mehreen mahmud denise gabuzda university college cork, ireland searching for helical magnetic fields...
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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.
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
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