Hidden Rotation Measures

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

2. 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 ca RM* l2 – but give the WRONG answer • RM synthesis can give the same WRONG answer as l2 fitting. • CAVEAT: RM(l2), P(l2). RM determinations must be tailored to the specific scientific goals, and must utilize multiple techniques, including RM synthesis, Q, U vs. l2and Q vs. U diagnostics. L. Rudnick: RM2010

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

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

5. Two simple components -- >polarization complexity Q, U vs.l2 P vs.l2 co,2 0o 20o 40o 60o 80o 100o AMP RM co 0.07 -5 90 0.05 -6 var L. Rudnick: RM2010

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

7. 3C33 Polarization -7 rad/m2 L. Rudnick: RM2010

8. 3C33 Polarization -7 rad/m2 L. Rudnick: RM2010

9. 3C33 Polarization -7 rad/m2 RM synthesis -13 -11 -15 -9 -7 -5 -7 rad/m2 -3 -1 +1 L. Rudnick: RM2010

10. 3C33 Polarization: -7 rad/m2 ? L. Rudnick: RM2010

11. 3C33 Polarization AMP RM co 0.07 -3.1 93 0.05 -046 L. Rudnick: RM2010

12. 3C33 Polarization AMP RM co 0.07 -3.1 93 0.05 -046 RM (l2) L. Rudnick: RM2010

13. -10 -5 0 5 10 -10 -5 0 5 10 3C33 Polarization – the range of fits c2 RM weaker component RM stronger component L. Rudnick: RM2010

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

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

16. 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 L. Rudnick: RM2010

17. Addressing the challenge… • Very broad l2 coverage • Check for RM(l2) and P(l2) • Use ALL of the available diagnostics L. Rudnick: RM2010

18. 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 ca RM* l2 – but give the WRONG answer • RM synthesis can give the same WRONG answer as l2 fitting. • CAVEAT: RM(l2), P(l2). RM determinations must be tailored to the specific scientific goals, and must utilize multiple techniques, including RM synthesis, Q, U vs. l2and Q vs. U diagnostics. Partial support for this work was provided by U.S. National Science Foundation GrantbAST-0908668 to the University of Minnesota. L. Rudnick: RM2010

19. 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 ca RM* l2 – but give the WRONG answer • RM synthesis can give the same WRONG answer as l2 fitting. • CAVEAT: RM(l2), P(l2). RM determinations must be tailored to the specific scientific goals, and must utilize multiple techniques, including RM synthesis, Q, U vs. l2and Q vs. U diagnostics. #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. L. Rudnick: RM2010

20. L. Rudnick: RM2010

21. L. Rudnick: RM2010