Faraday Rotation Measure Gradients from a Helical Magnetic Field in 3C 273

Using high frequency (12-22 GHz) VLBA observations we confirm the existenceof a Faraday rotation measure gradient of ~ 500 rad/m^2/mas transverse to thejet axis in the quasar 3C 273. The gradient is seen in two epochs spacedroughly six months apart. This stable transverse rotation measure gradient isexpected if a helical magnetic field wraps around the jet. The overall order tothe magnetic field in the inner projected 40 parsecs is consistent with ahelical field. However, we find an unexpected increase in fractionalpolarization along the edges of the source, contrary to expectations. This highfractional polarization rules out internal Faraday rotation, but is not readilyexplained by a helical field. After correcting for the rotation measure, theintrinsic magnetic field direction in the jet of 3C 273 changes from parallelto nearly perpendicular to the projected jet motion at two locations. If ahelical magnetic field causes the observed rotation measure gradient then thesynchrotron emitting electrons must be separate from the helical field region.The presence or absence of transverse rotation measure gradients in othersources is also discussed.Comment: Accepted to ApJ Letters; 11 pages, 4 figures (1 color figure