NICER observation list: Target ID 770201

X-ray polarization has proven to be effective in constraining the orientation of the innermost regions of black hole binaries (BHBs). However, polarimetric measurements can tell us little about the extension of the corona and how it interacts with the accretion disk during the outbursts of BHBs. On the other hand, X-ray reverberation lags, produced by the different light crossing time between the Comptonized photons and the reprocessed ones, is sensitive to the size of the corona and its separation from the accretion disk. We aim to combine these two analyses to study the evolution of GX 339-4 coronal geometry. We propose to observe the source in 3 states: hard, intermediate, and soft state with IXPE + NICER to measure the polarization evolution and reverberation lags during the outburst.