1111.3416 (J. C. Algaba)
J. C. Algaba
One of the tools that can provide evidence about the existence of helical
magnetic fields in AGN is the observation of rotation measure gradients across
the jet. Such observations have been previously made successfully, proving that
such gradients are far from being rare, but common and typically persistent
over several years, although some of them may show a reversal in the direction
along the jet. Further studies of rotation measure gradients can help us in our
understanding of the magnetic field properties and structure in the base of the
jets. We studied Very Long Baseline Array (VLBA) polarimetric observations of 8
sources consistent of some quasars and BL Lacs at 12, 15, 22, 24 and 43 GHz and
we find that all but two sources show indications of rotation measure
gradients, either parallel or perpendicular to the jet. We interpret gradients
perpendicular to the jet as indications of the change of the line of sight of
the magnetic field due to its helicity, and gradients parallel to the jet as
the decrease of magnetic field strength and/or electron density as we move
along the jet. When comparing our results with the literature, we find
tentative evidence of a rotation measure gradient flip, which can be explained
as a change of the pitch angle or jet bending.
View original:
http://arxiv.org/abs/1111.3416
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