1. Polarization at mm Telescopes Overview: 1- milestones of astronomical polarization 2- polarization fundamentals 3- intermezzo: a poor man’s polarimeter 4- polarimeters at mm telescopes 5- XPOL – the cross correlation polarimeter at the 30m telescope 6- XPOL - astronomical results 7- literature Clemens Thum IRAM

2. 1- milestones of astronomical polarization the story (mostly) of cosmic magnetic fields 1908 Hale: Solar magnetic field 1947 Babcock: Magnetic stars 1947 Hiltner, Hall: Interstellar polarization 1953 Ginzburg, Shklovsky: Cosmic synchrotron radiation 1969 Radhakrishnan & Cooke: Pulsars are oblique rotators 1975 Reid: Polarization of OH masers 1982 Antonucci: Seyfert 2 galaxies are obscured Seyfert 1 1990 Crutcher: Magnetic fields in star forming regions 2000 Stenflo: The second solar spectrum 2002 Carlstrom: Cosmic microwave background

3. milestones: the solar magnetic field ZIMPOL Zürich Imaging Polarimeter - diffraction (not seeing) limited - velocity resolved - measures all Stokes parameters - polarization purity 10 -5 section of “second solar spectrum”

4. milestones: interstellar polarization Cygnus Perseus Differential extinction by interstellar dust - grains are not spherical - aligned by interstellar magnetic field - field often has large scale order often nicely displayed in nearby spirals - field important in molecular clouds ?

5. milestones: pulsars Oblique rotators - misaligned magnetic and rotation axes - line of sight nearly (magnetic) pole-on - magn. field direction rotates across pulse - polarization angle rotates as well

6. milestones: type 1 / type 2 unification Early evidence: Antonucci 1982 type 1: strong broad lines type 2: broad lines weak or absent Unification: type 2 is an obscured type 1 Observational evidence: polarization spectra (Keck spectrum of radio galaxy 3C234 by Tran et al. 1995)

7. milestones: magnetic fields in star forming regions DR21(OH) Crutcher et al. 1999 CN(1-0) at IRAM 30m - Protagonist: R. Crutcher - different molecules probe different densities - Zeeman effect on paramagnetic transitions - CN(1-0) at 113 GHz is ideal - field increases with density: B  n  with  = 0.44 (ambipolar diffusion) - dense core are close to magnetic criticality

8. 2- Fundamentals: the Stokes parameters Question: Stokes parameters in the particle picture ?

9. 3- Intermezzo I: poor man’s polarimeter Observation of the SiO star S CrB at 30m telescope - using horizontally and vertically polarized receivers - sky rotates as seen from receivers - after 90 deg. of rotation: receivers exchange power Why is this not a good polarimeter ? - inefficient - inaccurate - incomplete - not feasible at low declinations

10. 3- Intermezzo II: waveplate polarimeters Birefringent material - ordinary, extraordinary rays have different refractive indices - after passage through material of thickness d Δ = d (n e – n o ) if Δ = λ/2 : halfwave plate, converts linear to linear if Δ = λ/4 : quarterwave plate, converts linear to circular Why is this not a really good polarimeter ? - often not very efficient (blanking when switched) - prone to instrumental polarization (indices are wavelength dependent) - incomplete (mostly either linear or circular) - moving elements in beam cause baseline ripples fast axis slow axis

11. 4- Polarimeters at mm telescope Waveplate polarimeters: NRAO 12m telescope: Millipol FCRAO: various waveplate polarimeters JCMT: broadband halfwave plates with bolometer arrays BIMA: single polarization receivers, time multiplexing Correlation polarimeters: Arecibo IRAM 30m: XPOL IRAM Plateau de Bure ?

12. Millipol at NRAO 12m Telescope Instrument: rotating half-wave plate 1.3mm wavelength Observation: Orion KL nebula dust is aligned “polarization hole”

13. Waveplate polarimeters at FCRAO - stepping half- and quarterwave plates - HCN maser in CIT 6 - circular polarization in SiO stars - linear polarization in AGNs

14. Waveplate polarimeters at JCMT - stepping half-wave plate - usable for broadband observations - many interesting continuum observations - field weaker in center of cloud

15. BIMA: first polarimetry with a mm interferometer First polarimetry on a mm interferometer - stepping waveplates - first dectection of Goldreich-Kylafis effect - confirmation of depolarization towards cloud centers - very high dust polarization - inefficient operation with one pol/antenna

16. IRAM Plateau de Bure interferometer - new generation dual polarization receivers in 2006 - efficient polarization observations will become possible - without competition until ALMA

17. 5- XPOL: The instrument Description: correlation spectropolarimeter all-digital mini-interferometer measure all Stokes parameters simultaneously line and continuum polarimetry People involved: D. Morris, S. Navarro, G. Paubert, C. Thum, H. Wiesemeyer Components: a pair of orthogonally polarized receivers special wiring of Local Oscillators correlator capable of auto- and cross-correlation scheme for calibrating the phase analysis software

18. XPOL: receivers and Nasmyth optics

19. XPOL – the instrument: VESPA in polimetry mode Polarimetry modes: - 40 kHz / 120 MHz - 1.25 MHz /480 MHz Highest spectral resolution polarimetry mode shown here: - 6 units of 20 MHz bandwidth - each board has 256 delay channels - spectral resolution: 40 kHz - twice the number of channels for cross correlation Note that auto and cross correlations are made in the same unit

20. XPOL: phase calibration I Principle: ✗ wire grid in front of hot/cold load generates coherent broad band power in V and H Rxs we measure the difference of phase between H and V Rxs towards phase cal unit front view phase calibration unit side view

21. XPOL: phase calibration II Amplitude & phase amplitude ripple due to reflection from grid bandpass of 480 MHz composed of 12 base- bands each with their own phase offsets & slope phase globally flat across bandpass: delay between H & V signal path is well balanced each spectral channel calibrated to < 1 ° auto-correlation: H and V receivers cross-correlation: real and imaginary parts

22. XPOL: a continuum observation medium strong AGN can be measured to < 1% precision in minutes polarization parameters are band-averages noise has same ampl. in all Stokes spectra S/N = 1 in p L or p C require S/N = 100 in Stokes I if p L,p C = 1%

23. 2001 2005 XPOL: the Stokes beams Why worry ? point sources: instrumental polarization due to on-axis response, pointing errors extended sources: false polarization in presence of source structure, velocity gradients (“beam squint”) Origins of polarization sidelobes: receiver misalignment non-optimum Nasmyth optics element ellipticity of feed horns

24. 6- XPOL: Astronomical results: Crab Nebula XPOL 86 GHz p L = 27 % p C < 0.3 %  = 155±1° 6- X

25. 6- Astronomical Results: The Moon

26. 6- Astronomical Results: methanol Wiesemeyer et al. 2004, A&A 428, 479 NGC7538 SCUBA 0.9mm dust, lines: CO outflow

27. 7- Literature on astronomical polarization Polarization is a basic property of photons, just like frequency. It is often almost as densely coded with information. Antonucci 2000