NICER observation list: Target ID 251101

PSR B0540-69 recently transitioned to a previously unknown state with a 36% increase in spin-down rate and a large change in the braking index from 2.1 to near 0, a rare value for any pulsar. On-going Swift/XRT observations show that the pulsar remains in this new state. With its large effective area, low background, rapid readout, and accurate timing, NICER is by far the most efficient instrument for extending the ephemeris. We propose 12 short (200 s), monthly observations to maintain a phase-connected ephemeris; to search for a possible return to its old spin-down state; to measure the evolution in the braking index; to search for a change in the gamma-ray flux or pulse profile; and to improve our understanding of this extraordinary pulsar.