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

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?

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

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

Introduction (3) The authors have reanalyzed 12-22 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 (2000.07) and 2000 August 11 (2000.61).

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.

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.

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

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°.

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.

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.

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

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 B1611+343 and B2251+158 (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.