Sensors

Very-long-baseline interferometry (VLBI) and polarimetry

What are the technological capabilities that allow the detection of magnetic fields from space structures located million light-years away? The example of the detection of the magnetic fields of M87's black hole.

Presentation "VLBI Polarimetry Techniques I" - Notes and figures

• Origin of polarization: https://youtu.be/J_oXou6QGpI?t=483

  • Unlike scalar waves (such as sound), EM waves have polarization i.e. directionality of oscillations (Figure 1). By convention, we refer to polarization as the orientation of the electric field.

  • Ordinary light is made of photons with random E field orientations (Figure 2). 

  • Certain media may select specific orientations of the electric field. This is the case, for instance, for polarized glasses.

• Two polarization modes: 

  • Linear polarization: The electric field has a preferred direction of oscillations

  • Circular polarization: The electric field rotates around the direction of propagation (Poynting vector)

• Description of polarization using the Stokes parameters

  • In order to fully describe polarization, the four Stokes parameters, I, Q, U and V, are needed. These describe how much polarized vs unpolarized light we have, how much circular polarization we have and what is the strength and orientation of the linear and circular polarization.

• Different analyses can be performed in space. 

  • For instance, electrons in very strong magnetic fields are rotating around magnetic field lines with acceleration, thereby emitting synchrotron light. This can be analyzed.

• Measuring Polarization: https://youtu.be/J_oXou6QGpI?t=1442

  • How polarizers register polarization (Figure 3 and 4).

• Polarization and interferometry/ https://youtu.be/J_oXou6QGpI?t=1684

• VLBI in a nutshell: https://youtu.be/J_oXou6QGpI?t=2024

  • We place an increasing number of radiotelescopes on a map and we appreciate how a representation is recontructed.

  • First we place one radiotelescope and we get a poor image. Same for two. When we place three telescopes, we have three baselines, meaning three different combinations for a plane wave, or three plane waves. These will interfere and will be added coherently to offer a better reconstruction (Figure 4).

  • By using the Earth rotation and performing "Earth rotation synthesis" we obtain a high-fidelity image (Figure 4).