Determining the Nature of the Extreme Off-States in ULX Pulsars with NGC5907

Following a series of remarkable discoveries, we now know that some ofthe most luminous ULXs are actually powered by highly super-Eddingtonpulsars (i.e. neutron stars), making them the most extreme accretorsknown. A number of the known ULX pulsars exhibit unusual 'off'-states inwhich their X-ray flux drops by ~2 orders of magnitude (or more). Theseoff-states are fairly rare, but recent monitoring with Swift shows thatNGC5907 ULX1, the most extreme of the ULX pulsars with L(X,peak)~1e41erg/s, is now entering one. Here we propose three 60ks observationsof NGC5907 ULX1 (180ks in total) designed to continue our efforts todetermine the nature of these events. This is of critical importance,as these off-states may offer a means to determine the B-fields ofthese systems.