7,023 research outputs found
Connection between inner jet kinematics and broadband flux variability in the BL Lac object S5 0716+714
We present a high-frequency very long baseline interferometry (VLBI)
kinematical study of the BL Lac object S5 0716+714 over the time period of
September 2008 to October 2010. The aim of the study is to investigate the
relation of the jet kinematics to the observed broadband flux variability. We
find significant non-radial motions in the jet outflow of the source. In the
radial direction, the highest measured apparent speed is \sim37 c, which is
exceptionally high, especially for a BL Lac object. Patterns in the jet flow
reveal a roughly stationary feature \sim0.15 mas downstream of the core. The
long-term fits to the component trajectories reveal acceleration in the sub-mas
region of the jet. The measured brightness temperature, T_{B}, follows a
continuous trend of decline with distance, T_B \propto
r_{jet}^{-(2.36\pm0.41)}, which suggests a gradient in Doppler factor along the
jet axis. Our analysis suggest that a moving disturbance (or a shock wave) from
the base of the jet produces the high-energy (optical to \gamma-ray) variations
upstream of the 7 mm core, and then later causes an outburst in the core.
Repetitive optical/\gamma-ray flares and the curved trajectories of the
associated components suggest that the shock front propagates along a bent
trajectory or helical path. Sharper \gamma-ray flares could be related to the
passage of moving disturbances through the stationary feature. Our analysis
suggests that the \gamma-ray and radio emission regions have different Doppler
factors.Comment: 12 pages, 8 figures, accepted for publication in A&
Dust sedimentation and self-sustained Kelvin-Helmholtz turbulence in protoplanetary disk mid-planes. I. Radially symmetric simulations
We perform numerical simulations of the Kelvin-Helmholtz instability in the
mid-plane of a protoplanetary disk. A two-dimensional corotating slice in the
azimuthal--vertical plane of the disk is considered where we include the
Coriolis force and the radial advection of the Keplerian rotation flow. Dust
grains, treated as individual particles, move under the influence of friction
with the gas, while the gas is treated as a compressible fluid. The friction
force from the dust grains on the gas leads to a vertical shear in the gas
rotation velocity. As the particles settle around the mid-plane due to gravity,
the shear increases, and eventually the flow becomes unstable to the
Kelvin-Helmholtz instability. The Kelvin-Helmholtz turbulence saturates when
the vertical settling of the dust is balanced by the turbulent diffusion away
from the mid-plane. The azimuthally averaged state of the self-sustained
Kelvin-Helmholtz turbulence is found to have a constant Richardson number in
the region around the mid-plane where the dust-to-gas ratio is significant.
Nevertheless the dust density has a strong non-axisymmetric component. We
identify a powerful clumping mechanism, caused by the dependence of the
rotation velocity of the dust grains on the dust-to-gas ratio, as the source of
the non-axisymmetry. Our simulations confirm recent findings that the critical
Richardson number for Kelvin-Helmholtz instability is around unity or larger,
rather than the classical value of 1/4Comment: Accepted for publication in ApJ. Some minor changes due to referee
report, most notably that the clumping mechanism has been identified as the
streaming instability of Youdin & Goodman (2005). Movies of the simulations
are still available at http://www.mpia.de/homes/johansen/research_en.ph
The role of the lateral prefrontal cortex and anterior cingulate in stimulus–response association reversals
Many complex tasks require us to flexibly switch between
behavioral rules, associations, and strategies. The prefrontal cerebral cortex is thought to be critical to the performance of such behaviors, although the relative contribution of different components of this structure and associated subcortical regions are not fully understood. We used functional magnetic resonance imaging to measure brain activity during a simple task which required repeated reversals of a rule linking a colored cue and a left/right motor response. Each trial comprised three discrete events separated by variable delay periods. A colored cue instructed which response was to be executed, followed by a go signal which told the subject to execute the response and a feedback instruction which indicated whether to ‘‘hold’’ or ‘‘f lip’’ the rule linking the colored cue and response. The design allowed us to determine which brain regions were recruited by the specific demands of
preparing a rule contingent motor response, executing such a
response, evaluating the significance of the feedback, and
reconfiguring stimulus–response (SR) associations. The results indicate that an increase in neural activity occurs within the anterior cingulate gyrus under conditions in which SR associations are labile. In contrast, lateral frontal regions are activated by unlikely/unexpected perceptual events regardless of their significance for behavior. A network of subcortical structures, including the mediodorsal nucleus of the thalamus and striatum were the only regions showing activity that was exclusively correlated with the neurocognitive demands of reversing SR associations. We conclude that lateral frontal regions act to evaluate the behavioral significance of perceptual
events, whereas medial frontal–thalamic circuits are involved in monitoring and reconfiguring SR associations when necessary
3 mm GMVA Observations of Total and Polarized Emission from Blazar and Radio Galaxy Core Regions
We present total and linearly polarized 3 mm Global mm-VLBI Array images of a
sample of blazars and radio galaxies from the VLBA-BU-BLAZAR 7 mm monitoring
program designed to probe the innermost regions of active galactic nuclei (AGN)
jets and locate the sites of gamma-ray emission observed by the Fermi-LAT. The
lower opacity at 3 mm and improved angular resolution, on the order of 50
microarcseconds, allow us to distinguish features in the jet not visible in the
7 mm VLBA data. We also compare two different methods used for the calibration
of instrumental polarisation and we analyze the resulting images for some of
the sources in the sample.Comment: Polarised Emission from Astrophysical Jets, June 12-16, 2017,
Ierapetra, Greec
Isospin splitting of the nucleon mean field
The isospin splitting of the nucleon mean field is derived from the Brueckner
theory extended to asymmetric nuclear matter. The Argonne V18 has been adopted
as bare interaction in combination with a microscopic three body force. The
isospin splitting of the effective mass is determined from the
Brueckner-Hartree-Fock self-energy: It is linear acording to the Lane ansatz
and such that for neutron-rich matter. The symmetry potential
is also determined and a comparison is made with the predictions of the
Dirac-Brueckner approach and the phenomenological interactions. The theoretical
predictions are also compared with the empirical parametrizations of neutron
and proton optical-model potentials based on the experimental nucleon-nucleus
scattering and the phenomenological ones adopted in transport-model simulations
of heavy-ion collisions. The direct contribution of the rearrangement term due
to three-body forces to the single particle potential and symmetry potential is
discussed.Comment: 8 pages, 10 figure
Sum rule of the correlation function
We discuss a sum rule satisfied by the correlation function of two particles
with small relative momenta. The sum rule, which results from the completeness
condition of the quantum states of the two particles, is first derived and then
we check how it works in practice. The sum rule is shown to be trivially
satisfied by free particle pair, and then there are considered three different
systems of interacting particles. We discuss a pair of neutron and proton in
the s-wave approximation and the case of the so-called hard spheres with the
phase shifts taken into account up to l=4. Finally, the Coulomb system of two
charged particles is analyzed.Comment: 18 pages, 18 figures, revised, to appear in Phys. Rev.
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