Effects of plasma on gravitational lensing

We study gravitational lensing when plasma surrounds the lens. An extra deflection angle is induced by the plasma in addition to the deflection generated by gravity. An inhomogeneous plasma distribution generates a greater effect than a homogeneous one, and may cause significant effects to be detected in low frequency radio observations (a few hundred MHz). In particular, the lensed image positions will be different between optical and radio observations. The change of position due to a plasma can reach a few tens of milli-arcsec, which is readily detectable. One can use the position difference in different frequencies to estimate the density of plasma in the lens. The magnification ratios between multiple images are mainly determined by other properties of the lens, and are only weakly affected by the plasma. More importantly, we find that the strong lensing time delay will be affected by the plasma. Estimation of the Hubble constant from the time delay in low radio frequency observation may be slightly biased due to plasma in the lens. Unfortunately, the ionosphere of the Earth strongly affects low frequency radio observations. Thus our ability to detect the effect depends on how well we are able to calibrate out the ionosphere.

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