated with galaxy clusters or the cosmic web.
Methods: We carried out GMRT 610 MHz continuum observations of unidentified
diffuse steep spectrum sources.
Results: We have constructed a sample of diffuse steep spectrum sources,
selected from the 74 MHz VLSS survey. We identified eight diffuse radio sources
probably all located in clusters. We found five radio relics, one cluster with
a giant radio halo and a radio relic, and one radio mini-halo. By complementing
our observations with measurements from the literature we find correlations
between the physical size of relics and the spectral index, in the sense that
smaller relics have steeper spectra. Furthermore, larger relics are mostly
located in the outskirts of clusters while smaller relics are located closer to
the cluster center.Comment: 20 pages, 26 figures, accepted for publication in A&A on October 7,
200
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
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