1. EVN2015 – Studies of AGN Polarisation Denise Gabuzda University College Cork

2. Key future observations for EVN:
Sensitive, multi-wavelength polarisation observations, aimed at detailed studies of
Magnetic field structures in weak or extended jets
Transverse polarisation structure of jets
“Subtle” polarisation effects such as Faraday rotation-measure gradients, either along the jet (e.g. distribution of electron density from nucleus) or across the jet (e.g. due to the presence of helical B fields or shear interactions with surrounding medium)
“Subtle” structures in spectral-index maps
Circular polarisation

3. B field structures in extended regions of jets
2cm VLBA map of ; Gabuzda & Lisakov, in prep.
18cm VLBA map of ; Gabuzda & Chernetskii 2003

4. Transverse polarisation structure, such as “sheath-like” structuresB
Pushkarev et al. 2005
Attridge, Roberts & Wardle 1999 B

5. B Negative RM (LOS B away from observer) ~ Zero RM
Powerful diagnostic for presence of toroidal/helical B fields: Faraday-rotation gradient across the jet – due to systematically changing line-of-sight component of B field across the jet. If jet is viewed at ~90° to jet axis in source frame (~ 1/γ in observer’s frame):
Negative RM (LOS B away from observer) B
~ Zero RM
Positive RM (LOS B towards observer) E

6. Gabuzda, Murray, Cronin 2004: Found expected behaviour for toroidal/helical B field viewed at 90º to jet axis in source frame in Mrk501
Croke & Gabuzda (in prep): RM gradient with same sense detected using cm VLBA data

7. Transverse RM gradient found in — but in the opposite sense to the gradient visible in RM image of Zavala & Taylor (2003) — change of winding direction for helix?
Zavala & Taylor 2003
Gabuzda & Mahmud 2007

8. • Transverse RM gradients appear to be fairly common, but detecting them depends on detecting small differences in RM across the jet:
– accurate polarisation angle calibration crucial – if thermal electron density is low, RM signal will be weak, making it difficult to detect RM without very sensitive measurements

9. If polarization “sheaths” are associated with helical B fields, some sources with “sheaths” should display RM gradients as well …
Pushkarev et al. 2005
Gabuzda, Murray & Cronin 2004
Mrk501

11. B
Zavala & Taylor 2003

12. Promising little studied technique for probing complex jet structures – spectral “tomography” (Katz-Stone & Rudnick 1997) – can be especially effective for studying layered sub-structures
Regions of flat/inverted spectral index trace out bands crossing the jet (Croke & Gabuzda, in prep)

13. Spectral index distribution can delineate jet sub-structures that are not (as) obvious in intensity maps
Regions of inverted spectral index seem to describe helical structures wrapped around the jet – evidence that the jet itself has helical sub-structures?

14. First MOJAVE Circular Polarization Measurements recently published (Homan and Lister 2006)
Prime suspect for mechanism generating circular polarization is Faraday conversion of LP to CP – but no obvious correlation between degree of CP and other source properties (degree of LP, spectral index, etc.).

15. 35 MOJAVE sources had detectable CP in first-epoch measurements – many of these display rich polarisation structures, such as transverse RM gradients and sheath-like polarization structures – linear polarisation structure must be connected with CP – but how?

16. CP also detected in jets of several sources!
, – have sheath polarization structures
3C273 – has transverse RM grad
also has transverse RM grad

17. Determining intrinsic radio core polarisation angle using RM of VLBI core – very high RMs sometimes detected at high frequencies (counterintuitive – Faraday rotation should become less important at higher frequency!)

18. Zavala & Taylor 2003
Reynolds, Cawthorne & Gabuzda 2000
Core RM measurements for BL Lac at 1.3-6cm and 2-4cm indicate MINUS several HUNDRED rad/m², but new measurements at 7mm-2cm yield PLUS several THOUSAND rad/m² (note also previous reports by Mutel & Denn)!
Gabuzda et al

19. How can this occur?
• Difference in direction of dominant LOS B field on different scales
• Observations at different frequencies with different resolutions predominantly sample different scales
— New 7mm+1.3cm+2cm+3.6cm+6cm VLBA polarization observations planned to directly test this hypothesis (O’Sullivan & Gabuzda)
— May be possible to use changes in sign of RM along jet to reconstruct 3D B field structure to some extent

20. Summary – The Future
• Wide range of somewhat “subtle” but very informative effects coming within realm of observation – RM gradients along and transverse to VLBI jets, transverse polarisation structure of jets, features in spectral index distribution, circular polarisation in core region and in jets – and connections between them all
— RM gradients transverse to VLBI jets provide evidence for helical B fields, helicity directly given by RM gradient – connection to rotation of central black hole?
— Different signs of RM (LOS B-field) on different scales – using sign of RM to reconstruct 3-D structure of B field
— Circular polarisation – provides a key piece of information, but not clear yet how it fits into the puzzle

21. This shows that there are still (fundamental) things to be learned by studying “strong” AGN jets. In addition, we should aim to
– extend detailed polarisation observations to much fainter AGN jets…
– … and even jets in microquasars and young stellar objects
All this requires
– high sensitivity
– multi-wavelength capability (simultaneous multi-λ polarisation capability would be great!)
– reliable and hassle-free D-term removal
– reliable, accurate and hassle-free polarisation angle calibration

22. Closing thoughts:
• A change of paradigm can lead to the need for a completely new type of observation (e.g. a need for high-sensitivity Faraday-rotation mapping was only dimly foreseen 10 years ago).
• Multi-wavelength polarisation observations, Faraday-rotation measurements etc. should no longer be considered of interest only to a few specialists – we should strive to make them as routine as possible

24. Schematics – how sign of RM can potentially change due to bends or kinks in jet with helical B field
We should not neglect opportunity to use the SIGN of the rotation measure to investigate the direction of the LOS magnetic field.