082432

We now know that some of the most luminous members of the ultraluminous X-ray source openParULXclosePar population are actually powered by highly super-Eddington pulsars comma making them the most extreme accretors known. Three such systems are now knowndoublePoint M82 X-2 comma NGC 7793 P13 comma and NGC 5907 ULX. These sources all exhibit unusual .off.-states in which their X-ray flux drops by virgul2 orders of magnitude openParor moreclosePar. These off-states are fairly rare comma but recent monitoring with Swift shows that NGC 5907 ULX is now entering one. Here we propose an ambitious series of six 60 ks observations of NGC 5907 ULX openPar360 ks in totalclosePar designed to determine the nature of these events. This is of critical importance comma as these off-states may offer a means to determine the magnetic fields of these systems.