1. L. Rudnick: RM2010 Hidden Rotation Measures Lawrence Rudnick University of Minnesota with Damon Farnsworth, UMN Shea Brown, CSIRO

2. L. Rudnick: RM2010 Take Home Messages Two components with small RM differences can generate a wide range of complex behaviors. Polarization angle behavior over wide bands can be reasonably fit with   RM* 2 – but give the WRONG answer RM synthesis can give the same WRONG answer as 2 fitting. CAVEAT: RM( 2 ), P( 2 ). RM determinations must be tailored to the specific scientific goals, and must utilize multiple techniques, including RM synthesis, Q, U vs. 2 and Q vs. U diagnostics.

3. L. Rudnick: RM2010 XRAY NVSS I NVSS P800

4. L. Rudnick: RM2010 A little pre-history…

5. L. Rudnick: RM2010 Two simple components -- > polarization complexity Q, U vs.   P vs.   0 o 20 o 40 o 60 o 80 o 100 o AMP RM  o 0.07 -5 90 0.05 -6 var  o,2

6. L. Rudnick: RM2010 3C33 Polarization Leahy & Perley, 1991 WSRT 352 MHz Q U 377.5 356.1 324.1 MHz MHz MHz

7. L. Rudnick: RM2010 3C33 Polarization -7 rad/m 2

8. L. Rudnick: RM2010 3C33 Polarization -7 rad/m 2

9. L. Rudnick: RM2010 3C33 Polarization -7 rad/m 2 -7-5 -3+1 -9 -15 -13-11 -7 rad/m 2 RM synthesis

10. L. Rudnick: RM2010 3C33 Polarization: ? -7 rad/m 2

11. L. Rudnick: RM2010 3C33 Polarization AMP RM  o 0.07 -3.1 93 0.05 -0 46

12. L. Rudnick: RM2010 3C33 Polarization AMP RM  o 0.07 -3.1 93 0.05 -0 46 RM ( 2 )

13. L. Rudnick: RM2010 3C33 Polarization – the range of fits -10 -5 0 5 10 RM stronger component RM weaker component 22

14. L. Rudnick: RM2010 Other 3C33 fits… Exclude 20cm Note: FACTOR of 2-3 in RMs -10 -5 0 5 10 RMs: -3.1, -0.52 RMs: -7.3, -9.4

15. L. Rudnick: RM2010 RM synthesis and two components -10,-2 [b]

16. L. Rudnick: RM2010 RM synthesis and two components Faraday spectrum peaks (P) are NOT at the input RMs Whether two RMs are resolved depends on the PHASES of the components

17. L. Rudnick: RM2010 Addressing the challenge… Very broad 2 coverage Check for RM( 2 ) and P( 2 ) Use ALL of the available diagnostics

18. L. Rudnick: RM2010 Take Home Messages Two components with small RM differences can generate a wide range of complex behaviors. Polarization angle behavior over wide bands can be reasonably fit with   RM* 2 – but give the WRONG answer RM synthesis can give the same WRONG answer as 2 fitting. CAVEAT: RM( 2 ), P( 2 ). RM determinations must be tailored to the specific scientific goals, and must utilize multiple techniques, including RM synthesis, Q, U vs. 2 and Q vs. U diagnostics. Partial support for this work was provided by U.S. National Science Foundation GrantbAST-0908668 to the University of Minnesota.

19. L. Rudnick: RM2010 Take Home Messages # COROLLARY: When looking at trends – e.g. redshift, cluster impact parameter, size, etc. systematic errors in RM can be fatal. Partial support for this work was provided by U.S. National Science Foundation GrantbAST-0908668 to the University of Minnesota. Two components with small RM differences can generate a wide range of complex behaviors. Polarization angle behavior over wide bands can be reasonably fit with   RM* 2 – but give the WRONG answer RM synthesis can give the same WRONG answer as 2 fitting. CAVEAT: RM( 2 ), P( 2 ). RM determinations must be tailored to the specific scientific goals, and must utilize multiple techniques, including RM synthesis, Q, U vs. 2 and Q vs. U diagnostics.

20. L. Rudnick: RM2010