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

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

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

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

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 0954+658 -740 +160 1418+546 +410 +480 or -960 1749+096 -675 +255 2007+777 +780 -200 2200+420 +6100 -430

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 (2200+420) up to 5000rad/m2 nine epochs from 1998 to 2002

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

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

Optical Thick-Thin Transition 1418+546: 2cm-6cm RM = +480rad/m2 EVPA at 2cm is rotated by 90o, indicated by the hollow circle, provides a clear linear dependence on l2

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

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

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

Possible Explanations of RM Sign Changes with Distance from Centre

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

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 1. Possible transverse RM gradients across the jet 2. Projected path of the jet in the plane of the sky 3. Distribution of linear polarisation 4. 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