1. Shane O’Sullivan University College Cork
Using Faraday Rotation Sign Reversals To Study Magnetic Fields in AGN Jets
Shane O’Sullivan
University College Cork

2. Faraday Rotation
Rotation of observed plane of polarisation due to propagation of the radiation through a magnetised plasma
Slope of the line is the Rotation Measure (RM)
Depends linearly on the electron density ne and the integrated LoS component of the B-field along the path length dl through the plasma

3. Previous Observations
Multi wavelength VLBA polarisation observations used to study Faraday rotation
Previous studies focused on enhancements in magnitude of the RM in the VLBI core compared to values in the VLBI jet
Interpreted as reflecting an increase in the density of free electrons with approach toward the central engine

4. Current Observations VLBI core RMs of 34 BL Lac objects
Observations obtained at 2cm, 3.6cm, 6cm simultaneously and at 7mm, 1.3cm, 2cm simultaneously
Enhancement in magnitude of RM in the VLBI core compared to the VLBI jet was found in many cases, consistent with ↑ in the density of free e-s towards centre of the AGN
Core RMs at shorter wavelengths have larger magnitudes but the same sign as those at longer wavelengths, consistent with higher resolution observations probing scales closer to centre of activity of the AGN

5. RM Sign Reversals
Several sources whose VLBI core components display RMs with different signs in our 7mm-2cm and 2-6cm dataset
7mm-2cm
Core RM (rad/m2)
2cm-6cm
-740
+160
+410
+480 or -960
-675
+255
+780
-200
+6100
-430

6. RM Sign Reversals Reynolds et al. 2003 Gabuzda et al. 2006
-430rad/m2 (1.3, 2, 3.6, and 6cm)
Gabuzda et al. 2006
+6100rad/m2 (7mm, 1.3cm & 2cm)
l2 dependence expected for Faraday rotation
But both the magnitude and sign of the inferred RMs are different
Mutel & Denn 2003
high core RMs for BL Lac ( ) up to 5000rad/m2
nine epochs from 1998 to 2002

7. np Ambiguity 1418+546: problem caused by the np ambiguity in the EVPAs
black squares do not display a linear dependence on l2
good linear relationship is obtained if p is subtracted from the observed c = 88o at 6cm (hollow diamond)
RM of -960rad/m2 signified by the dotted line

8. Optically Thick-Thin Transition
Optically thin regime, the EVPA is ┴ to the B-field
Optically thick regime, the EVPA is ║ to the B-field
If optically thick-thin transition occurs within our observed frequency range it can disrupt the l2 behaviour of Faraday Rotation
 to ensure that EVPAs are all in the same regime, one EVPA must be rotated by 90o

9. Optical Thick-Thin Transition
: 2cm-6cm RM = +480rad/m2
EVPA at 2cm is rotated by 90o, indicated by the hollow circle, provides a clear linear dependence on l2

10. Use of RM Measurements
Sign of RM → LoS compnt of the B-field is orientated towards (+ RM) or away (- RM) from Earth
Gabuzda, Murray & Cronin 2004
RM gradient across the jet
Blue: - RM → LoS B-field is moving away
Green: + RM → LoS B-field is moving towards us
Excellent evidence for presence of helical B-fields surrounding AGN jets

11. RM Sign Reversals
2 possibilities of why we observe core RM sign reversals
Orientation of LoS B-field in core region of AGN changes with (i) distance from centre of activity or
(ii) time
Single set of simultaneous VLBA polarisation observations at 7mm, 1.3cm, 2cm, 3.6cm, 6cm
Used to discriminate between (i) & (ii)
If core RM sign reversals found with this dataset we can assume sign reversals are not due to changes between observing epochs

12. Possible Explanations of RM Sign Changes with Distance from Centre
dirn of LoS B-field changes because of a change of helicity due to a kink or reconnection in helical B-field along jet

13. Possible Explanations of RM Sign Changes with Distance from Centre

14. New Observations
Single set of simultaneous VLBA polarisation observations at 7mm, 1.3, 2, 3.6, 6cm
Observations at 2 wavelengths in each of the 2cm, 3.6cm & 6cm bands
v. important because we can make several RM maps providing resolution on a range of scales each with a minimum of four wavelengths (7mm-2cm, 2cm-3.6cm, 3.6cm-6cm)
Enable us to reliably map out the RM distribution and its sign in the core region and out along the jet
Determine if sign of the core region RM is same on all scales or if it changes sign at some points along inner jet

15. 3-D Structure of Jet
Presence of RM sign reversals in innermost VLBI jets can yield information about 3-D paths of curved jets through space
Interpretation of RM sign changes will be aided by
Possible transverse RM gradients across the jet
Projected path of the jet in the plane of the sky
Distribution of linear polarisation
Distribution of spectral index
Theoretically, changes in sign of RM with distance from centre of activity of AGN is possible
 We should not neglect the opportunity to use the SIGN of the rotation measure to investigate the dirn of the LoS B-field