NICER observation list: Target ID 366701

The puzzling variety of super-orbital modulations seen in the long term light-curves of Supergiant High Mass X-ray binaries (sgHMXBs), opens up new questions on the stellar wind interaction with a highly magnetised neutron star. We propose a joint 40 ks NuSTAR + 10 ks NICER observation of the persistent sgHMXB 4U 1538-52 at its super-orbital maximum and minimum to conclusively understand the mechanisms driving these flux modulations. The results of energy resolved pulsation studies, pulse phase resolved spectroscopic study of photon indices, absorption column density and cyclotron lines will be used to differentiate or constrain the various models explaining these super-orbital modulations.