Polarimetry in Astronomy

1. Polarimetry in Astronomy Or Do you know where your photons are coming from? Elizabeth Corbett AAO

2. Polarimetry: The Basics Light be described in terms of two components: Taken from Hecht (1987) “Optics” Polarimetry in Astronomy

3. Polarimetry: The Basics In general light is elliptically polarised Special cases: x = 0 or np - linearly polarised light x = p/2  np and Ex=Ey circularly polarised light Unpolarised light has a well-defined E which fluctuates rapidly, hence no net polarisation is measured. q Polarimetry in Astronomy

4. Introducing: - The Stokes Vectors • Electromagnetic radiation can be described in terms of the Stokes Vectors; I, Q, U & V. • I - total intensity • Q & U - describing linear polarisation • V - circular polarisation • Polarisation PA: • Degree of Polarisation: • For linear polarisation V=0 Polarimetry in Astronomy

5. Why Stokes Vectors? • Easy to describe polarisations: • Additive - e.g. Polarimetry in Astronomy

6. Sources of Polarised Emission • Synchrotron: • dominant radiation mechanism in the optical - radio continua of the blazar class of AGN, also seen in SNR, pulsars • emitted by charged particles, generally electrons accelerated by a magnetic field • produces a high degree of linear polarisation (up to 45% in some blazars) • polarisation position angle is aligned with the E vector perpendicular to the local magnetic field Polarimetry in Astronomy

7. Dichroic Absorption: • also known as interstellar polarisation • dichroic absorbers preferentially absorb radiation with one polarisation state and transmit the orthogonal state • due to anisotropic dust grains aligning in the presence of a magnetic field • radiation passing through such a cloud becomes polarised with an E vector parallel to the magnetic field Polarimetry in Astronomy

8. Scattering: • Light can be scattered by electrons or dust • High degrees of linear polarisation can result • Polarisation PA is perpendicular to the scattering plane • Degree of polarisation depends on the scattering angle,c • Circular polarisation can result from multiple scatters from dust 0% 60% c 100% Polarimetry in Astronomy

9. Polarisation in AGN Dichroic absorption by dust Absorbed by torus BLR Torus NLR Direct view Synchrotron emission Polarimetry in Astronomy

10. Synchrotron Polarimetry in Astronomy

11. Polarisation in AGN Scattered in the NLR Dichroic absorption by dust Absorbed by torus BLR Torus NLR Scattered off torus Direct view Synchrotron emission Polarimetry in Astronomy

12. Scattered in the NLR Circinus: Alexander et al (2000) Polarimetry in Astronomy

13. Young et al(1996) Polarimetry in Astronomy

14. Polarisation in AGN Scattered by moving scatterers Scattered in the NLR Dichroic absorption by dust Absorbed by torus BLR Torus NLR Direct view Scattered off torus Synchrotron emission Polarimetry in Astronomy

15. Scattered by moving scatterers Polarimetry in Astronomy

16. l/4 plate Polarimeter To TV guider l/2 plate Calcite To spectrograph or imager Tilted slit/dekker Analyser Arc lamp Polarimetry in Astronomy

17. Polarimetry o e o e o e o e } Sky { } Object { Imaging Spectroscopy Polarimetry in Astronomy

18. Measuring the Stokes Parameters • Rotate the wave-plate to 0o, 45o, 22.5o and 67.5o • Subtract the sky from each ray on each frame • Take the ratio of the intensities of the o- and e- ray for each frame • For Q, and • For U, and • Polarization PA • Degree of polarization Polarimetry in Astronomy

19. Summary • Polarimetry provides information on where your photons originated • Have they been scattered? • Have they been through dust? • Have they (perhaps) come from a jet? • Important for inclination dependent systems - eg AGN, YSO • “Not as hard as it used to be” - easy data reduction • But - very “photon hungry” • so for a dP~0.1% you need SNR ~1400 or 2E6 photons! Polarimetry in Astronomy

20. Scattered by NLR Scattered by torus Polarimetry in Astronomy