Gamma- and X-ray polarization Follow-up
Mózsi Kiss
June 1 2007 – 5A5461: Experimental techniques for particle astrophysics
Crump Institute for Molecular Imaging
x
z y
Rybicki & Lightman, Fig. 4.11d Wikipedia
Quantum mechanical interpretation of polarization?
Classical physics: electromagnetic waves.
Polarization is the direction of the E-field
Crump Institute for Molecular Imaging
P ol ar iza ti on v ec tor (E)
x
z y
Quantum mechanics: photons. Plane wave solution of electromagnetic wave equation:
where is given by
E: amplitude of the electric field
Polarization given by the “Jones vector”
Quantum mechanical description (state vectors, Hermitian operators, probability amplitudes, etc.) follow naturally from Maxwell’s equations
Reference kindly provided by Jacob Trier Frederiksen
What processes can yield polarization at source?
1. Compton scattering:
• Klein-Nishina formula photons have a higher probability to scatter perpendicular to the polarization vector of the incident photons
• “Selecting” photons scattering at a certain angle “selecting” photons with a certain polarization
Radioactive source
Lead block Scattering
material
Emitted photons (unpolarized)
Scattered photons (polarized)
Cold outer disc
Hot inner disc
Black hole
Emitted photons (unpolarized) Scattered
photons
(polarized)
What processes can yield polarization at source?
2. Synchrotron radiation
In the frame of the electron: emitted radiation has dipole character
In the frame of the observer: emitted radiation is beamed forward
Rybicki & Lightman, Fig. 3.5 Rybicki & Lightman, Fig. 4.11d
M. M. Nikitin, Russian Physics Journal,
Vol. 15, No. 4 (1972)
What processes can yield polarization at source?
3. Strong magnetic fields
From J. S. Heyl, et al., MNRAS, Vol. 311, No. 3 (2000):
“Extremely strong magnetic fields change the vacuum index of refraction.
Although this polarization-dependent effect is small for typical neutron stars, it is large enough to decouple the polarization states of photons travelling within the field. The photon states evolve adiabatically and follow the changing magnetic field direction. The combination of a rotating magnetosphere and a frequency- dependent-state decoupling predicts polarization phase lags between different wavebands, if the emission process takes place well within the light cylinder. This QED effect may allow observations to distinguish between different pulsar-
emission mechanisms and to reconstruct the structure of the magnetosphere.“
QED strong magnetic fields refractive index n 1 frequency-dependent
decoupling of photon polarization states frequency-dependent polarization
phase lags different polarization angles for different wavelengths
Sources of background in a Compton-based detector?
PoGOLite collab.
PoGOLite – Polarized Gamma-ray Observer
PoGOLite collab.
• Neutrons (atmospheric and structure- induced): use paraffin shield to remove
• Cosmic rays: high energy deposition
(1.8 MeV/(g/cm
2) for muons) easy to reject (low energy resolution sufficient)
• Gamma-rays in field of view: use Compton kinematics (high energy resolution needed, can be difficult)
• Gamma-rays near field of view: use
collimation and good pointing accuracy to minimize source confusion
• Gamma-rays from other directions: use a
high-Z shield to detect or absorb
Loss/change of polarization from distant sources?
Wikipedia
