1. Faraday Rotation Measure Gradients From A Helical Magnetic Field In 3C273
Zavala & Taylor 2005, ApJ, 626, L73

2. Abstract Using high-frequency (12-22GHz) VLBA observations, the
existence of a Faraday rotation measure gradient of ~500
rad m-2 mas-1 transverse to the jet axis is confirmed in the
quasar 3C 273.
This gradient is seen in two epochs spaced roughly 6 months
apart.  A helical magnetic field warping around the jet?

3. Introduction (1) The jet launching mechanism : unresolved problem
Magnetic field is thought to contribute to the collimation.
“Blandford (1993) urged observers to search for Faraday
rotation measure (RM) gradients transverse to the relativistic
jet.”
χ : Observed polarization angle
χ0 : Intrinsic polarization angle

4. Introduction (2) Line of sight component : minus and maximum
Plus and maximum
Asada et al. 2002
K. Asada private
communication

5. Introduction (3)
The authors have reanalyzed GHz VLBA polarimetry
observations for two epochs on 3C273 to obtain higher
angular resolution transverse to the jet axis.
A scale for 3C273 : 1 mas = 2.52pc
The observations were made on 2000 January 27 ( )
and 2000 August 11 ( ).

6. Results (1) RM : Figure 1a (2000/1/27), Figure 1b (2000/8/11)
RM:gradient
from –1000
to 1000
RM~1800rad m-2
Main changing point : the region immediately southwest of the
Stokes I peak.

7. Results (2)
Beyond 3 mas from the Stokes I peak, the RM distribution does
not change dramatically between the two epochs.
An RM gradient transverse to the jet is apparent.

8. Results (3) The RM-corrected intrinsic B-vectors under the assumption
That the source is
optically thin from 12
to 22GHz.

9. Results (4) They simply took the Faraday-corrected
E-vectors and rotated
them by 90°.
This does not take
relativistic aberration
into account (Lyutikov
et al. 2003, 2005),
which could change
the orientation of the
B-vectors by up to 20°.

10. Results (5) Initially oriented roughly parallel
to the projected jet direction
Rotating to almost a
north-south orientation
6 mas
This B// and B⊥
structure appears
again.
Parallel (?) to the jet.

11. Faraday Screen in 3C273
Changes of the RM near the core : A region of that size is
unlikely to change over a 6 month time interval.
Changes in the observed RM are caused by jet components
sampling different sight lines as they move out from the core.
In general, the Faraday screen in front of the jet appears to
be constant in time, as the RM does not significantly change
over the 6 month time interval.

12. Evidence for a Systematic RM Gradient
RM image slices perpendicular to the jet axis for both epoch.
A transverse gradient of
approximately 500 rad m-2
mas-1 is visible.
The southern edge : mostly negative.
The northern edge : mostly positive.
An ordered helical field.
2000/8/11
2000/1/27

13. RM Distributions in Other Sources
M87 : The RM data cover a relatively small area.
There is no obvious RM gradient, but the sign changes.
consistent with a helical magnetic field.
Gabuzda et al. (2004) report RM gradients in four BL
Lacertae sources.
3C147 (Zhang et al. 2004) : An RM gradient observed at 8GHz.
The quasars B and B (Zavala & Taylor
2003) : No evidence of a transverse RM gradient.
At present we can say that transverse RM gradients do exist,
but these gradients are not a universal feature.