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

We now know that some of the most luminous members of theultraluminous X-ray source (ULX) population are actually powered byhighly super-Eddington pulsars, making them the most extreme accretorsknown. Three such systems are now known: M82 X-2, NGC 7793 P13, and NGC5907 ULX. These sources all exhibit unusual `off'-states in which theirX-ray flux drops by ~2 orders of magnitude (or more). These off-statesare fairly rare, but recent monitoring with Swift shows that NGC 5907ULX is now entering one. Here we propose an ambitious series of six 60ks observations of NGC 5907 ULX (360 ks in total) designed to determinethe nature of these events. This is of critical importance, as theseoff-states may offer a means to determine the magnetic fields of thesesystems.