3,288 research outputs found
Probing the Circular Polarization of Relativistic Jets on VLBI Scales
High resolution studies of circular polarization allow us see where it arises
in a jet, study its local fractional level and spectrum, and compare these
results to local measures of linear polarization and Faraday rotation. Here we
not only review past results from Very Long Baseline Array (VLBA) circular
polarization studies, but we also present preliminary new results on two
quasars. In the core of PKS 0607-157, we find strong circular polarization at 8
GHz and much weaker levels at 15 GHz. Combined with the linear polarization
data, we favor a simple model where the circular is produced by Faraday
conversion driven by a small amount of Faraday rotation. In the core of 3C345,
we find strong circular polarization at 15 GHz in a component with distinct
linear polarization. This core component is optically thick at 8 GHz, where we
detect no circular polarization. With opposite trends in frequency for PKS
0607-157 and 3C345, it seems clear that local conditions in a jet can have a
strong effect on circular polarization and need to be taken into account when
studying inhomogeneous objects with multi-frequency observations.Comment: To appear in the proceedings of "Circular Polarization of
Relativistic Jet Sources", eds R. P. Fender and J.-P. Macquart, in
Astrophysics and Space Science. 11 pgs, 2 fig
Theoretical Models for Producing Circularly Polarized Radiation in Extragalactic Radio Sources
We discuss the production of circular polarization in compact radio sources
both by the intrinsic mechanism and by Faraday conversion. We pay particular
attention to the magnetic field structure, considering partially ordered fiel
ds and Laing sheets, and distinguishing between uniform and unidirectional
fields. (The latter can be constrained b y flux conservation arguments.) In
most cases, Faraday conversion is the more important mechanism. Conversion
opera tes on Stokes U, which can be generated by internal Faraday rotation, or
by magnetic field fluctuations, which can therefore produce circular
polarization even in a pure pair plasma. We also show that the spectrum of
circular pola rization in an inhomogeneous jet can be quite different from that
in a uniform source, being flat or even inverted.Comment: To appear in the proceedings of "Circular Polarization of
Relativistic Jet Sources", eds R. P. Fender and J.-P. Macquart, in
Astrophysics and Space Science. 12 pgs, 0 fig
VLA Observations of a Complete Sample of Radio Loud Quasars between redshifts 2.5 and 5.28: I. high-redshift sample summary and the radio images
We present high resolution (arcsecond or better) observations made with the
Karl G. Jansky Very Large Array of 123 radio-loud quasars with redshifts in the
range that form a complete flux limited sample ( mJy at 1.4 GHz or 5 GHz). Where possible, we used previous high resolution
VLA observations (mainly A array at 1.4, 5 and 8 GHz) from the NRAO archive and
re-imaged them (43 sources). For the remainder, new observations were made in
the A array at 1.4 and 5 GHz. We show images of the 61 resolved sources, and
list structural properties of all of them. Optical data from the SDSS are
available for nearly every source. This work represents a significant increase
in the number of high redshift quasars with published radio structures, and
will be used to study the properties and evolution of luminous radio sources in
the high redshift universe
Concurrent 43 and 86 GHz Very Long Baseline Polarimetry of 3C273
We present sub-milliarcsecond resolution total intensity and linear
polarization VLBI images of 3C273, using concurrent 43 and 86 GHz data taken
with the Very Long Baseline Array in May 2002. The structure seen in the
innermost jet suggest that we have fortuitously caught the jet in the act of
changing direction. The polarization images confirm that the core is
unpolarized (fractional polarization m < 1 %) at 86 GHz, but also show well
ordered magnetic fields (m ~ 15 %) in the inner jet, at a projected distance of
2.3 pc from the core. In this strongly polarized region, the rotation measure
changes across the jet by 4.2 x 10^{4} rad m^{-2} over an angular width of
about 0.3 milliarcseconds. If the lack of polarization in the core is also
attributed to a Faraday screen, then a rotation measure dispersion > 5.2 x
10^{4} rad m^{-2} must be present in or in front of that region. These are
among the highest rotation measures reported so far in the nucleus of any
active galaxy or quasar, and must occur outside (but probably close to) the
radio emitting region. The transverse rotation measure gradient is in the same
sense as that observed by Asada et al and by Zavala and Taylor at greater core
distances. The magnitude of the transverse gradient decreases rapidly with
distance down the jet, and appears to be variable.Comment: 4 pages, LaTeX, 3 postscript figures, submitted to Astrophysical
Journal Letter
High Resolution Linear Polarimetric Imaging for the Event Horizon Telescope
Images of the linear polarization of synchrotron radiation around Active
Galactic Nuclei (AGN) identify their projected magnetic field lines and provide
key data for understanding the physics of accretion and outflow from
supermassive black holes. The highest resolution polarimetric images of AGN are
produced with Very Long Baseline Interferometry (VLBI). Because VLBI
incompletely samples the Fourier transform of the source image, any image
reconstruction that fills in unmeasured spatial frequencies will not be unique
and reconstruction algorithms are required. In this paper, we explore
extensions of the Maximum Entropy Method (MEM) to linear polarimetric VLBI
imaging. In contrast to previous work, our polarimetric MEM algorithm combines
a Stokes I imager that uses only bispectrum measurements that are immune to
atmospheric phase corruption with a joint Stokes Q and U imager that operates
on robust polarimetric ratios. We demonstrate the effectiveness of our
technique on 7- and 3-mm wavelength quasar observations from the VLBA and
simulated 1.3-mm Event Horizon Telescope observations of Sgr A* and M87.
Consistent with past studies, we find that polarimetric MEM can produce
superior resolution compared to the standard CLEAN algorithm when imaging
smooth and compact source distributions. As an imaging framework, MEM is highly
adaptable, allowing a range of constraints on polarization structure.
Polarimetric MEM is thus an attractive choice for image reconstruction with the
EHT.Comment: 19 pages, 9 figures. Accepted for publication in ApJ. Imaging code
available at https://github.com/achael/eht-imaging
Relative Astrometry of Compact Flaring Structures in Sgr A* with Polarimetric VLBI
We demonstrate that polarimetric interferometry can be used to extract
precise spatial information about compact polarized flares of Sgr A*. We show
that, for a faint dynamical component, a single interferometric baseline
suffices to determine both its polarization and projected displacement from the
quiescent intensity centroid. A second baseline enables two-dimensional
reconstruction of the displacement, and additional baselines can self-calibrate
using the flare, enhancing synthesis imaging of the quiescent emission. We
apply this technique to simulated 1.3-mm wavelength observations of a "hot
spot" embedded in a radiatively inefficient accretion disk around Sgr A*. Our
results indicate that, even with current sensitivities, polarimetric
interferometry with the Event Horizon Telescope can achieve ~5 microarcsecond
relative astrometry of compact flaring structures near Sgr A* on timescales of
minutes.Comment: 9 Pages, 4 Figures, accepted for publication in Ap
Radio Jet-Ambient Medium Interactions on Parsec Scales in the Blazar 1055+018
As part of our study of the magnetic fields of AGN we have recently observed
a large sample of blazars with the Very Long Baseline Array. Here we report the
discovery of a striking two-component jet in the source 1055+018, consisting of
an inner spine with a transverse magnetic field, and a fragmentary but distinct
boundary layer with a longitudinal magnetic field. The polarization
distribution in the spine strongly supports shocked-jet models while that in
the boundary layer suggests interaction with the surrounding medium. This
behavior suggests a new way to understand the differing polarization properties
of strong- and weak-lined blazars.Comment: LaTex; 10 pages; 6 figures; reference fix; to appear in ApJL, 518,
1999 June 2
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