091308

Following a series of remarkable recent discoveries we now know that some of the most luminous members of the ultraluminous X-ray source (ULX) population are actually powered by highly super-Eddington pulsars (i.e. neutron stars) making them the most extreme sustained accretors known. Six such systems are now known but NGC5907 ULX1 stands out as the most luminous peaking at Lx virgul 1e41 erg/s. Here we propose a pair of simultaneous NuSTAR+XMM observations of NGC5907 ULX1. These will allow us to continue to track the long-term evolution of the neutron star spin help investigate why the pulsations are transient in some ULX pulsars and explore the nature of the long-term 78d X-ray period present in this system. [truncated! Please see actual data for full text]