1,509 research outputs found
Jet opening angles and gamma-ray brightness of AGN
We have investigated the differences in apparent opening angles between the
parsec-scale jets of the active galactic nuclei (AGN) detected by the Fermi
Large Area Telescope (LAT) during its first three months of operations and
those of non-LAT-detected AGN. We used 15.4 GHz VLBA observations of sources
from the 2 cm VLBA MOJAVE program, a subset of which comprise the statistically
complete flux density limited MOJAVE sample. We determined the apparent opening
angles by analyzing transverse jet profiles from the data in the image plane
and by applying a model fitting technique to the data in the (u,v) plane. Both
methods provided comparable opening angle estimates. The apparent opening
angles of gamma-ray bright blazars are preferentially larger than those of
gamma-ray weak sources. At the same time, we have found the two groups to have
similar intrinsic opening angle distributions, based on a smaller subset of
sources. This suggests that the jets in gamma-ray bright AGN are oriented at
preferentially smaller angles to the line of sight resulting in a stronger
relativistic beaming. The intrinsic jet opening angle and bulk flow Lorentz
factor are found to be inversely proportional, as predicted by standard models
of compact relativistic jets. If a gas dynamical jet acceleration model is
assumed, the ratio of the initial pressure of the plasma in the core region P_0
to the external pressure P_ext lies within the range 1.1 to 34.6, with a best
fit estimate of P_0/P_ext=2.Comment: 4 pages, 3 figures; accepted for publication in the A&A Letters;
table in electronic form can be extracted from the preprint sourc
The Relation between Radio Polarization and Gamma-ray Emission in AGN Jets
We have compared the parsec-scale jet linear polarization properties of the
Fermi LAT-detected and non-detected sources in the complete
flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE
sources, 30 were detected by the LAT during its first three months of
operation. We find that during the era since the launch of Fermi, the
unresolved core components of the LAT-detected jets have significantly higher
median fractional polarization at 15 GHz. This complements our previous
findings that these LAT sources have higher apparent jet speeds, brightness
temperatures and Doppler factors, and are preferentially found in higher
activity states.Comment: 6 pages, 3 figures, to appear in the proceedings of "High Energy
Phenomena In Relativistic Outflows II" (Buenos Aires, Argentina, October
26-30, 2009) International Journal of Modern Physics
Magnetoelastic nature of solid oxygen epsilon-phase structure
For a long time a crystal structure of high-pressure epsilon-phase of solid
oxygen was a mistery. Basing on the results of recent experiments that have
solved this riddle we show that the magnetic and crystal structure of
epsilon-phase can be explained by strong exchange interactions of
antiferromagnetic nature. The singlet state implemented on quaters of O2
molecules has the minimal exchange energy if compared to other possible singlet
states (dimers, trimers). Magnetoelastic forces that arise from the spatial
dependence of the exchange integral give rise to transformation of 4(O2)
rhombuses into the almost regular quadrates. Antiferromagnetic character of the
exchange interactions stabilizes distortion of crystal lattice in epsilon-phase
and impedes such a distortion in long-range alpha- and delta-phases.Comment: 11 pages, 4 figures, Changes: corrected typos, reference to the
recent paper is adde
Magnetic field dependence of antiferromagnetic resonance in NiO
We report on measurements of magnetic field and temperature dependence of antiferromagnetic resonances in the prototypical antiferromagnet NiO. The frequencies of the magnetic resonances in the vicinity of 1 THz have been determined in the time-domain via time-resolved Faraday measurements after selective excitation by narrow-band superradiant terahertz (THz) pulses at temperatures down to 3 K and in magnetic fields up to 10 T. The measurements reveal two antiferromagnetic resonance modes, which can be distinguished by their characteristic magnetic field dependencies. The nature of the two modes is discussed by comparison to an eight-sublattice antiferromagnetic model, which includes superexchange between the next-nearest-neighbor Ni spins, magnetic dipolar interactions, cubic magneto-crystalline anisotropy, and Zeeman interaction with the external magnetic field. Our study indicates that a two-sublattice model is insufficient for the description of spin dynamics in NiO, while the magnetic-dipolar interactions and magneto-crystalline anisotropy play important roles
Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine Scale Structure
We have used VLBA fringe visibility data obtained at 15 GHz to examine the
compact structure in 250 extragalactic radio sources. For 171 sources in our
sample, more than half of the total flux density seen by the VLBA remains
unresolved on the longest baselines. There are 163 sources in our list with a
median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest
baselines. For about 60% of the sources, we have at least one observation in
which the core component appears unresolved (generally smaller than 0.05 mas)
in one direction, usually transverse to the direction into which the jet
extends. BL Lacs are on average more compact than quasars, while active
galaxies are on average less compact. Also, in an active galaxy the
sub-milliarcsecond core component tends to be less dominant. IDV sources
typically have a more compact, more core-dominated structure on
sub-milliarcsecond scales than non-IDV sources, and sources with a greater
amplitude of intra-day variations tend to have a greater unresolved VLBA flux
density. The objects known to be GeV gamma-ray loud appear to have a more
compact VLBA structure than the other sources in our sample. This suggests that
the mechanisms for the production of gamma-ray emission and for the generation
of compact radio synchrotron emitting features are related. The brightness
temperature estimates and lower limits for the cores in our sample typically
range between 10^11 and 10^13 K, but they extend up to 5x10^13 K, apparently in
excess of the equipartition brightness temperature, or the inverse Compton
limit for stationary synchrotron sources. The largest component speeds are
observed in radio sources with high observed brightness temperatures, as would
be expected from relativistic beaming (abridged).Comment: 31 pages, 13 figures, 4 tables, accepted for publication in the
Astronomical Journal; minor changes to the text are mad
Nonlinear acoustic waves in channels with variable cross sections
The point symmetry group is studied for the generalized Webster-type equation
describing non-linear acoustic waves in lossy channels with variable cross
sections. It is shown that, for certain types of cross section profiles, the
admitted symmetry group is extended and the invariant solutions corresponding
to these profiles are obtained. Approximate analytic solutions to the
generalized Webster equation are derived for channels with smoothly varying
cross sections and arbitrary initial conditions.Comment: Revtex4, 10 pages, 2 figure. This is an enlarged contribution to
Acoustical Physics, 2012, v.58, No.3, p.269-276 with modest stylistic
corrections introduced mainly in the Introduction and References. Several
typos were also correcte
Survey of Instantaneous 1-22 GHz Spectra of 550 Compact Extragalactic Objects with Declinations from -30deg to +43deg
We present observational results for extragalactic radio sources with
milliarcsecond components, obtained with the 600 meter ring radio telescope
RATAN-600 from 1st to 22nd December, 1997. For each source, a six frequency
broad band radio spectrum was obtained by observing simultaneously with an
accuracy up to a minute at 1.4, 2.7, 3.9, 7.7, 13 and 31 cm. The observed list
is selected from Preston et al. (1985) VLBI survey and contains all the sources
in the declinations between -30deg and +43deg with a correlated flux density
exceeding 0.1 Jy at 13 cm. The sample includes the majority of sources to be
studied in the current VSOP survey and the future RadioAstron Space VLBI
mission.Comment: 26 pages, 4 figures, 5 tables, published in the A&AS; figure 4 with
the broad-band spectra plots is included in the preprint; tables 1 and 5, in
electronic form, as well as the ReadMe file can be extracted from the
preprint sourc
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