13,775 research outputs found
Physical properties of the gamma-ray binary LS 5039 through low and high frequency radio observations
We have studied in detail the 0.15-15 GHz radio spectrum of the gamma-ray
binary LS 5039 to look for a possible turnover and absorption mechanisms at low
frequencies, and to constrain the physical properties of its emission. We have
analysed two archival VLA monitorings, all the available archival GMRT data and
a coordinated quasi-simultaneous observational campaign conducted in 2013 with
GMRT and WSRT. The data show that the radio emission of LS 5039 is persistent
on day, week and year timescales, with a variability at all
frequencies, and no signature of orbital modulation. The obtained spectra
reveal a power-law shape with a curvature below 5 GHz and a turnover at
GHz, which can be reproduced by a one-zone model with synchrotron
self-absorption plus Razin effect. We obtain a coherent picture for a size of
the emitting region of , setting a magnetic field of
, an electron density of and a mass-loss rate of . These values imply a significant mixing of the stellar wind with the
relativistic plasma outflow from the compact companion. At particular epochs
the Razin effect is negligible, implying changes in the injection and the
electron density or magnetic field. The Razin effect is reported for first time
in a gamma-ray binary, giving further support to the young non-accreting pulsar
scenario.Comment: 16 pages, 9 figures, accepted for publication in MNRA
On the distribution of barriers in the spin glasses
We discuss a general formalism that allows study of transitions over barriers
in spin glasses with long-range interactions that contain large but finite
number, , of spins. We apply this formalism to the Sherrington-Kirkpatrick
model with finite and derive equations for the dynamical order parameters
which allow ''instanton'' solutions describing transitions over the barriers
separating metastable states. Specifically, we study these equations for a
glass state that was obtained in a slow cooling process ending a little below
and show that these equations allow ''instanton'' solutions which erase
the response of the glass to the perturbations applied during the slow cooling
process. The corresponding action of these solutions gives the energy of the
barriers, we find that it scales as where is the reduced
temperature.Comment: 8 pages, LaTex, 2 Postscript figure
Why don't clumps of cirrus dust gravitationally collapse?
We consider the Herschel-Planck infrared observations of presumed
condensations of interstellar material at a measured temperature of
approximately 14 K (Juvela et al., 2012), the triple point temperature of
hydrogen. The standard picture is challenged that the material is cirrus-like
clouds of ceramic dust responsible for Halo extinction of cosmological sources
(Finkbeiner, Davis, and Schlegel 1999). Why would such dust clouds not collapse
gravitationally to a point on a gravitational free-fall time scale of
years? Why do the particles not collide and stick together, as is fundamental
to the theory of planet formation (Blum 2004; Blum and Wurm, 2008) in pre-solar
accretion discs? Evidence from 3.3 m and UIB emissions as well as ERE
(extended red emission) data point to the dominance of PAH-type macromolecules
for cirrus dust, but such fractal dust will not spin in the manner of rigid
grains (Draine & Lazarian, 1998). IRAS dust clouds examined by Herschel-Planck
are easily understood as dark matter Proto-Globular-star-Cluster (PGC) clumps
of primordial gas planets, as predicted by Gibson (1996) and observed by Schild
(1996).Comment: 8 pages, 2 figures, Conference FQMT'1
Competition between hidden order and antiferromagnetism in URu_2Si_2 under uniaxial stress studied by neutron scattering
We have performed elastic neutron scattering experiments under uniaxial
stress sigma applied along the tetragonal [100], [110] and [001] directions for
the heavy electron compound URu2Si2. We found that antiferromagnetic (AF) order
with large moment is developed with sigma along the [100] and [110] directions.
If the order is assumed to be homogeneous, the staggered ordered moment mu_o
continuously increases from 0.02 mu_B (sigma=0) to 0.22 mu_B (0.25 GPa). The
rate of increase partial mu_o/partial sigma is ~ 1.0 mu_B/GPa, which is four
times larger than that for the hydrostatic pressure (partial mu_o/partial P sim
0.25 mu_B/GPa). Above 0.25 GPa, mu_o shows a tendency to saturate, similar to
the hydrostatic pressure behavior. For sigma||[001], mu_o shows only a slight
increase to 0.028 mu_B (sigma = 0.46 GPa) with a rate of ~ 0.02 mu_B/GPa,
indicating that the development of the AF state highly depends on the direction
of sigma. We have also found a clear hysteresis loop in the isothermal
mu_o(sigma) curve obtained for sigma||[110] under the zero-stress-cooled
condition at 1.4 K. This strongly suggests that the sigma-induced AF phase is
metastable, and separated from the "hidden order" phase by a first-order phase
transition. We discuss these experimental results on the basis of crystalline
strain effects and elastic energy calculations, and show that the c/a ratio
plays a key role in the competition between these two phases.Comment: 9 pages, 7 figures, to appear in Physical Review
GMRT Discovery of A Millisecond Pulsar in a Very Eccentric Binary System
We report the discovery of the binary millisecond pulsar J0514-4002A, which
is the first known pulsar in the globular cluster NGC 1851 and the first pulsar
discovered using the Giant Metrewave Radio Telescope (GMRT). The pulsar has a
rotational period of 4.99 ms, an orbital period of 18.8 days, and the most
eccentric pulsar orbit yet measured (e = 0.89). The companion has a minimum
mass of 0.9 M_sun and its nature is presently unclear. After accreting matter
from a low-mass companion star which spun it up to a (few) millisecond spin
period, the pulsar eventually exchanged the low-mass star for its more massive
present companion. This is exactly the same process that could form a system
containing a millisecond pulsar and a black hole; the discovery of NGC 1851A
demonstrates that such systems might exist in the Universe, provided that
stellar mass black holes exist in globular clusters.Comment: 12 pages (referee format), 3 figures, accepted for publication in
Astrophysical Journal Letter
Exchange Bias Effect in Au-Fe3O4 Nanocomposites
We report exchange bias (EB) effect in the Au-Fe3O4 composite nanoparticle
system, where one or more Fe3O4 nanoparticles are attached to an Au seed
particle forming dimer and cluster morphologies, with the clusters showing much
stronger EB in comparison with the dimers. The EB effect develops due to the
presence of stress in the Au-Fe3O4 interface which leads to the generation of
highly disordered, anisotropic surface spins in the Fe3O4 particle. The EB
effect is lost with the removal of the interfacial stress. Our atomistic
Monte-Carlo studies are in excellent agreement with the experimental results.
These results show a new path towards tuning EB in nanostructures, namely
controllably creating interfacial stress, and open up the possibility of tuning
the anisotropic properties of biocompatible nanoparticles via a controllable
exchange coupling mechanism.Comment: 28 pages, 6 figures, submitted to Nanotechnolog
New insights into the evolution of the FR I radio galaxy 3C 270 (NGC 4261) from VLA and GMRT radio observations
We present Giant Metrewave Radio Telescope (GMRT) 240 MHz observations of the
nearby luminous FR I radio source 3C 270, in the group-central elliptical NGC
4261. Combining these data with reprocessed Very Large Array (VLA) 1.55 and 4.8
GHz observations, we produce spectral index maps that reveal a constant
spectral index along the jets and a gradual steepening from the ends of the
jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted
to the integrated spectrum of the source gives an asymptotic low-frequency
index of , while JP models fitted to the
observed spectral index trend along the lobes allow us to estimate radiative
ages of Myr and Myr for the west and east lobes respectively.
Our age estimates are a factor of two lower than the 75-Myr upper limit derived
from X-ray data (O'Sullivan et al. 2011). We find unlikely the scenario of an
early supersonic phase in which the lobe expanded into the ISM at approximately
Mach 6 (3500 km s), and suggest that either the source underwent
multiple AGN outbursts with possible large changes in jet power, or possibly
that the source age that we find is due to a backflow that transports young
electrons from the jet tips through the lobes toward the nucleus relatively
quickly. We calculate that in the lobes the energy ratio of non-radiating to
radiating particles is indicating significant gas entrainment. If
the lobes are in pressure balance with their surroundings, the total energy
required to heat the entrained material is erg, 40% of the
total enthalpy of the lobes.Comment: 14 pages, 11 figures, 8 tables. Accepted for publication by MNRAS.
Revised throughout in response to referee's comment
A rapid and gentle method for the salt extraction of chromatin core histones H2A, H2B, H3 and H4 from rat liver nuclei
A complex of histones H2A, H2B, H3 and H4 has been isolated from purified rat liver nuclei by a method which is both gentle and rapid. Nuclei were homogenised in 0.25 I sucrose and the residual nuclear material obtained after centrifligation was adsorbed on calcium phosphate gel. After removing histone H1 from the adsorbed material by washing with 1M NaCl in 25 mM sodium phosphate buffer, pH 6.0, histones H2A, H2B, H3 and H4 were eluted together, with 2 I NaCl in 25 mM sodium phosphate buffer, pH 7.0. The core histones so obtained migrated as a single sharp band on polyacrylamide gel electrophoresis under non-denaturing conditions. Fractionation of the freshly prepared core histones on a Sephadex G-100 column yielded two major protein peaks. The peak having the larger elution volume contained histones H2A and H2B in equal amounts while the peak with the smaller elution volume contained all the four histones. Histones H3 and H4 were present in larger proportions in the second peak
Quantum Glass Transition in a Periodic Long-Range Josephson Array
We show that the ground state of the periodic long range Josephson array
frustrated by magnetic field is a glass for a sufficiently large Josephson
energies despite the absence of a quenched disorder. Like superconductors, this
glass state has non-zero phase stiffness and Meissner response; for smaller
Josephson energies the glass "melts" and the ground state loses the phase
stiffness and becomes insulating. We find the critical scaling behavior near
this quantum phase transition: the excitation gap vanishes as (J-J_c)^2, the
frequency-dependent magnetic susceptibility behaves as \chi(\omega) ~
\sqrt{\omega}\ln{\omega}.Comment: 9 pages, Latex, 3 figures in separated eps-file
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