2,229 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
The Cross-Wavelet Transform and Analysis of Quasiperiodic Behavior in the Pearson-Readhead VLBI Survey Sources
We introduce an algorithm for applying a cross-wavelet transform to analysis
of quasiperiodic variations in a time-series, and introduce significance tests
for the technique. We apply a continuous wavelet transform and the
cross-wavelet algorithm to the Pearson-Readhead VLBI survey sources using data
obtained from the University of Michigan 26-m parabloid at observing
frequencies of 14.5, 8.0, and 4.8 GHz. Thirty of the sixty-two sources were
chosen to have sufficient data for analysis, having at least 100 data points
for a given time-series. Of these thirty sources, a little more than half
exhibited evidence for quasiperiodic behavior in at least one observing
frequency, with a mean characteristic period of 2.4 yr and standard deviation
of 1.3 yr. We find that out of the thirty sources, there were about four time
scales for every ten time series, and about half of those sources showing
quasiperiodic behavior repeated the behavior in at least one other observing
frequency.Comment: Revised version, accepted by ApJ. 17 pages, 13 figures, color figures
included as gifs, seperate from the text. The addition of statistical
significance tests has resulted in modifying the technique and results, but
the broad conclusion remain the same. A high resolution version may be found
at http://www.astro.lsa.umich.edu/obs/radiotel/prcwdata.htm
The extreme flare in III Zw 2: Evolution of a radio jet in a Seyfert galaxy
A very detailed monitoring of a radio flare in the Seyfert I galaxy III Zw 2
with the VLA and the VLBA is presented. The relative astrometry in the VLBA
observations was precise on a level of a few microarcseconds. Spectral and
spatial evolution of the source are closely linked and these observations
allowed us to study in great detail a textbook example of a synchrotron
self-absorbed jet. We observe a phase where the jet gets frustrated, without
expansion and no spectral evolution. Then the jet breaks free and starts to
expand with apparent superluminal motion. This expansion is accompanied by a
strong spectral evolution. The results are a good confirmation of synchrotron
theory and equipartition for jets.Comment: Astronomy & Astrophysics, accepted, 11 pages, 14 Figures, also
available at http://www.jive.nl/~brunthal/pub.shtm
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