L. Rudnick: RM2010

Hidden Rotation Measures

Lawrence RudnickUniversity of Minnesota

withDamon Farnsworth, UMN

Shea Brown, CSIRO

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.

L. Rudnick: RM2010

XRAYNVSS INVSS P800

L. Rudnick: RM2010

A little pre-history…

L. Rudnick: RM2010

Two simple components -- >polarization complexity

Q, U vs.

P vs.

0o 20o 40o 60o 80o 100o

AMP RM o

0.07 -5 90

0.05 -6 var

o,2

L. Rudnick: RM2010

3C33 Polarization

Leahy & Perley, 1991

WSRT352 MHz

Q

U

377.5 356.1 324.1 MHz MHz MHz

L. Rudnick: RM2010

3C33 Polarization

-7 rad/m2

L. Rudnick: RM2010

3C33 Polarization

-7 rad/m2

L. Rudnick: RM2010

3C33 Polarization

-7 rad/m2

-7 -5

-3 -1 +1

-9

-15 -13 -11

-7 rad/m2

RM synthesis

L. Rudnick: RM2010

3C33 Polarization:

?

-7 rad/m2

L. Rudnick: RM2010

3C33 Polarization

AMP RM o

0.07 -3.1 93

0.05 -0 46

L. Rudnick: RM2010

3C33 Polarization

AMP RM o

0.07 -3.1 93

0.05 -0 46

RM (2)

L. Rudnick: RM2010

3C33 Polarization – the range of fits

-10 -5 0 5 10

-10

-

5

0

5

10

RM stronger component

RM

wea

ker

com

pone

nt

2

L. Rudnick: RM2010

Other 3C33 fits…Exclude 20cm

Note: FACTOR of 2-3 in RMs

-10 -5 0 5 10

-10

-

5

0

5

10

RM

s:

-3.1

, -

0.52

RM

s: -7.3, -9.4

L. Rudnick: RM2010

RM synthesis and two components

-10,-2[b]

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

L. Rudnick: RM2010

Addressing the challenge…

• Very broad 2 coverage

• Check for RM(2) and P(2)

• Use ALL of the available diagnostics

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.

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.

L. Rudnick: RM2010

L. Rudnick: RM2010