Where Has All the Polarization Gone?
نویسنده
چکیده
Circumnuclear tori are a central ingredient in the unification of the AGN phenomenon, but the conditions in the tori, the jet collimation, and the accretion mechanisms are still poorly constrained. Magnetic fields are involved in jet collimation and probably in feeding material into the nucleus, but those that are derived using equipartition are uncertain since equipartition conditions are not known to hold. A more direct measurement of magnetic field strength can be made using Faraday rotation (FR) and free-free absorption (FFA). FR changes the electric vector position angle of a polarized wave passing through a magnetized plasma by an angle χ = Rm λ , where the rotation measure Rm is the path integral over the line-of-sight component of the magnetic field, B‖, and the density of thermal electrons, ne. FFA depends on ne, the path length, L, and the electron temperature, Te. The value of L can be estimated by assuming it to be the same as the width of the region of FFA in the VLBI images. Making reasonable assumptions about Te, estimates of ne can also be derived from the FFA measurements. Thus, a joint analysis of FR and FFA measurements can provide direct diagnostics of the magnetic-field strength B‖ with minimum imposed assumptions.
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