polarimetry in astronomy
DESCRIPTION
Polarimetry in Astronomy. Or Do you know where your photons are coming from ?. Elizabeth Corbett AAO. Polarimetry: The Basics. Light be described in terms of two components:. Taken from Hecht (1987) “Optics”. Polarimetry: The Basics. In general light is elliptically polarised. - PowerPoint PPT PresentationTRANSCRIPT
Polarimetry in Astronomy
Or
Do you know where your photons are coming from?
Elizabeth CorbettAAO
Polarimetry in Astronomy 2
Polarimetry: The Basics
)cos( tkExx zE
)cos( tkE yy zE
yx EEE
Light be described in terms of two components:
Taken from Hecht (1987) “Optics”
Polarimetry in Astronomy 3
Polarimetry: The Basics
Special cases:
= 0 or nlinearly polarised light
= /2 n and Ex=Ey 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 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
Q
U1tan2
1
I
VUQ 2 22 P
Polarimetry in Astronomy 5
Why Stokes Vectors?
• Easy to describe polarisations:
• Additive - e.g.
light polarisedcircular right 100% 1,0,0,1
)(90at polarisedlinearly 100% )0,0,1,1(
45at polarisedlinearly 100% 0,1,0,1
)(0at polarisedlinearly 100% (1,1,0,0)
light ed)(unpolaris Natural 0,0,0,1
o
o
o
S
S
S
S
S
WestEast
SouthNorth
)0,0,0,2()0,0,1,1()0,0,1,1(
%705.22 )0,1,1,2()0,1,0,1()0,0,1,1(
)0,0,1,1()0,0,0,9()0,0,1,10(
21
021
SS
SS
S
P
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,
– Circular polarisation can result from multiple scatters from dust
100%
0%
60%
Polarimetry in Astronomy 9
Polarisation in AGN
Absorbed by torus
Synchrotron emission
Direct view
Dichroic absorptionby dust
BLR
NLR
Torus
Polarimetry in Astronomy 10
Synchrotron
Polarimetry in Astronomy 11
Polarisation in AGN
Scattered off torus
Absorbed by torus
Synchrotron emission
Scattered in the NLR
Direct view
Dichroic absorptionby dust
BLR
NLR
Torus
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 off torus
Absorbed by torus
Synchrotron emission
Scattered in the NLR
Direct view
Dichroic absorptionby dust
BLR
NLR
Scattered by moving scatterers
Torus
Polarimetry in Astronomy 15
Scattered by moving scatterers
Polarimetry in Astronomy 16
Polarimeter
To spectrograph or imager
To TV guider
Tilted slit/dekker
Arc lamp
/2 plate
Analyser
Calcite
/4 plate
Polarimetry in Astronomy 17
Polarimetry
} Object {
} Sky {oe
oe
oeoe
SpectroscopyImaging
Polarimetry in Astronomy 18
• 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
Measuring the Stokes Parameters
45
02
I
IRQ
1
1
U
U
R
RU
5.67
5.222
I
IRU
1
1
Q
Q
R
RQ
Q
U1tan2
1
I
UQ 2 2P
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 P~0.1% you need SNR ~1400 or 2E6 photons!
SNRP
2100
Polarimetry in Astronomy 20
Scattered by NLR Scattered by torus