344 research outputs found
Stimulated wave of polarization in spin chains
Stimulated wave of polarization, triggered by a flip of a single spin,
presents a simple model of quantum amplification. Previously, it has been found
that such wave can be excited in a 1D Ising chain with nearest-neighbor
interactions, irradiated by a weak resonant transverse field. Here we explore
models with more realistic Hamiltonians, in particular, with natural
dipole-dipole interactions. Results of simulations for 1D spin chains and rings
with up to nine spins are presented.Comment: 15 pages, 5 figure
The TeV blazar Markarian 421 at the highest spatial resolution
We report the results obtained for the AGN Markarian 421 by model-fitting the
data in the visibility plane, studing the proper motion of jet components, the
light curve, and the spectral index of the jet features. We compare the radio
data with optical light curves obtained at the Steward Observatory, considering
also the optical polarization information. Mrk 421 has a bright nucleus and a
one-sided jet extending towards the north-west for a few parsecs. The
model-fits show that brightness distribution is well described using 6-7
circular Gaussian components, four of which are reliably identified at all
epochs; all components are effectively stationary except for component D, at
~0.4 mas from the core, whose motion is however subluminal. Analysis of the
light curve shows two different states, with the source being brighter and more
variable in the first half of 2011 than in the second half. The highest flux
density is reached in February. A comparison with the optical data reveals an
increase of the V magnitude and of the fractional polarization simultaneous
with the enhancement of the radio activity.Comment: 11 pages, 6 figure
Parsec scale polarization properties of the TeV blazar Markarian 421
In this work we present a polarization analysis at radio frequencies of
Markarian 421, one of the closest (z=0.03) TeV blazars. The observations were
obtained, both in total and in polarized intensity, with the Very Long Baseline
Array (VLBA) at 15, 24, and 43 GHz throughout 2011, with one observation per
month (for a total of twelve epochs). We investigate the magnetic field
topology and the polarization structure on parsec scale and their evolution
with time. We detect polarized emission both in the core and in the jet region,
and it varies with frequency, location and time. In the core region we measure
a mean fractional polarization of about 1-2%, with a peak of about 4% in March
at 43 GHz; the polarization angle is almost stable at 43 GHz, but it shows
significant variability in the range 114-173 deg at 15 GHz. In the jet region
the polarization properties show a more stable behavior; the fractional
polarization is about 16% and the polarization angle is nearly perpendicular to
the jet axis. The higher EVPA variability observed at 15 GHz is due both to a
variable Faraday rotation effect and to opacity. The residual variability
observed in the intrinsic polarization angle, together with the low degree of
polarization in the core region, could be explained with the presence of a
blend of variable cross-polarized subcomponents within the beam.Comment: 6 pages, 4 figures, proceedings of the 12th European VLBI Network
Symposium and Users Meeting (7-10 October 2014, Cagliari, Italy
Effect of pre-heating and post-weld heat treatment on structure and mechanical properties of laser beam-welded Ti₂AlNb-based joints
The effect of pre-heating and post-weld heat treatment on microstructure and mechanical properties of laserwelded joints in a Ti-23Al-23Nb-1.4V-0.8Zr-0.4Mo-0.4Si (at.%) alloy was studied. Laser beam-welding was carried out at room temperature as well as after pre-heating up to 800◦С. The post-weld heat treatment comprised either air quenching from 920◦С followed by aging at 800◦С or only aging at 800◦
Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron II. Observations of 3C 273 at minimum activity
RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R
satellite, launched in 2011, performing Space Very Long Baseline Interferometry
(SVLBI) observations supported by a global ground array of radio telescopes.
With an apogee of about 350 000 km, it is offering for the first time the
possibility to perform {\mu}as-resolution imaging in the cm-band. We present
observations at 22 GHz of 3C 273, performed in 2014, designed to reach a
maximum baseline of approximately nine Earth diameters. Reaching an angular
resolution of 0.3 mas, we study a particularly low-activity state of the
source, and estimate the nuclear region brightness temperature, comparing with
the extreme one detected one year before during the RadioAstron early science
period. We also make use of the VLBA-BU-BLAZAR survey data, at 43 GHz, to study
the kinematics of the jet in a 1.5-year time window. We find that the nuclear
brightness temperature is two orders of magnitude lower than the exceptionally
high value detected in 2013 with RadioAstron at the same frequency (1.4x10^13
K, source-frame), and even one order of magnitude lower than the equipartition
value. The kinematics analysis at 43 GHz shows that a new component was ejected
2 months after the 2013 epoch, visible also in our 22 GHz map presented here.
Consequently this was located upstream of the core during the brightness
temperature peak. These observations confirm that the previously detected
extreme brightness temperature in 3C 273, exceeding the inverse Compton limit,
is a short-lived phenomenon caused by a temporary departure from equipartition.
Thus, the availability of interferometric baselines capable of providing
{\mu}as angular resolution does not systematically imply measured brightness
temperatures over the known physical limits for astrophysical sources.Comment: Accepted for publication in A&
Effect of hot rolling on the microstructure and mechanical properties of a Ti-15Mo/TiB metal-matrix composite
A Ti-15Mo/TiB metal matrix composite was produced by the spark plasma sintering process at 1400 °C using a Ti-14.25 wt.% Mo-5 wt.% TiB2 powder mixture. The microstructure and mechanical properties of the composite were studied after non-isothermal rolling of specimens heated to 1000 °C to a thickness strain of ~0.
Effect of hot rolling on the microstructure and mechanical properties of a Ti-15Mo/TiB metal-matrix composite
A Ti-15Mo/TiB metal matrix composite was produced by the spark plasma sintering process at 1400°C using a Ti-14.25 wt.% Mo-5 wt.% TiB2 powder mixture. The microstructure and mechanical properties of the composite were studied after non-isothermal rolling of specimens heated to 1000°C to a thickness strain of ~0.7. Transmission and scanning electron microscopy, as well as X-ray analysis were used for microstructure examinatio
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