Abstract
We briefly review how opacity affects the observed polarization in synchrotron emitting jets. We show some new multi-frequency observations of 3C 273 made with the VLBA in 1999–2000, which add significantly to the available rotation measure (RM) observations of this source. Our findings can be summarized as follows: (1) The transverse gradient in RM is amply confirmed. This implies a toroidal component to the magnetic field, which in turn requires a current of 1017–1018 A flowing down the jet. (2) The net magnetic field in the jet is longitudinal; however, whether or not the longitudinal component is vector-ordered is an open question. (3) The RM distribution is variable on timescales of months to years. We attribute this to the motion of superluminal components behind a turbulent Faraday screen that surrounds the jet. (4) Finally, we suggest that Faraday rotation measurements at higher resolution and higher frequencies, with the Event Horizon Telescope, may enable useful constraints to be placed on the accretion rate onto the central black hole.
Highlights
We show some new multi-frequency observations of 3C 273 made with the VLBA in 1999–2000, which add significantly to the available rotation measure (RM) observations of this source
(4) we suggest that Faraday rotation measurements at higher resolution and higher frequencies, with the Event Horizon Telescope, may enable useful constraints to be placed on the accretion rate onto the central black hole
As the optical depth increases, Q steadily decreases, eventually passing through zero and becoming negative. This corresponds to a change of 90◦ in the EVPA, and at large enough optical depth, the fractional polarization becomes m = 3/(6p + 13)
Summary
Similar to what is measured by the core-shift technique [6,7], which measures the position of the peak in total intensity as a function of frequency The minimum in fractional polarization near the center of the jet is a persistent feature, best seen in the movie sequence of images on the MOJAVE website [15], and was first remarked on by [10] It is not a Faraday effect, but a simple consequence of the magnetic field geometry, where the toroidal and longitudinal field components give comparable but orthogonal polarizations. Important support for very high RMs close to the core is given by the authors of [21], who observed 3C 273 with ALMA at a wavelength of 1 mm and measured an RM of 3.8 × 105 rad/m2 . (They could not, resolve the jet with ALMA, so this value corresponds most closely to the ridge RMs in Column 4 of Table 1.)
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