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

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 : , , , , ● Conclusions and current work

● 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 ● Shock Model: Series of relativistic shocks each of which enhances local transverse B field. Introduction

Gabuzda, D., Murray,E. & Cronin,P. (2004) ● RM map of 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

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.

● VLBA polarisation observations of 37 BL Lac objects observed between August 2003 and September ● 'Snap shot' mode, each source observed for about 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

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.

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

Detection of transverse gradients in ● 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.

Conclusions and current/future work ● Transverse rotation measure gradients in 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 , , , ● Simplest explanation is of helical magnetic fields wrapped around jet.

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.

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