Mehreen Mahmud

1
Searching for Helical Magnetic Fields in Several
BL Lac Objects
Mehreen Mahmud Denise Gabuzda University
College Cork, Ireland
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
• 0256075, 0735178 ,1418546, 1803784,
  2155-152
• Conclusions and current work

3
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.

4

• RM map of 1652398 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)
5
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.
6
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.

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 1803784 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.

9
Detection of transverse gradients in 1803784

• 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
2155-152
0735178
11
0256075
1418546
12
Conclusions and current/future work

• Transverse rotation measure gradients in
  1803784 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 0256075,
  0735178,1418546, 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