On the possibility of magnetic field detection of a source star at caustic crossing

In a microlensing event, at caustic crossing, flux of a spot can be magnified several times with respect of the source star. This magnification contrast provide a unique opportunity to measure the magnetic field over a source star. In this work we investigate possibility of magnetic field detection through the Zeeman effect when a source crosses a caustic line. Using Fourier analysis, one can separate other broadening agents from actual Zeeman broadening. At next stage we use microlensing formalism to measure the magnification contrast of spot and source and finally using two reliable strategy, we find the delectability of magnetic field as a function of maximum magnification for spots with different sizes. The resolution and the signal to noise in each strategy determine which magnetic fields are detectable.