6,369 research outputs found
The University of Michigan Centimeter-Band All Stokes Blazar Monitoring Program: Single-Dish Polarimetry as a Probe of Parsec-Scale Magnetic Fields
The University of Michigan 26-m paraboloid was dedicated to obtaining linear
polarization and total flux density observations of blazars from the mid-1960s
until June 2012 providing an unprecedented record tracking centimeter-band
variability over decades at 14.5, 8.0, and 4.8 GHz for both targeted objects
and members of flux-limited samples. In the mid-1970s through the mid-1980s,
and during the last decade of the program, observations were additionally
obtained of circular polarization for a small sample of radio-bright (S>5Jy),
active sources. Key program results include evidence supporting class-dependent
differences in the magnetic field geometry of BL Lac and QSO jets,
identification of linear polarization changes temporally associated with flux
outbursts supporting a shock-in-jet scenario, and determination of the spectral
evolution of the Stokes V amplitude and polarity for testing proposed models.
Recent radiative transfer modeling during large flares supports a jet scenario
with a kinetically-dominated, relativistic flow at parsec scales with embedded
turbulent magnetic fields and dynamically-weak ordered components which may be
helical; the circular polarization observations are consistent with
linear-to-circular mode conversion within this turbulent jet environment.Comment: 8 pages, 4 figures, Proceedings of the conference "Polarised Emission
from Astrophysical Jets", June 12-16, 2017, Ierapetra, Greece, eds. E.
Angelakis, M. Boettcher, and J.-L. Gome
Constraints on Blazar Jet Conditions During Gamma-Ray Flaring from Radiative Transfer Modeling
As part of a program to investigate jet flow conditions during GeV gamma-ray
flares detected by Fermi, we are using UMRAO multi-frequency, centimeter-band
total flux density and linear polarization monitoring observations to constrain
radiative transfer models incorporating propagating shocks orientated at an
arbitrary angle to the flow direction. We describe the characteristics of the
model, illustrate how the data are used to constrain the models, and present
results for three program sources with diverse characteristics: PKS 0420-01, OJ
287, and 1156+295. The modeling of the observed spectral behavior yields
information on the sense, strength and orientation of the shocks producing the
radio-band flaring; on the energy distribution of the radiating particles; and
on the observer's viewing angle with respect to the jet independent of VLBI
data. We present evidence that, while a random component dominates the jet
magnetic field, a distinguishing feature of those radio events with an
associated gamma-ray flare is the presence of a weak but non-negligible ordered
magnetic field component along the jet axis.Comment: 6 pages, 4 figures. To appear in the proceedings of "The Innermost
Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spai
A new method for estimating frequency-dependent core shifts in active galactic nucleus jets
We discuss the opacity in the core regions of active galactic nuclei observed
with Very Long Baseline Interferometry (VLBI), and describe a new method for
deriving the frequency-dependent shifts of the VLBI core from the
frequency-dependent time lags of flares observed with single-dish observations.
Application of the method to the core shifts of the quasar 3C 345 shows a very
good agreement between the core shifts directly measured from VLBI observations
and derived from flares in the total flux density using the proposed method.
The frequency-dependent time lags of flares can be used to derive physical
parameters of the jets, such as distance from the VLBI core to the base of the
jet and the magnetic fields in the core region. Our estimates for 3C 345
indicate core magnetic fields ~0.1 G and magnetic field at 1 pc ~0.4 G.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Opacity, variability and kinematics of AGN jets
Synchrotron self-absorption in active galactic nuclei (AGN) jets manifests
itself as a time delay between flares observed at high and low radio
frequencies. It is also responsible for the observing frequency dependent
change in size and position of the apparent base of the jet, aka the core shift
effect, detected with very long baseline interferometry (VLBI). We measure the
time delays and the core shifts in 11 radio-loud AGN to estimate the speed of
their jets without relying on multi-epoch VLBI kinematics analysis. The 158
GHz total flux density time lags are obtained using Gaussian process
regression, the core shift values are measured using VLBI observations and
adopted from the literature. A strong correlation is found between the apparent
core shift and the observed time delay. Our estimate of the jet speed is higher
than the apparent speed of the fastest VLBI components by the median
coefficient of 1.4. The coefficient ranges for individual sources from 0.5 to
20. We derive Doppler factors, Lorentz factors and viewing angles of the jets,
as well as the corresponding de-projected distance from the jet base to the
core. The results support evidence for acceleration of the jets with bulk
motion Lorentz factor on de-projected scales
of 0.5500 parsecs.Comment: Accepted by MNRAS; 11 pages, 11 figures, 3 table
Systems engineering considerations for operational support systems
Operations support as considered here is the infrastructure of people, procedures, facilities and systems that provide NASA with the capability to conduct space missions. This infrastructure involves most of the Centers but is concentrated principally at the Johnson Space Center, the Kennedy Space Center, the Goddard Space Flight Center, and the Jet Propulsion Laboratory. It includes mission training and planning, launch and recovery, mission control, tracking, communications, data retrieval and data processing
The chemical composition of normal stellar atmospheres
Spectroscopic analysis of stellar atmosphere chemical compositio
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