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

Polarimetry in Astronomy2 Polarimetry: The Basics Light be described in terms of two components: Taken from Hecht (1987) “Optics”

Polarimetry in Astronomy3 Polarimetry: The Basics Special cases:  = 0 or n  linearly polarised light  =  /2  n  and E x =E y circularly polarised light Unpolarised light has a well-defined E which fluctuates rapidly, hence no net polarisation is measured. In general light is elliptically polarised 

Polarimetry in Astronomy4 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 Astronomy5 Why Stokes Vectors? Easy to describe polarisations: Additive - e.g.

Polarimetry in Astronomy6 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 Astronomy7 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 Astronomy8 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,  –Circular polarisation can result from multiple scatters from dust 100% 0% 60% 

Polarimetry in Astronomy9 Polarisation in AGN Absorbed by torus Synchrotron emission Direct view Dichroic absorption by dust BLR NLR Torus

Polarimetry in Astronomy10 Synchrotron

Polarimetry in Astronomy11 Polarisation in AGN Scattered off torus Absorbed by torus Synchrotron emission Scattered in the NLR Direct view Dichroic absorption by dust BLR NLR Torus

Polarimetry in Astronomy12 Scattered in the NLR Circinus: Alexander et al (2000)

Polarimetry in Astronomy13 Young et al (1996)

Polarimetry in Astronomy14 Polarisation in AGN Scattered off torus Absorbed by torus Synchrotron emission Scattered in the NLR Direct view Dichroic absorption by dust BLR NLR Scattered by moving scatterers Torus

Polarimetry in Astronomy15 Scattered by moving scatterers

Polarimetry in Astronomy16 Polarimeter To spectrograph or imager To TV guider Tilted slit/dekker Arc lamp /2 plate Analyser Calcite /4 plate

Polarimetry in Astronomy17 Polarimetry } Object { } Sky { oeoeoeoe oeoeoeoe Spectroscopy Imaging

Polarimetry in Astronomy18 Rotate the wave-plate to 0 o, 45 o, 22.5 o and 67.5 o –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 Measuring the Stokes Parameters

Polarimetry in Astronomy19 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  P~0.1% you need SNR ~1400 or 2E6 photons!

Polarimetry in Astronomy20 Scattered by NLR Scattered by torus