Mehreen Mahmud Denise Gabuzda

University College Cork, Ireland

Searching for Helical Magnetic Fields in Several BL Lac Objects

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

● 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

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/m2 to 131 rads/m2

Faraday Rotation

The amount of rotation is proportional to the integral of the density of free electrons ne 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 ne 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 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

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 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/m2 at core, 14 rad/m2 in jet.

● Compare RM map to my map observed ~ 4 years later.

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.

0735+178 2155-152

0256+075 1418+546

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.

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