093141

Following a series of remarkable recent discoveries we now know thatsome 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 accretorsknown. Six such systems are now known but NGC5907 ULX1 stands out asthe most luminous peaking at Lx ~ 1e41 erg/s. Here we propose a pair ofsimultaneous NuSTAR+XMM observations of NGC5907 ULX1. These will allowus to continue to track the long-term evolution of the neutron star spinhelp investigate why the pulsations are transient in some ULX pulsars andexplore the nature of the long-term 78d X-ray period present in this system. [truncated! Please see actual data for full text]