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% 

N. Manset / CFHTPolarization of Light: Basics to Instruments9 Part IV: Polarimeters Polaroid-type polarimeters Dual-beam polarimeters

N. Manset / CFHTPolarization of Light: Basics to Instruments10 Polaroid-type polarimeter for linear polarimetry (I) Use a linear polarizer (polaroid) to measure linear polarization... [another cool applet] Location: cool applet] Polarization percentage and position angle: Part IV: Polarimeters, polaroid-type

N. Manset / CFHTPolarization of Light: Basics to Instruments11 Polaroid-type polarimeter for linear polarimetry (II) Advantage: very simple to make Disadvantage: half of the light is cut out Other disadvantages: non-simultaneous measurements, cross-talk... Move the polaroid to 2 positions, 0º and 45º (to measure Q, then U) Part IV: Polarimeters, polaroid-type

N. Manset / CFHTPolarization of Light: Basics to Instruments12 Polaroid-type polarimeter for circular polarimetry Polaroids are not sensitive to circular polarization, so convert circular polarization to linear first, by using a quarter-wave plate Polarimeter now uses a quarter-wave plate and a polaroid Same disadvantages as before Part IV: Polarimeters, polaroid-type

N. Manset / CFHTPolarization of Light: Basics to Instruments13 Dual-beam polarimeters Principle Instead of cutting out one polarization and keeping the other one (polaroid), split the 2 polarization states and keep them both Use a Wollaston prism as an analyzer Disadvantages: need 2 detectors (PMTs, APDs) or an array; end up with 2 ‘pixels’ with different gain Solution: rotate the Wollaston or keep it fixed and use a half-wave plate to switch the 2 beams Part IV: Polarimeters, dual-beam type

N. Manset / CFHTPolarization of Light: Basics to Instruments14 Dual-beam polarimeters Switching beams Part IV: Polarimeters, dual-beam type Unpolarized light: two beams have identical intensities whatever the prism’s position if the 2 pixels have the same gain To compensate different gains, switch the 2 beams and average the 2 measurements

N. Manset / CFHTPolarization of Light: Basics to Instruments15 Dual-beam polarimeters Switching beams by rotating the prism rotate by 180º Part IV: Polarimeters, dual-beam type

N. Manset / CFHTPolarization of Light: Basics to Instruments16 Dual-beam polarimeters Switching beams using a ½ wave plate Rotated by 45º Part IV: Polarimeters, dual-beam type

N. Manset / CFHTPolarization of Light: Basics to Instruments17 Dual-beam polarimeter for circular polarization - Wollaston and quarter-wave plate Part IV: Polarimeters, dual-beam type The measurements V/I is: Switch the beams to compensate the gain effects

N. Manset / CFHTPolarization of Light: Basics to Instruments18 A real circular polarimeter Semel, Donati, Rees (1993) Quarter-wave plate, rotated at -45º and +45º Analyser: double calcite crystal Part IV: Polarimeters, example of circular polarimeter

N. Manset / CFHTPolarization of Light: Basics to Instruments19 A real circular polarimeter free from gain (g) and atmospheric transmission (  ) variation effects First measurement with quarter-wave plate at -45º, signal in the (r)ight and (l)eft beams: Second measurement with quarter-wave plate at +45º, signal in the (r)ight and (l)eft beams: Measurements of the signals: Part IV: Polarimeters, example of circular polarimeter

N. Manset / CFHTPolarization of Light: Basics to Instruments20 A real circular polarimeter free from gain and atmospheric transmission variation effects Build a ratio of measured signals which is free of gain and variable atmospheric transmission effects: average of the 2 measurements Part IV: Polarimeters, example of circular polarimeter

N. Manset / CFHTPolarization of Light: Basics to Instruments21 Polarimeters - Summary 2 types: –polaroid-type: easy to make but ½ light is lost, and affected by variable atmospheric transmission –dual-beam type: no light lost but affected by gain differences and variable transmission problems Linear polarimetry: –analyzer, rotatable –analyzer + half-wave plate Circular polarimetry: –analyzer + quarter-wave plate 2 positions minimum 1 position minimum Part IV: Polarimeters, summary

Polarimetry in Astronomy22 Polarimeter To spectrograph or imager To TV guider Tilted slit/dekker Arc lamp /2 plate Analyser Calcite /4 plate

Polarimetry in Astronomy23 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!