The effects of plasma on the magnification and time delay of strongly lensed fast radio bursts

The number of identified Fast Radio Bursts (FRBs) is increasing rapidly with current and future facilities. Strongly lensed FRBs are expected to be found as well, which can provide precise time delays and thus have rich applications in cosmology and fundamental physics. However, the radio signal of lensed FRBs will be deflected by plasma in lens galaxies in addition to gravity. Such deflections by both gravity and plasma will cause frequency dependent time delays, which are different from the dispersion delay and the geometric delay caused by gravitational lensing. Depending on the lensing and plasma models, the frequency-time delay relation of the lensed images can show distinguishing behaviours either between the multiple images, or from the dispersion relation. Such phenomena cannot be neglected in future studies, especially at low radio frequency, as plasma exists in lens galaxies in general. More importantly, such information provides not only a potential way to search for lensed FRBs, but also constraints on the mass and plasma distributions in lens galaxy. In particular, plasma may make the missing central images observable at low radio frequency.