1. Mehreen Mahmud Denise Gabuzda University College Cork, Ireland Searching for Helical Magnetic Fields in Several BL Lac Objec ts

2. Outline ● Introduction - Overview of previous work ● Faraday Rotation ● Data Reduction - Observations, Calibration, Imaging and Rotation Measure (RM) determination ● Results To Date - Sources with transverse rotation measure gradients : 0256+075, 0735+178,1418+546, 1803+784, 2155-152 ● Conclusions and current work

3. ● Polarization important because it shows the ordering of the magnetic field associated with the radio emission. ● BL Lac Objects show a tendency for the magnetic fields in their parsec-scale jets to be perpendicular to the jet direction. ● Gabuzda, Murray and Cronin (2004), showed systematic Faraday- Rotation gradients across the parsec-scale jets of several BL Lac Objects, - Interpreted as evidence for helical magnetic fields – the gradients were taken to be due to the systematic variation of the line-of-sight magnetic field component across the jet. - Used three frequencies at 2 cm, 4 cm and 6 cm, observed in 1997. ● Shock Model: Series of relativistic shocks each of which enhances local transverse B field. Introduction

4. Gabuzda, D., Murray,E. & Cronin,P. (2004) ● RM map of 1652+398 observed at 2cm, 4cm and 6cm. ● Example of 'spine-sheath' B-field structure ● Transverse RM gradient ranging from ~ -63 rads/m 2 to 131 rads/m 2

5. Faraday Rotation The amount of rotation is proportional to the integral of the density of free electrons n e multiplied by the line-of-sight magnetic field B dl, the square of the observing wavelength, and various physical constants; the coefficient of 2 is called the rotation measure, RM:   2  n e B dl  RM 2 Thus, the intrinsic polarization of the source,   0 can be obtained:  obs =   0 + RM ( 2 ) where  obs is the observed polarization angle,   0 is the intrinsic polarization angle observed if no rotation occurred and  is the observing wavelength.

6. ● VLBA polarisation observations of 37 BL Lac objects observed between August 2003 and September 2004. ● 'Snap shot' mode, each source observed for about 25-30 minutes, several scans over the observing time period. ● 6 wavelengths; 2 at each of the 2cm, 4cm and 6cm bands. ● Objective to verify earlier results and get more refined Faraday Rotation gradients and identify new sources with the FR gradients. Data Observation and Reduction I

7. Data Observation and Reduction II ● After calibration, for each wavelength, total intensity (I) and polarization images (distribution of Stokes parameters Q and U) mapped ● Polarization angle images combined to make rotation measure maps after matching their parameters (beam size, image size, cell size) ● Before final RM maps made, contributions from known integrated (Galactic) Faraday Rotation subtracted at each wavelength. ● Calibration, Imaging and Rotation Measure determination done with AIPS package using standard techniques.

8. Map of 1803+784 by Zavala R. & Taylor G. (2003) ● Observed at seven frequencies between 8.1 and 15.2 GHz on June 27th, 2000. ● RM of -201 rad/m 2 at core, 14 rad/m 2 in jet. ● Compare RM map to my map observed ~ 4 years later.

9. Detection of transverse gradients in 1803+784 ● North-South RM gradient at ~2 mas from core. ● Hints of transverse RM gradient further along jet (at ~ 5 mas). ● Direction of North-South gradient reversed as compared to Zavala and Taylor (2003) ● Possible explanation: Kink in magnetic field followed by reconnection.

10. 0735+178 2155-152

11. 0256+075 1418+546

12. Conclusions and current/future work ● Transverse rotation measure gradients in 1803+784 verified, and evidence for gradient further out in the jet (which still need verification). Orientation of rotation measure gradients may change over time (may indicate kinks in B-field). ● Possible transverse gradients in 0256+075, 0735+178,1418+546, 2155- 152 ● Simplest explanation is of helical magnetic fields wrapped around jet.

13. Acknowledgments Radio Astronomy Lab at UCC This work is supported by a Basic Research Grant from Science Foundation Ireland. The VLBA is operated by the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

14. References Gabuzda, D., Murray,E. & Cronin,P. 2004, MNRAS,351,L90 Zavala R. & Taylor G. 2003, ApJ, 589, 126Z “Searching for Helical Magnetic Fields in Several BL Lac objects” Denise Gabuzda, Mehreen Mahmud and Askea O'Dowd ( Poster presented at conference Ultra-Relativistic Jets in Astrophysics, Banff, Canada, July 2005 ) Pushkarev A. 2001, Astron. Rep., 45, 667 Rusk R. 1988, PhD Thesis, University of Toronto