Abstract
We report observations of Faraday rotation measures for a sample of 191
extragalactic radio jets observed within the MOJAVE program.
Multifrequency Very Long Baseline Array observations were carried out
over 12 epochs in 2006 at four frequencies between 8 and 15 GHz. We
detect parsec-scale Faraday rotation measures in 149 sources and find
the quasars to have larger rotation measures on average than BL Lac
objects. The median core rotation measures are significantly higher than
in the jet components. This is especially true for quasars where we
detect a significant negative correlation between the magnitude of the
rotation measure and the de-projected distance from the core. We perform
detailed simulations of the observational errors of total intensity,
polarization, and Faraday rotation, and concentrate on the errors of
transverse Faraday rotation measure gradients in unresolved jets. Our
simulations show that the finite image restoring beam size has a
significant effect on the observed rotation measure gradients, and
spurious gradients can occur due to noise in the data if the jet is less
than two beams wide in polarization. We detect significant transverse
rotation measure gradients in four sources (0923+392, 1226+023,
2230+114, and 2251+158). In 1226+023 the rotation measure is for the
first time seen to change sign from positive to negative over the
transverse cuts, which supports the presence of a helical magnetic field
in the jet. In this source we also detect variations in the jet rotation
measure over a timescale of three months, which are difficult to explain
with external Faraday screens and suggest internal Faraday rotation. By
comparing fractional polarization changes in jet components between the
four frequency bands to depolarization models, we find that an external
purely random Faraday screen viewed through only a few lines of sight
can explain most of our polarization observations, but in some sources,
such as 1226+023 and 2251+158, internal Faraday rotation is needed.
Original language | English |
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Pages (from-to) | 105 |
Number of pages | 4 |
Journal | Astronomical Journal |
Volume | 144 |
Issue number | 4 |
DOIs | |
Publication status | Published - 2012 |
MoE publication type | A1 Journal article-refereed |
Keywords
- BL Lacertae objects: general
- galaxies: active
- galaxies: jets
- polarization
- quasars: general
- radio continuum: galaxies