088422

Following a series of remarkable discoveries comma we now know that some of the most luminous ULXs are actually powered by highly super-Eddington pulsars openPari.e. neutron starsclosePar comma making them the most extreme accretors known. A number of the known ULX pulsars 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 NGC5907 ULX1 comma the most extreme of the ULX pulsars with LopenParX comma peakcloseParvirgul1e41 erg-s comma is now entering one. Here we propose three 60ks observations of NGC5907 ULX1 openPar180ks in totalclosePar designed to continue our efforts to determine the nature of these events. This is of critical importance comma as these off-states may offer a means to determine the B-fields of these systems.