NICER observation list: Target ID 354401

In Dec. 2011, B0540-69, the most luminous gamma-ray pulsar known, transitioned to a previously unknown state with a 36% increase in spin-down rate and a large change in the braking index (n) from 2.1 to ~0, much smaller than for almost any other pulsar. The transition is likely due to a reconfiguration of the global magnetic field or currents. After the transition, first XRT and now NICER observations show that the pulsar remains in this new state with a slowly evolving n. NICER is by far the most efficient instrument to extend the ephemeris of B0540-69. We propose short observations to maintain a phase-connected ephemeris; search for a return to its old spin-down state; measure the evolving n; search for changes in the flux; and improve our understanding of this extraordinary pulsar.