MULTI-EPOCH BROADBAND MONITORING OF NGC5907 ULX1

Following a series of remarkable recent discoveries, we now know thatsome of the most luminous members of the ultraluminous Xray source(ULX) population are actually powered by highly superEddington 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 virgul 1e41 erg/s. Here we propose a pair ofsimultaneous NuSTAR+XMM observations of NGC5907 ULX1. These will allowus to continue to track the longterm evolution of the neutron star spin,help investigate why the pulsations are transient in some ULX pulsars, andexplore the nature of the longterm 78d Xray period present in this system. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]