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

We now know that some of the most luminous members of the ultraluminous Xraysource (ULX) population are actually powered by highly superEddington pulsars,making them the most extreme accretors known. Three such systems are now known\\:M82 X2, NGC 7793 P13, and NGC 5907 ULX. These sources all exhibit unusualoff.states in which their Xray flux drops by virgul2 orders of magnitude (ormore). These offstates are fairly rare, but recent monitoring with Swift showsthat NGC 5907 ULX is now entering one. Here we propose an ambitious series ofsix 60 ks observations of NGC 5907 ULX (360 ks in total) designed to determinethe nature of these events. This is of critical importance, as these offstatesmay offer a means to determine the magnetic fields of these systems. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]