Searching for Helical Magnetic Fields in Several BL Lac Objects Mehreen Mahmud Denise Gabuzda University College Cork, Ireland
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
Introduction • 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.
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 Gabuzda, D., Murray,E. & Cronin,P. (2004)
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 2is called the rotation measure, RM: 2 neB • dl RM 2 Thus, the intrinsic polarization of the source, 0can be obtained: obs = 0 + RM ( 2) where obs is the observed polarization angle, 0is the intrinsic polarization angle observed if no rotation occurred and is the observing wavelength.
Data Observation and Reduction I • 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 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.
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